merge(glittershark/achilles): Subtree import at 'b93268085a' r/2279

Imported from https://github.com/glittershark/achilles/

git-subtree-dir: users/glittershark/achilles
git-subtree-mainline: 4d193f239525954631ba9d789ca3aea9a2f4e14d
git-subtree-split: b93268085aab14c80a400c299da5d04d2781098e
Change-Id: I64a583b454bbe03e20358ad7808939a4cbc212ba

22 files changed, 3567 insertions, 0 deletions

diff --git a/users/glittershark/achilles/src/ast/hir.rs b/users/glittershark/achilles/src/ast/hir.rs
new file mode 100644
index 000000000000..6859174a2dd0
--- /dev/null
+++ b/users/glittershark/achilles/src/ast/hir.rs
@@ -0,0 +1,252 @@
+use itertools::Itertools;
+
+use super::{BinaryOperator, Ident, Literal, UnaryOperator};
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Binding<'a, T> {
+    pub ident: Ident<'a>,
+    pub type_: T,
+    pub body: Expr<'a, T>,
+}
+
+impl<'a, T> Binding<'a, T> {
+    fn to_owned(&self) -> Binding<'static, T>
+    where
+        T: Clone,
+    {
+        Binding {
+            ident: self.ident.to_owned(),
+            type_: self.type_.clone(),
+            body: self.body.to_owned(),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Expr<'a, T> {
+    Ident(Ident<'a>, T),
+
+    Literal(Literal<'a>, T),
+
+    UnaryOp {
+        op: UnaryOperator,
+        rhs: Box<Expr<'a, T>>,
+        type_: T,
+    },
+
+    BinaryOp {
+        lhs: Box<Expr<'a, T>>,
+        op: BinaryOperator,
+        rhs: Box<Expr<'a, T>>,
+        type_: T,
+    },
+
+    Let {
+        bindings: Vec<Binding<'a, T>>,
+        body: Box<Expr<'a, T>>,
+        type_: T,
+    },
+
+    If {
+        condition: Box<Expr<'a, T>>,
+        then: Box<Expr<'a, T>>,
+        else_: Box<Expr<'a, T>>,
+        type_: T,
+    },
+
+    Fun {
+        args: Vec<(Ident<'a>, T)>,
+        body: Box<Expr<'a, T>>,
+        type_: T,
+    },
+
+    Call {
+        fun: Box<Expr<'a, T>>,
+        args: Vec<Expr<'a, T>>,
+        type_: T,
+    },
+}
+
+impl<'a, T> Expr<'a, T> {
+    pub fn type_(&self) -> &T {
+        match self {
+            Expr::Ident(_, t) => t,
+            Expr::Literal(_, t) => t,
+            Expr::UnaryOp { type_, .. } => type_,
+            Expr::BinaryOp { type_, .. } => type_,
+            Expr::Let { type_, .. } => type_,
+            Expr::If { type_, .. } => type_,
+            Expr::Fun { type_, .. } => type_,
+            Expr::Call { type_, .. } => type_,
+        }
+    }
+
+    pub fn traverse_type<F, U, E>(self, f: F) -> Result<Expr<'a, U>, E>
+    where
+        F: Fn(T) -> Result<U, E> + Clone,
+    {
+        match self {
+            Expr::Ident(id, t) => Ok(Expr::Ident(id, f(t)?)),
+            Expr::Literal(lit, t) => Ok(Expr::Literal(lit, f(t)?)),
+            Expr::UnaryOp { op, rhs, type_ } => Ok(Expr::UnaryOp {
+                op,
+                rhs: Box::new(rhs.traverse_type(f.clone())?),
+                type_: f(type_)?,
+            }),
+            Expr::BinaryOp {
+                lhs,
+                op,
+                rhs,
+                type_,
+            } => Ok(Expr::BinaryOp {
+                lhs: Box::new(lhs.traverse_type(f.clone())?),
+                op,
+                rhs: Box::new(rhs.traverse_type(f.clone())?),
+                type_: f(type_)?,
+            }),
+            Expr::Let {
+                bindings,
+                body,
+                type_,
+            } => Ok(Expr::Let {
+                bindings: bindings
+                    .into_iter()
+                    .map(|Binding { ident, type_, body }| {
+                        Ok(Binding {
+                            ident,
+                            type_: f(type_)?,
+                            body: body.traverse_type(f.clone())?,
+                        })
+                    })
+                    .collect::<Result<Vec<_>, E>>()?,
+                body: Box::new(body.traverse_type(f.clone())?),
+                type_: f(type_)?,
+            }),
+            Expr::If {
+                condition,
+                then,
+                else_,
+                type_,
+            } => Ok(Expr::If {
+                condition: Box::new(condition.traverse_type(f.clone())?),
+                then: Box::new(then.traverse_type(f.clone())?),
+                else_: Box::new(else_.traverse_type(f.clone())?),
+                type_: f(type_)?,
+            }),
+            Expr::Fun { args, body, type_ } => Ok(Expr::Fun {
+                args: args
+                    .into_iter()
+                    .map(|(id, t)| Ok((id, f.clone()(t)?)))
+                    .collect::<Result<Vec<_>, E>>()?,
+                body: Box::new(body.traverse_type(f.clone())?),
+                type_: f(type_)?,
+            }),
+            Expr::Call { fun, args, type_ } => Ok(Expr::Call {
+                fun: Box::new(fun.traverse_type(f.clone())?),
+                args: args
+                    .into_iter()
+                    .map(|e| e.traverse_type(f.clone()))
+                    .collect::<Result<Vec<_>, E>>()?,
+                type_: f(type_)?,
+            }),
+        }
+    }
+
+    pub fn to_owned(&self) -> Expr<'static, T>
+    where
+        T: Clone,
+    {
+        match self {
+            Expr::Ident(id, t) => Expr::Ident(id.to_owned(), t.clone()),
+            Expr::Literal(lit, t) => Expr::Literal(lit.to_owned(), t.clone()),
+            Expr::UnaryOp { op, rhs, type_ } => Expr::UnaryOp {
+                op: *op,
+                rhs: Box::new((**rhs).to_owned()),
+                type_: type_.clone(),
+            },
+            Expr::BinaryOp {
+                lhs,
+                op,
+                rhs,
+                type_,
+            } => Expr::BinaryOp {
+                lhs: Box::new((**lhs).to_owned()),
+                op: *op,
+                rhs: Box::new((**rhs).to_owned()),
+                type_: type_.clone(),
+            },
+            Expr::Let {
+                bindings,
+                body,
+                type_,
+            } => Expr::Let {
+                bindings: bindings.into_iter().map(|b| b.to_owned()).collect(),
+                body: Box::new((**body).to_owned()),
+                type_: type_.clone(),
+            },
+            Expr::If {
+                condition,
+                then,
+                else_,
+                type_,
+            } => Expr::If {
+                condition: Box::new((**condition).to_owned()),
+                then: Box::new((**then).to_owned()),
+                else_: Box::new((**else_).to_owned()),
+                type_: type_.clone(),
+            },
+            Expr::Fun { args, body, type_ } => Expr::Fun {
+                args: args
+                    .into_iter()
+                    .map(|(id, t)| (id.to_owned(), t.clone()))
+                    .collect(),
+                body: Box::new((**body).to_owned()),
+                type_: type_.clone(),
+            },
+            Expr::Call { fun, args, type_ } => Expr::Call {
+                fun: Box::new((**fun).to_owned()),
+                args: args.into_iter().map(|e| e.to_owned()).collect(),
+                type_: type_.clone(),
+            },
+        }
+    }
+}
+
+pub enum Decl<'a, T> {
+    Fun {
+        name: Ident<'a>,
+        args: Vec<(Ident<'a>, T)>,
+        body: Box<Expr<'a, T>>,
+        type_: T,
+    },
+}
+
+impl<'a, T> Decl<'a, T> {
+    pub fn type_(&self) -> &T {
+        match self {
+            Decl::Fun { type_, .. } => type_,
+        }
+    }
+
+    pub fn traverse_type<F, U, E>(self, f: F) -> Result<Decl<'a, U>, E>
+    where
+        F: Fn(T) -> Result<U, E> + Clone,
+    {
+        match self {
+            Decl::Fun {
+                name,
+                args,
+                body,
+                type_,
+            } => Ok(Decl::Fun {
+                name,
+                args: args
+                    .into_iter()
+                    .map(|(id, t)| Ok((id, f(t)?)))
+                    .try_collect()?,
+                body: Box::new(body.traverse_type(f.clone())?),
+                type_: f(type_)?,
+            }),
+        }
+    }
+}
diff --git a/users/glittershark/achilles/src/ast/mod.rs b/users/glittershark/achilles/src/ast/mod.rs
new file mode 100644
index 000000000000..3a2261aeda23
--- /dev/null
+++ b/users/glittershark/achilles/src/ast/mod.rs
@@ -0,0 +1,410 @@
+pub(crate) mod hir;
+
+use std::borrow::Cow;
+use std::collections::HashMap;
+use std::convert::TryFrom;
+use std::fmt::{self, Display, Formatter};
+
+use itertools::Itertools;
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct InvalidIdentifier<'a>(Cow<'a, str>);
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+pub struct Ident<'a>(pub Cow<'a, str>);
+
+impl<'a> From<&'a Ident<'a>> for &'a str {
+    fn from(id: &'a Ident<'a>) -> Self {
+        id.0.as_ref()
+    }
+}
+
+impl<'a> Display for Ident<'a> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl<'a> Ident<'a> {
+    pub fn to_owned(&self) -> Ident<'static> {
+        Ident(Cow::Owned(self.0.clone().into_owned()))
+    }
+
+    pub fn from_str_unchecked(s: &'a str) -> Self {
+        debug_assert!(is_valid_identifier(s));
+        Self(Cow::Borrowed(s))
+    }
+
+    pub fn from_string_unchecked(s: String) -> Self {
+        debug_assert!(is_valid_identifier(&s));
+        Self(Cow::Owned(s))
+    }
+}
+
+pub fn is_valid_identifier<S>(s: &S) -> bool
+where
+    S: AsRef<str> + ?Sized,
+{
+    s.as_ref()
+        .chars()
+        .any(|c| !c.is_alphanumeric() || !"_".contains(c))
+}
+
+impl<'a> TryFrom<&'a str> for Ident<'a> {
+    type Error = InvalidIdentifier<'a>;
+
+    fn try_from(s: &'a str) -> Result<Self, Self::Error> {
+        if is_valid_identifier(s) {
+            Ok(Ident(Cow::Borrowed(s)))
+        } else {
+            Err(InvalidIdentifier(Cow::Borrowed(s)))
+        }
+    }
+}
+
+impl<'a> TryFrom<String> for Ident<'a> {
+    type Error = InvalidIdentifier<'static>;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        if is_valid_identifier(&s) {
+            Ok(Ident(Cow::Owned(s)))
+        } else {
+            Err(InvalidIdentifier(Cow::Owned(s)))
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum BinaryOperator {
+    /// `+`
+    Add,
+
+    /// `-`
+    Sub,
+
+    /// `*`
+    Mul,
+
+    /// `/`
+    Div,
+
+    /// `^`
+    Pow,
+
+    /// `==`
+    Equ,
+
+    /// `!=`
+    Neq,
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum UnaryOperator {
+    /// !
+    Not,
+
+    /// -
+    Neg,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Literal<'a> {
+    Int(u64),
+    Bool(bool),
+    String(Cow<'a, str>),
+}
+
+impl<'a> Literal<'a> {
+    pub fn to_owned(&self) -> Literal<'static> {
+        match self {
+            Literal::Int(i) => Literal::Int(*i),
+            Literal::Bool(b) => Literal::Bool(*b),
+            Literal::String(s) => Literal::String(Cow::Owned(s.clone().into_owned())),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Binding<'a> {
+    pub ident: Ident<'a>,
+    pub type_: Option<Type<'a>>,
+    pub body: Expr<'a>,
+}
+
+impl<'a> Binding<'a> {
+    fn to_owned(&self) -> Binding<'static> {
+        Binding {
+            ident: self.ident.to_owned(),
+            type_: self.type_.as_ref().map(|t| t.to_owned()),
+            body: self.body.to_owned(),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Expr<'a> {
+    Ident(Ident<'a>),
+
+    Literal(Literal<'a>),
+
+    UnaryOp {
+        op: UnaryOperator,
+        rhs: Box<Expr<'a>>,
+    },
+
+    BinaryOp {
+        lhs: Box<Expr<'a>>,
+        op: BinaryOperator,
+        rhs: Box<Expr<'a>>,
+    },
+
+    Let {
+        bindings: Vec<Binding<'a>>,
+        body: Box<Expr<'a>>,
+    },
+
+    If {
+        condition: Box<Expr<'a>>,
+        then: Box<Expr<'a>>,
+        else_: Box<Expr<'a>>,
+    },
+
+    Fun(Box<Fun<'a>>),
+
+    Call {
+        fun: Box<Expr<'a>>,
+        args: Vec<Expr<'a>>,
+    },
+
+    Ascription {
+        expr: Box<Expr<'a>>,
+        type_: Type<'a>,
+    },
+}
+
+impl<'a> Expr<'a> {
+    pub fn to_owned(&self) -> Expr<'static> {
+        match self {
+            Expr::Ident(ref id) => Expr::Ident(id.to_owned()),
+            Expr::Literal(ref lit) => Expr::Literal(lit.to_owned()),
+            Expr::UnaryOp { op, rhs } => Expr::UnaryOp {
+                op: *op,
+                rhs: Box::new((**rhs).to_owned()),
+            },
+            Expr::BinaryOp { lhs, op, rhs } => Expr::BinaryOp {
+                lhs: Box::new((**lhs).to_owned()),
+                op: *op,
+                rhs: Box::new((**rhs).to_owned()),
+            },
+            Expr::Let { bindings, body } => Expr::Let {
+                bindings: bindings.iter().map(|binding| binding.to_owned()).collect(),
+                body: Box::new((**body).to_owned()),
+            },
+            Expr::If {
+                condition,
+                then,
+                else_,
+            } => Expr::If {
+                condition: Box::new((**condition).to_owned()),
+                then: Box::new((**then).to_owned()),
+                else_: Box::new((**else_).to_owned()),
+            },
+            Expr::Fun(fun) => Expr::Fun(Box::new((**fun).to_owned())),
+            Expr::Call { fun, args } => Expr::Call {
+                fun: Box::new((**fun).to_owned()),
+                args: args.iter().map(|arg| arg.to_owned()).collect(),
+            },
+            Expr::Ascription { expr, type_ } => Expr::Ascription {
+                expr: Box::new((**expr).to_owned()),
+                type_: type_.to_owned(),
+            },
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Arg<'a> {
+    pub ident: Ident<'a>,
+    pub type_: Option<Type<'a>>,
+}
+
+impl<'a> Arg<'a> {
+    pub fn to_owned(&self) -> Arg<'static> {
+        Arg {
+            ident: self.ident.to_owned(),
+            type_: self.type_.as_ref().map(Type::to_owned),
+        }
+    }
+}
+
+impl<'a> TryFrom<&'a str> for Arg<'a> {
+    type Error = <Ident<'a> as TryFrom<&'a str>>::Error;
+
+    fn try_from(value: &'a str) -> Result<Self, Self::Error> {
+        Ok(Arg {
+            ident: Ident::try_from(value)?,
+            type_: None,
+        })
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Fun<'a> {
+    pub args: Vec<Arg<'a>>,
+    pub body: Expr<'a>,
+}
+
+impl<'a> Fun<'a> {
+    pub fn to_owned(&self) -> Fun<'static> {
+        Fun {
+            args: self.args.iter().map(|arg| arg.to_owned()).collect(),
+            body: self.body.to_owned(),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Decl<'a> {
+    Fun { name: Ident<'a>, body: Fun<'a> },
+    Ascription { name: Ident<'a>, type_: Type<'a> },
+}
+
+////
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct FunctionType<'a> {
+    pub args: Vec<Type<'a>>,
+    pub ret: Box<Type<'a>>,
+}
+
+impl<'a> FunctionType<'a> {
+    pub fn to_owned(&self) -> FunctionType<'static> {
+        FunctionType {
+            args: self.args.iter().map(|a| a.to_owned()).collect(),
+            ret: Box::new((*self.ret).to_owned()),
+        }
+    }
+}
+
+impl<'a> Display for FunctionType<'a> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "fn {} -> {}", self.args.iter().join(", "), self.ret)
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Type<'a> {
+    Int,
+    Float,
+    Bool,
+    CString,
+    Var(Ident<'a>),
+    Function(FunctionType<'a>),
+}
+
+impl<'a> Type<'a> {
+    pub fn to_owned(&self) -> Type<'static> {
+        match self {
+            Type::Int => Type::Int,
+            Type::Float => Type::Float,
+            Type::Bool => Type::Bool,
+            Type::CString => Type::CString,
+            Type::Var(v) => Type::Var(v.to_owned()),
+            Type::Function(f) => Type::Function(f.to_owned()),
+        }
+    }
+
+    pub fn alpha_equiv(&self, other: &Self) -> bool {
+        fn do_alpha_equiv<'a>(
+            substs: &mut HashMap<&'a Ident<'a>, &'a Ident<'a>>,
+            lhs: &'a Type,
+            rhs: &'a Type,
+        ) -> bool {
+            match (lhs, rhs) {
+                (Type::Var(v1), Type::Var(v2)) => substs.entry(v1).or_insert(v2) == &v2,
+                (
+                    Type::Function(FunctionType {
+                        args: args1,
+                        ret: ret1,
+                    }),
+                    Type::Function(FunctionType {
+                        args: args2,
+                        ret: ret2,
+                    }),
+                ) => {
+                    args1.len() == args2.len()
+                        && args1
+                            .iter()
+                            .zip(args2)
+                            .all(|(a1, a2)| do_alpha_equiv(substs, a1, a2))
+                        && do_alpha_equiv(substs, ret1, ret2)
+                }
+                _ => lhs == rhs,
+            }
+        }
+
+        let mut substs = HashMap::new();
+        do_alpha_equiv(&mut substs, self, other)
+    }
+}
+
+impl<'a> Display for Type<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Type::Int => f.write_str("int"),
+            Type::Float => f.write_str("float"),
+            Type::Bool => f.write_str("bool"),
+            Type::CString => f.write_str("cstring"),
+            Type::Var(v) => v.fmt(f),
+            Type::Function(ft) => ft.fmt(f),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn type_var(n: &str) -> Type<'static> {
+        Type::Var(Ident::try_from(n.to_owned()).unwrap())
+    }
+
+    mod alpha_equiv {
+        use super::*;
+
+        #[test]
+        fn trivial() {
+            assert!(Type::Int.alpha_equiv(&Type::Int));
+            assert!(!Type::Int.alpha_equiv(&Type::Bool));
+        }
+
+        #[test]
+        fn simple_type_var() {
+            assert!(type_var("a").alpha_equiv(&type_var("b")));
+        }
+
+        #[test]
+        fn function_with_type_vars_equiv() {
+            assert!(Type::Function(FunctionType {
+                args: vec![type_var("a")],
+                ret: Box::new(type_var("b")),
+            })
+            .alpha_equiv(&Type::Function(FunctionType {
+                args: vec![type_var("b")],
+                ret: Box::new(type_var("a")),
+            })))
+        }
+
+        #[test]
+        fn function_with_type_vars_non_equiv() {
+            assert!(!Type::Function(FunctionType {
+                args: vec![type_var("a")],
+                ret: Box::new(type_var("a")),
+            })
+            .alpha_equiv(&Type::Function(FunctionType {
+                args: vec![type_var("b")],
+                ret: Box::new(type_var("a")),
+            })))
+        }
+    }
+}
diff --git a/users/glittershark/achilles/src/codegen/llvm.rs b/users/glittershark/achilles/src/codegen/llvm.rs
new file mode 100644
index 000000000000..ee087845b640
--- /dev/null
+++ b/users/glittershark/achilles/src/codegen/llvm.rs
@@ -0,0 +1,365 @@
+use std::convert::{TryFrom, TryInto};
+use std::path::Path;
+use std::result;
+
+use inkwell::basic_block::BasicBlock;
+use inkwell::builder::Builder;
+pub use inkwell::context::Context;
+use inkwell::module::Module;
+use inkwell::support::LLVMString;
+use inkwell::types::{BasicType, BasicTypeEnum, FunctionType, IntType};
+use inkwell::values::{AnyValueEnum, BasicValueEnum, FunctionValue};
+use inkwell::IntPredicate;
+use thiserror::Error;
+
+use crate::ast::hir::{Binding, Decl, Expr};
+use crate::ast::{BinaryOperator, Ident, Literal, Type, UnaryOperator};
+use crate::common::env::Env;
+
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum Error {
+    #[error("Undefined variable {0}")]
+    UndefinedVariable(Ident<'static>),
+
+    #[error("LLVM Error: {0}")]
+    LLVMError(String),
+}
+
+impl From<LLVMString> for Error {
+    fn from(s: LLVMString) -> Self {
+        Self::LLVMError(s.to_string())
+    }
+}
+
+pub type Result<T> = result::Result<T, Error>;
+
+pub struct Codegen<'ctx, 'ast> {
+    context: &'ctx Context,
+    pub module: Module<'ctx>,
+    builder: Builder<'ctx>,
+    env: Env<&'ast Ident<'ast>, AnyValueEnum<'ctx>>,
+    function_stack: Vec<FunctionValue<'ctx>>,
+    identifier_counter: u32,
+}
+
+impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
+    pub fn new(context: &'ctx Context, module_name: &str) -> Self {
+        let module = context.create_module(module_name);
+        let builder = context.create_builder();
+        Self {
+            context,
+            module,
+            builder,
+            env: Default::default(),
+            function_stack: Default::default(),
+            identifier_counter: 0,
+        }
+    }
+
+    pub fn new_function<'a>(
+        &'a mut self,
+        name: &str,
+        ty: FunctionType<'ctx>,
+    ) -> &'a FunctionValue<'ctx> {
+        self.function_stack
+            .push(self.module.add_function(name, ty, None));
+        let basic_block = self.append_basic_block("entry");
+        self.builder.position_at_end(basic_block);
+        self.function_stack.last().unwrap()
+    }
+
+    pub fn finish_function(&mut self, res: &BasicValueEnum<'ctx>) -> FunctionValue<'ctx> {
+        self.builder.build_return(Some(res));
+        self.function_stack.pop().unwrap()
+    }
+
+    pub fn append_basic_block(&self, name: &str) -> BasicBlock<'ctx> {
+        self.context
+            .append_basic_block(*self.function_stack.last().unwrap(), name)
+    }
+
+    pub fn codegen_expr(&mut self, expr: &'ast Expr<'ast, Type>) -> Result<AnyValueEnum<'ctx>> {
+        match expr {
+            Expr::Ident(id, _) => self
+                .env
+                .resolve(id)
+                .cloned()
+                .ok_or_else(|| Error::UndefinedVariable(id.to_owned())),
+            Expr::Literal(lit, ty) => {
+                let ty = self.codegen_int_type(ty);
+                match lit {
+                    Literal::Int(i) => Ok(AnyValueEnum::IntValue(ty.const_int(*i, false))),
+                    Literal::Bool(b) => Ok(AnyValueEnum::IntValue(
+                        ty.const_int(if *b { 1 } else { 0 }, false),
+                    )),
+                    Literal::String(_) => todo!(),
+                }
+            }
+            Expr::UnaryOp { op, rhs, .. } => {
+                let rhs = self.codegen_expr(rhs)?;
+                match op {
+                    UnaryOperator::Not => unimplemented!(),
+                    UnaryOperator::Neg => Ok(AnyValueEnum::IntValue(
+                        self.builder.build_int_neg(rhs.into_int_value(), "neg"),
+                    )),
+                }
+            }
+            Expr::BinaryOp { lhs, op, rhs, .. } => {
+                let lhs = self.codegen_expr(lhs)?;
+                let rhs = self.codegen_expr(rhs)?;
+                match op {
+                    BinaryOperator::Add => Ok(AnyValueEnum::IntValue(self.builder.build_int_add(
+                        lhs.into_int_value(),
+                        rhs.into_int_value(),
+                        "add",
+                    ))),
+                    BinaryOperator::Sub => Ok(AnyValueEnum::IntValue(self.builder.build_int_sub(
+                        lhs.into_int_value(),
+                        rhs.into_int_value(),
+                        "add",
+                    ))),
+                    BinaryOperator::Mul => Ok(AnyValueEnum::IntValue(self.builder.build_int_sub(
+                        lhs.into_int_value(),
+                        rhs.into_int_value(),
+                        "add",
+                    ))),
+                    BinaryOperator::Div => {
+                        Ok(AnyValueEnum::IntValue(self.builder.build_int_signed_div(
+                            lhs.into_int_value(),
+                            rhs.into_int_value(),
+                            "add",
+                        )))
+                    }
+                    BinaryOperator::Pow => unimplemented!(),
+                    BinaryOperator::Equ => {
+                        Ok(AnyValueEnum::IntValue(self.builder.build_int_compare(
+                            IntPredicate::EQ,
+                            lhs.into_int_value(),
+                            rhs.into_int_value(),
+                            "eq",
+                        )))
+                    }
+                    BinaryOperator::Neq => todo!(),
+                }
+            }
+            Expr::Let { bindings, body, .. } => {
+                self.env.push();
+                for Binding { ident, body, .. } in bindings {
+                    let val = self.codegen_expr(body)?;
+                    self.env.set(ident, val);
+                }
+                let res = self.codegen_expr(body);
+                self.env.pop();
+                res
+            }
+            Expr::If {
+                condition,
+                then,
+                else_,
+                type_,
+            } => {
+                let then_block = self.append_basic_block("then");
+                let else_block = self.append_basic_block("else");
+                let join_block = self.append_basic_block("join");
+                let condition = self.codegen_expr(condition)?;
+                self.builder.build_conditional_branch(
+                    condition.into_int_value(),
+                    then_block,
+                    else_block,
+                );
+                self.builder.position_at_end(then_block);
+                let then_res = self.codegen_expr(then)?;
+                self.builder.build_unconditional_branch(join_block);
+
+                self.builder.position_at_end(else_block);
+                let else_res = self.codegen_expr(else_)?;
+                self.builder.build_unconditional_branch(join_block);
+
+                self.builder.position_at_end(join_block);
+                let phi = self.builder.build_phi(self.codegen_type(type_), "join");
+                phi.add_incoming(&[
+                    (&BasicValueEnum::try_from(then_res).unwrap(), then_block),
+                    (&BasicValueEnum::try_from(else_res).unwrap(), else_block),
+                ]);
+                Ok(phi.as_basic_value().into())
+            }
+            Expr::Call { fun, args, .. } => {
+                if let Expr::Ident(id, _) = &**fun {
+                    let function = self
+                        .module
+                        .get_function(id.into())
+                        .or_else(|| self.env.resolve(id)?.clone().try_into().ok())
+                        .ok_or_else(|| Error::UndefinedVariable(id.to_owned()))?;
+                    let args = args
+                        .iter()
+                        .map(|arg| Ok(self.codegen_expr(arg)?.try_into().unwrap()))
+                        .collect::<Result<Vec<_>>>()?;
+                    Ok(self
+                        .builder
+                        .build_call(function, &args, "call")
+                        .try_as_basic_value()
+                        .left()
+                        .unwrap()
+                        .into())
+                } else {
+                    todo!()
+                }
+            }
+            Expr::Fun { args, body, .. } => {
+                let fname = self.fresh_ident("f");
+                let cur_block = self.builder.get_insert_block().unwrap();
+                let env = self.env.save(); // TODO: closures
+                let function = self.codegen_function(&fname, args, body)?;
+                self.builder.position_at_end(cur_block);
+                self.env.restore(env);
+                Ok(function.into())
+            }
+        }
+    }
+
+    pub fn codegen_function(
+        &mut self,
+        name: &str,
+        args: &'ast [(Ident<'ast>, Type)],
+        body: &'ast Expr<'ast, Type>,
+    ) -> Result<FunctionValue<'ctx>> {
+        self.new_function(
+            name,
+            self.codegen_type(body.type_()).fn_type(
+                args.iter()
+                    .map(|(_, at)| self.codegen_type(at))
+                    .collect::<Vec<_>>()
+                    .as_slice(),
+                false,
+            ),
+        );
+        self.env.push();
+        for (i, (arg, _)) in args.iter().enumerate() {
+            self.env.set(
+                arg,
+                self.cur_function().get_nth_param(i as u32).unwrap().into(),
+            );
+        }
+        let res = self.codegen_expr(body)?.try_into().unwrap();
+        self.env.pop();
+        Ok(self.finish_function(&res))
+    }
+
+    pub fn codegen_decl(&mut self, decl: &'ast Decl<'ast, Type>) -> Result<()> {
+        match decl {
+            Decl::Fun {
+                name, args, body, ..
+            } => {
+                self.codegen_function(name.into(), args, body)?;
+                Ok(())
+            }
+        }
+    }
+
+    pub fn codegen_main(&mut self, expr: &'ast Expr<'ast, Type>) -> Result<()> {
+        self.new_function("main", self.context.i64_type().fn_type(&[], false));
+        let res = self.codegen_expr(expr)?.try_into().unwrap();
+        if *expr.type_() != Type::Int {
+            self.builder
+                .build_return(Some(&self.context.i64_type().const_int(0, false)));
+        } else {
+            self.finish_function(&res);
+        }
+        Ok(())
+    }
+
+    fn codegen_type(&self, type_: &'ast Type) -> BasicTypeEnum<'ctx> {
+        // TODO
+        self.context.i64_type().into()
+    }
+
+    fn codegen_int_type(&self, type_: &'ast Type) -> IntType<'ctx> {
+        // TODO
+        self.context.i64_type()
+    }
+
+    pub fn print_to_file<P>(&self, path: P) -> Result<()>
+    where
+        P: AsRef<Path>,
+    {
+        Ok(self.module.print_to_file(path)?)
+    }
+
+    pub fn binary_to_file<P>(&self, path: P) -> Result<()>
+    where
+        P: AsRef<Path>,
+    {
+        if self.module.write_bitcode_to_path(path.as_ref()) {
+            Ok(())
+        } else {
+            Err(Error::LLVMError(
+                "Error writing bitcode to output path".to_owned(),
+            ))
+        }
+    }
+
+    fn fresh_ident(&mut self, prefix: &str) -> String {
+        self.identifier_counter += 1;
+        format!("{}{}", prefix, self.identifier_counter)
+    }
+
+    fn cur_function(&self) -> &FunctionValue<'ctx> {
+        self.function_stack.last().unwrap()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use inkwell::execution_engine::JitFunction;
+    use inkwell::OptimizationLevel;
+
+    use super::*;
+
+    fn jit_eval<T>(expr: &str) -> anyhow::Result<T> {
+        let expr = crate::parser::expr(expr).unwrap().1;
+
+        let expr = crate::tc::typecheck_expr(expr).unwrap();
+
+        let context = Context::create();
+        let mut codegen = Codegen::new(&context, "test");
+        let execution_engine = codegen
+            .module
+            .create_jit_execution_engine(OptimizationLevel::None)
+            .unwrap();
+
+        codegen.codegen_function("test", &[], &expr)?;
+
+        unsafe {
+            let fun: JitFunction<unsafe extern "C" fn() -> T> =
+                execution_engine.get_function("test")?;
+            Ok(fun.call())
+        }
+    }
+
+    #[test]
+    fn add_literals() {
+        assert_eq!(jit_eval::<i64>("1 + 2").unwrap(), 3);
+    }
+
+    #[test]
+    fn variable_shadowing() {
+        assert_eq!(
+            jit_eval::<i64>("let x = 1 in (let x = 2 in x) + x").unwrap(),
+            3
+        );
+    }
+
+    #[test]
+    fn eq() {
+        assert_eq!(
+            jit_eval::<i64>("let x = 1 in if x == 1 then 2 else 4").unwrap(),
+            2
+        );
+    }
+
+    #[test]
+    fn function_call() {
+        let res = jit_eval::<i64>("let id = fn x = x in id 1").unwrap();
+        assert_eq!(res, 1);
+    }
+}
diff --git a/users/glittershark/achilles/src/codegen/mod.rs b/users/glittershark/achilles/src/codegen/mod.rs
new file mode 100644
index 000000000000..8ef057dba04f
--- /dev/null
+++ b/users/glittershark/achilles/src/codegen/mod.rs
@@ -0,0 +1,25 @@
+pub mod llvm;
+
+use inkwell::execution_engine::JitFunction;
+use inkwell::OptimizationLevel;
+pub use llvm::*;
+
+use crate::ast::hir::Expr;
+use crate::ast::Type;
+use crate::common::Result;
+
+pub fn jit_eval<T>(expr: &Expr<Type>) -> Result<T> {
+    let context = Context::create();
+    let mut codegen = Codegen::new(&context, "eval");
+    let execution_engine = codegen
+        .module
+        .create_jit_execution_engine(OptimizationLevel::None)
+        .map_err(Error::from)?;
+    codegen.codegen_function("test", &[], &expr)?;
+
+    unsafe {
+        let fun: JitFunction<unsafe extern "C" fn() -> T> =
+            execution_engine.get_function("eval").unwrap();
+        Ok(fun.call())
+    }
+}
diff --git a/users/glittershark/achilles/src/commands/check.rs b/users/glittershark/achilles/src/commands/check.rs
new file mode 100644
index 000000000000..0bea482c1478
--- /dev/null
+++ b/users/glittershark/achilles/src/commands/check.rs
@@ -0,0 +1,39 @@
+use clap::Clap;
+use std::path::PathBuf;
+
+use crate::ast::Type;
+use crate::{parser, tc, Result};
+
+/// Typecheck a file or expression
+#[derive(Clap)]
+pub struct Check {
+    /// File to check
+    path: Option<PathBuf>,
+
+    /// Expression to check
+    #[clap(long, short = 'e')]
+    expr: Option<String>,
+}
+
+fn run_expr(expr: String) -> Result<Type<'static>> {
+    let (_, parsed) = parser::expr(&expr)?;
+    let hir_expr = tc::typecheck_expr(parsed)?;
+    Ok(hir_expr.type_().to_owned())
+}
+
+fn run_path(path: PathBuf) -> Result<Type<'static>> {
+    todo!()
+}
+
+impl Check {
+    pub fn run(self) -> Result<()> {
+        let type_ = match (self.path, self.expr) {
+            (None, None) => Err("Must specify either a file or expression to check".into()),
+            (Some(_), Some(_)) => Err("Cannot specify both a file and expression to check".into()),
+            (None, Some(expr)) => run_expr(expr),
+            (Some(path), None) => run_path(path),
+        }?;
+        println!("type: {}", type_);
+        Ok(())
+    }
+}
diff --git a/users/glittershark/achilles/src/commands/compile.rs b/users/glittershark/achilles/src/commands/compile.rs
new file mode 100644
index 000000000000..be8767575ab5
--- /dev/null
+++ b/users/glittershark/achilles/src/commands/compile.rs
@@ -0,0 +1,31 @@
+use std::path::PathBuf;
+
+use clap::Clap;
+
+use crate::common::Result;
+use crate::compiler::{self, CompilerOptions};
+
+/// Compile a source file
+#[derive(Clap)]
+pub struct Compile {
+    /// File to compile
+    file: PathBuf,
+
+    /// Output file
+    #[clap(short = 'o')]
+    out_file: PathBuf,
+
+    #[clap(flatten)]
+    options: CompilerOptions,
+}
+
+impl Compile {
+    pub fn run(self) -> Result<()> {
+        eprintln!(
+            ">>> {} -> {}",
+            &self.file.to_string_lossy(),
+            self.out_file.to_string_lossy()
+        );
+        compiler::compile_file(&self.file, &self.out_file, &self.options)
+    }
+}
diff --git a/users/glittershark/achilles/src/commands/eval.rs b/users/glittershark/achilles/src/commands/eval.rs
new file mode 100644
index 000000000000..61a712c08a8e
--- /dev/null
+++ b/users/glittershark/achilles/src/commands/eval.rs
@@ -0,0 +1,32 @@
+use clap::Clap;
+
+use crate::codegen;
+use crate::interpreter;
+use crate::parser;
+use crate::tc;
+use crate::Result;
+
+/// Evaluate an expression and print its result
+#[derive(Clap)]
+pub struct Eval {
+    /// JIT-compile with LLVM instead of interpreting
+    #[clap(long)]
+    jit: bool,
+
+    /// Expression to evaluate
+    expr: String,
+}
+
+impl Eval {
+    pub fn run(self) -> Result<()> {
+        let (_, parsed) = parser::expr(&self.expr)?;
+        let hir = tc::typecheck_expr(parsed)?;
+        let result = if self.jit {
+            codegen::jit_eval::<i64>(&hir)?.into()
+        } else {
+            interpreter::eval(&hir)?
+        };
+        println!("{}", result);
+        Ok(())
+    }
+}
diff --git a/users/glittershark/achilles/src/commands/mod.rs b/users/glittershark/achilles/src/commands/mod.rs
new file mode 100644
index 000000000000..fd0a822708c2
--- /dev/null
+++ b/users/glittershark/achilles/src/commands/mod.rs
@@ -0,0 +1,7 @@
+pub mod check;
+pub mod compile;
+pub mod eval;
+
+pub use check::Check;
+pub use compile::Compile;
+pub use eval::Eval;
diff --git a/users/glittershark/achilles/src/common/env.rs b/users/glittershark/achilles/src/common/env.rs
new file mode 100644
index 000000000000..59a5e46c466f
--- /dev/null
+++ b/users/glittershark/achilles/src/common/env.rs
@@ -0,0 +1,59 @@
+use std::borrow::Borrow;
+use std::collections::HashMap;
+use std::hash::Hash;
+use std::mem;
+
+/// A lexical environment
+#[derive(Debug, PartialEq, Eq)]
+pub struct Env<K: Eq + Hash, V>(Vec<HashMap<K, V>>);
+
+impl<K, V> Default for Env<K, V>
+where
+    K: Eq + Hash,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K, V> Env<K, V>
+where
+    K: Eq + Hash,
+{
+    pub fn new() -> Self {
+        Self(vec![Default::default()])
+    }
+
+    pub fn push(&mut self) {
+        self.0.push(Default::default());
+    }
+
+    pub fn pop(&mut self) {
+        self.0.pop();
+    }
+
+    pub fn save(&mut self) -> Self {
+        mem::take(self)
+    }
+
+    pub fn restore(&mut self, saved: Self) {
+        *self = saved;
+    }
+
+    pub fn set(&mut self, k: K, v: V) {
+        self.0.last_mut().unwrap().insert(k, v);
+    }
+
+    pub fn resolve<'a, Q>(&'a self, k: &Q) -> Option<&'a V>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq + ?Sized,
+    {
+        for ctx in self.0.iter().rev() {
+            if let Some(res) = ctx.get(k) {
+                return Some(res);
+            }
+        }
+        None
+    }
+}
diff --git a/users/glittershark/achilles/src/common/error.rs b/users/glittershark/achilles/src/common/error.rs
new file mode 100644
index 000000000000..51575a895e91
--- /dev/null
+++ b/users/glittershark/achilles/src/common/error.rs
@@ -0,0 +1,59 @@
+use std::{io, result};
+
+use thiserror::Error;
+
+use crate::{codegen, interpreter, parser, tc};
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    IOError(#[from] io::Error),
+
+    #[error("Error parsing input: {0}")]
+    ParseError(#[from] parser::Error),
+
+    #[error("Error evaluating expression: {0}")]
+    EvalError(#[from] interpreter::Error),
+
+    #[error("Compile error: {0}")]
+    CodegenError(#[from] codegen::Error),
+
+    #[error("Type error: {0}")]
+    TypeError(#[from] tc::Error),
+
+    #[error("{0}")]
+    Message(String),
+}
+
+impl From<String> for Error {
+    fn from(s: String) -> Self {
+        Self::Message(s)
+    }
+}
+
+impl<'a> From<&'a str> for Error {
+    fn from(s: &'a str) -> Self {
+        Self::Message(s.to_owned())
+    }
+}
+
+impl<'a> From<nom::Err<nom::error::Error<&'a str>>> for Error {
+    fn from(e: nom::Err<nom::error::Error<&'a str>>) -> Self {
+        use nom::error::Error as NomError;
+        use nom::Err::*;
+
+        Self::ParseError(match e {
+            Incomplete(i) => Incomplete(i),
+            Error(NomError { input, code }) => Error(NomError {
+                input: input.to_owned(),
+                code,
+            }),
+            Failure(NomError { input, code }) => Failure(NomError {
+                input: input.to_owned(),
+                code,
+            }),
+        })
+    }
+}
+
+pub type Result<T> = result::Result<T, Error>;
diff --git a/users/glittershark/achilles/src/common/mod.rs b/users/glittershark/achilles/src/common/mod.rs
new file mode 100644
index 000000000000..8368a6dd180f
--- /dev/null
+++ b/users/glittershark/achilles/src/common/mod.rs
@@ -0,0 +1,6 @@
+pub(crate) mod env;
+pub(crate) mod error;
+pub(crate) mod namer;
+
+pub use error::{Error, Result};
+pub use namer::{Namer, NamerOf};
diff --git a/users/glittershark/achilles/src/common/namer.rs b/users/glittershark/achilles/src/common/namer.rs
new file mode 100644
index 000000000000..016e9f6ed99a
--- /dev/null
+++ b/users/glittershark/achilles/src/common/namer.rs
@@ -0,0 +1,122 @@
+use std::fmt::Display;
+use std::marker::PhantomData;
+
+pub struct Namer<T, F> {
+    make_name: F,
+    counter: u64,
+    _phantom: PhantomData<T>,
+}
+
+impl<T, F> Namer<T, F> {
+    pub fn new(make_name: F) -> Self {
+        Namer {
+            make_name,
+            counter: 0,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+impl Namer<String, Box<dyn Fn(u64) -> String>> {
+    pub fn with_prefix<T>(prefix: T) -> Self
+    where
+        T: Display + 'static,
+    {
+        Namer::new(move |i| format!("{}{}", prefix, i)).boxed()
+    }
+
+    pub fn with_suffix<T>(suffix: T) -> Self
+    where
+        T: Display + 'static,
+    {
+        Namer::new(move |i| format!("{}{}", i, suffix)).boxed()
+    }
+
+    pub fn alphabetic() -> Self {
+        Namer::new(|i| {
+            if i <= 26 {
+                std::char::from_u32((i + 96) as u32).unwrap().to_string()
+            } else {
+                format!(
+                    "{}{}",
+                    std::char::from_u32(((i % 26) + 96) as u32).unwrap(),
+                    i - 26
+                )
+            }
+        })
+        .boxed()
+    }
+}
+
+impl<T, F> Namer<T, F>
+where
+    F: Fn(u64) -> T,
+{
+    pub fn make_name(&mut self) -> T {
+        self.counter += 1;
+        (self.make_name)(self.counter)
+    }
+
+    pub fn boxed(self) -> NamerOf<T>
+    where
+        F: 'static,
+    {
+        Namer {
+            make_name: Box::new(self.make_name),
+            counter: self.counter,
+            _phantom: self._phantom,
+        }
+    }
+
+    pub fn map<G, U>(self, f: G) -> NamerOf<U>
+    where
+        G: Fn(T) -> U + 'static,
+        T: 'static,
+        F: 'static,
+    {
+        Namer {
+            counter: self.counter,
+            make_name: Box::new(move |x| f((self.make_name)(x))),
+            _phantom: PhantomData,
+        }
+    }
+}
+
+pub type NamerOf<T> = Namer<T, Box<dyn Fn(u64) -> T>>;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn prefix() {
+        let mut namer = Namer::with_prefix("t");
+        assert_eq!(namer.make_name(), "t1");
+        assert_eq!(namer.make_name(), "t2");
+    }
+
+    #[test]
+    fn suffix() {
+        let mut namer = Namer::with_suffix("t");
+        assert_eq!(namer.make_name(), "1t");
+        assert_eq!(namer.make_name(), "2t");
+    }
+
+    #[test]
+    fn alphabetic() {
+        let mut namer = Namer::alphabetic();
+        assert_eq!(namer.make_name(), "a");
+        assert_eq!(namer.make_name(), "b");
+        (0..25).for_each(|_| {
+            namer.make_name();
+        });
+        assert_eq!(namer.make_name(), "b2");
+    }
+
+    #[test]
+    fn custom_callback() {
+        let mut namer = Namer::new(|n| n + 1);
+        assert_eq!(namer.make_name(), 2);
+        assert_eq!(namer.make_name(), 3);
+    }
+}
diff --git a/users/glittershark/achilles/src/compiler.rs b/users/glittershark/achilles/src/compiler.rs
new file mode 100644
index 000000000000..f925b267df57
--- /dev/null
+++ b/users/glittershark/achilles/src/compiler.rs
@@ -0,0 +1,86 @@
+use std::fmt::{self, Display};
+use std::path::Path;
+use std::str::FromStr;
+use std::{fs, result};
+
+use clap::Clap;
+use test_strategy::Arbitrary;
+
+use crate::codegen::{self, Codegen};
+use crate::common::Result;
+use crate::{parser, tc};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Arbitrary)]
+pub enum OutputFormat {
+    LLVM,
+    Bitcode,
+}
+
+impl Default for OutputFormat {
+    fn default() -> Self {
+        Self::Bitcode
+    }
+}
+
+impl FromStr for OutputFormat {
+    type Err = String;
+
+    fn from_str(s: &str) -> result::Result<Self, Self::Err> {
+        match s {
+            "llvm" => Ok(Self::LLVM),
+            "binary" => Ok(Self::Bitcode),
+            _ => Err(format!(
+                "Invalid output format {}, expected one of {{llvm, binary}}",
+                s
+            )),
+        }
+    }
+}
+
+impl Display for OutputFormat {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            OutputFormat::LLVM => f.write_str("llvm"),
+            OutputFormat::Bitcode => f.write_str("binary"),
+        }
+    }
+}
+
+#[derive(Clap, Debug, PartialEq, Eq, Default)]
+pub struct CompilerOptions {
+    #[clap(long, short = 'f', default_value)]
+    format: OutputFormat,
+}
+
+pub fn compile_file(input: &Path, output: &Path, options: &CompilerOptions) -> Result<()> {
+    let src = fs::read_to_string(input)?;
+    let (_, decls) = parser::toplevel(&src)?; // TODO: statements
+    let decls = tc::typecheck_toplevel(decls)?;
+
+    let context = codegen::Context::create();
+    let mut codegen = Codegen::new(
+        &context,
+        &input
+            .file_stem()
+            .map_or("UNKNOWN".to_owned(), |s| s.to_string_lossy().into_owned()),
+    );
+    for decl in &decls {
+        codegen.codegen_decl(decl)?;
+    }
+    match options.format {
+        OutputFormat::LLVM => codegen.print_to_file(output)?,
+        OutputFormat::Bitcode => codegen.binary_to_file(output)?,
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test_strategy::proptest;
+
+    #[proptest]
+    fn output_format_display_from_str_round_trip(of: OutputFormat) {
+        assert_eq!(OutputFormat::from_str(&of.to_string()), Ok(of));
+    }
+}
diff --git a/users/glittershark/achilles/src/interpreter/error.rs b/users/glittershark/achilles/src/interpreter/error.rs
new file mode 100644
index 000000000000..268d6f479a1e
--- /dev/null
+++ b/users/glittershark/achilles/src/interpreter/error.rs
@@ -0,0 +1,19 @@
+use std::result;
+
+use thiserror::Error;
+
+use crate::ast::{Ident, Type};
+
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum Error {
+    #[error("Undefined variable {0}")]
+    UndefinedVariable(Ident<'static>),
+
+    #[error("Unexpected type {actual}, expected type {expected}")]
+    InvalidType {
+        actual: Type<'static>,
+        expected: Type<'static>,
+    },
+}
+
+pub type Result<T> = result::Result<T, Error>;
diff --git a/users/glittershark/achilles/src/interpreter/mod.rs b/users/glittershark/achilles/src/interpreter/mod.rs
new file mode 100644
index 000000000000..3bfeeb52e85c
--- /dev/null
+++ b/users/glittershark/achilles/src/interpreter/mod.rs
@@ -0,0 +1,176 @@
+mod error;
+mod value;
+
+pub use self::error::{Error, Result};
+pub use self::value::{Function, Value};
+use crate::ast::hir::{Binding, Expr};
+use crate::ast::{BinaryOperator, FunctionType, Ident, Literal, Type, UnaryOperator};
+use crate::common::env::Env;
+
+#[derive(Debug, Default)]
+pub struct Interpreter<'a> {
+    env: Env<&'a Ident<'a>, Value<'a>>,
+}
+
+impl<'a> Interpreter<'a> {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    fn resolve(&self, var: &'a Ident<'a>) -> Result<Value<'a>> {
+        self.env
+            .resolve(var)
+            .cloned()
+            .ok_or_else(|| Error::UndefinedVariable(var.to_owned()))
+    }
+
+    pub fn eval(&mut self, expr: &'a Expr<'a, Type>) -> Result<Value<'a>> {
+        let res = match expr {
+            Expr::Ident(id, _) => self.resolve(id),
+            Expr::Literal(Literal::Int(i), _) => Ok((*i).into()),
+            Expr::Literal(Literal::Bool(b), _) => Ok((*b).into()),
+            Expr::Literal(Literal::String(s), _) => Ok(s.clone().into()),
+            Expr::UnaryOp { op, rhs, .. } => {
+                let rhs = self.eval(rhs)?;
+                match op {
+                    UnaryOperator::Neg => -rhs,
+                    _ => unimplemented!(),
+                }
+            }
+            Expr::BinaryOp { lhs, op, rhs, .. } => {
+                let lhs = self.eval(lhs)?;
+                let rhs = self.eval(rhs)?;
+                match op {
+                    BinaryOperator::Add => lhs + rhs,
+                    BinaryOperator::Sub => lhs - rhs,
+                    BinaryOperator::Mul => lhs * rhs,
+                    BinaryOperator::Div => lhs / rhs,
+                    BinaryOperator::Pow => todo!(),
+                    BinaryOperator::Equ => Ok(lhs.eq(&rhs).into()),
+                    BinaryOperator::Neq => todo!(),
+                }
+            }
+            Expr::Let { bindings, body, .. } => {
+                self.env.push();
+                for Binding { ident, body, .. } in bindings {
+                    let val = self.eval(body)?;
+                    self.env.set(ident, val);
+                }
+                let res = self.eval(body)?;
+                self.env.pop();
+                Ok(res)
+            }
+            Expr::If {
+                condition,
+                then,
+                else_,
+                ..
+            } => {
+                let condition = self.eval(condition)?;
+                if *(condition.as_type::<bool>()?) {
+                    self.eval(then)
+                } else {
+                    self.eval(else_)
+                }
+            }
+            Expr::Call { ref fun, args, .. } => {
+                let fun = self.eval(fun)?;
+                let expected_type = FunctionType {
+                    args: args.iter().map(|_| Type::Int).collect(),
+                    ret: Box::new(Type::Int),
+                };
+
+                let Function {
+                    args: function_args,
+                    body,
+                    ..
+                } = fun.as_function(expected_type)?;
+                let arg_values = function_args.iter().zip(
+                    args.iter()
+                        .map(|v| self.eval(v))
+                        .collect::<Result<Vec<_>>>()?,
+                );
+                let mut interpreter = Interpreter::new();
+                for (arg_name, arg_value) in arg_values {
+                    interpreter.env.set(arg_name, arg_value);
+                }
+                Ok(Value::from(*interpreter.eval(body)?.as_type::<i64>()?))
+            }
+            Expr::Fun { args, body, type_ } => {
+                let type_ = match type_ {
+                    Type::Function(ft) => ft.clone(),
+                    _ => unreachable!("Function expression without function type"),
+                };
+
+                Ok(Value::from(value::Function {
+                    // TODO
+                    type_,
+                    args: args.iter().map(|(arg, _)| arg.to_owned()).collect(),
+                    body: (**body).to_owned(),
+                }))
+            }
+        }?;
+        debug_assert_eq!(&res.type_(), expr.type_());
+        Ok(res)
+    }
+}
+
+pub fn eval<'a>(expr: &'a Expr<'a, Type>) -> Result<Value<'a>> {
+    let mut interpreter = Interpreter::new();
+    interpreter.eval(expr)
+}
+
+#[cfg(test)]
+mod tests {
+    use std::convert::TryFrom;
+
+    use super::value::{TypeOf, Val};
+    use super::*;
+    use BinaryOperator::*;
+
+    fn int_lit(i: u64) -> Box<Expr<'static, Type<'static>>> {
+        Box::new(Expr::Literal(Literal::Int(i), Type::Int))
+    }
+
+    fn do_eval<T>(src: &str) -> T
+    where
+        for<'a> &'a T: TryFrom<&'a Val<'a>>,
+        T: Clone + TypeOf,
+    {
+        let expr = crate::parser::expr(src).unwrap().1;
+        let hir = crate::tc::typecheck_expr(expr).unwrap();
+        let res = eval(&hir).unwrap();
+        res.as_type::<T>().unwrap().clone()
+    }
+
+    #[test]
+    fn simple_addition() {
+        let expr = Expr::BinaryOp {
+            lhs: int_lit(1),
+            op: Mul,
+            rhs: int_lit(2),
+            type_: Type::Int,
+        };
+        let res = eval(&expr).unwrap();
+        assert_eq!(*res.as_type::<i64>().unwrap(), 2);
+    }
+
+    #[test]
+    fn variable_shadowing() {
+        let res = do_eval::<i64>("let x = 1 in (let x = 2 in x) + x");
+        assert_eq!(res, 3);
+    }
+
+    #[test]
+    fn conditional_with_equals() {
+        let res = do_eval::<i64>("let x = 1 in if x == 1 then 2 else 4");
+        assert_eq!(res, 2);
+    }
+
+    #[test]
+    #[ignore]
+    fn function_call() {
+        let res = do_eval::<i64>("let id = fn x = x in id 1");
+        assert_eq!(res, 1);
+    }
+}
diff --git a/users/glittershark/achilles/src/interpreter/value.rs b/users/glittershark/achilles/src/interpreter/value.rs
new file mode 100644
index 000000000000..55ba42f9de58
--- /dev/null
+++ b/users/glittershark/achilles/src/interpreter/value.rs
@@ -0,0 +1,203 @@
+use std::borrow::Cow;
+use std::convert::TryFrom;
+use std::fmt::{self, Display};
+use std::ops::{Add, Div, Mul, Neg, Sub};
+use std::rc::Rc;
+use std::result;
+
+use derive_more::{Deref, From, TryInto};
+
+use super::{Error, Result};
+use crate::ast::hir::Expr;
+use crate::ast::{FunctionType, Ident, Type};
+
+#[derive(Debug, Clone)]
+pub struct Function<'a> {
+    pub type_: FunctionType<'a>,
+    pub args: Vec<Ident<'a>>,
+    pub body: Expr<'a, Type<'a>>,
+}
+
+#[derive(From, TryInto)]
+#[try_into(owned, ref)]
+pub enum Val<'a> {
+    Int(i64),
+    Float(f64),
+    Bool(bool),
+    String(Cow<'a, str>),
+    Function(Function<'a>),
+}
+
+impl<'a> TryFrom<Val<'a>> for String {
+    type Error = ();
+
+    fn try_from(value: Val<'a>) -> result::Result<Self, Self::Error> {
+        match value {
+            Val::String(s) => Ok(s.into_owned()),
+            _ => Err(()),
+        }
+    }
+}
+
+impl<'a> fmt::Debug for Val<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Val::Int(x) => f.debug_tuple("Int").field(x).finish(),
+            Val::Float(x) => f.debug_tuple("Float").field(x).finish(),
+            Val::Bool(x) => f.debug_tuple("Bool").field(x).finish(),
+            Val::String(s) => f.debug_tuple("String").field(s).finish(),
+            Val::Function(Function { type_, .. }) => {
+                f.debug_struct("Function").field("type_", type_).finish()
+            }
+        }
+    }
+}
+
+impl<'a> PartialEq for Val<'a> {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Val::Int(x), Val::Int(y)) => x == y,
+            (Val::Float(x), Val::Float(y)) => x == y,
+            (Val::Bool(x), Val::Bool(y)) => x == y,
+            (Val::Function(_), Val::Function(_)) => false,
+            (_, _) => false,
+        }
+    }
+}
+
+impl<'a> From<u64> for Val<'a> {
+    fn from(i: u64) -> Self {
+        Self::from(i as i64)
+    }
+}
+
+impl<'a> Display for Val<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Val::Int(x) => x.fmt(f),
+            Val::Float(x) => x.fmt(f),
+            Val::Bool(x) => x.fmt(f),
+            Val::String(s) => write!(f, "{:?}", s),
+            Val::Function(Function { type_, .. }) => write!(f, "<{}>", type_),
+        }
+    }
+}
+
+impl<'a> Val<'a> {
+    pub fn type_(&self) -> Type {
+        match self {
+            Val::Int(_) => Type::Int,
+            Val::Float(_) => Type::Float,
+            Val::Bool(_) => Type::Bool,
+            Val::String(_) => Type::CString,
+            Val::Function(Function { type_, .. }) => Type::Function(type_.clone()),
+        }
+    }
+
+    pub fn as_type<'b, T>(&'b self) -> Result<&'b T>
+    where
+        T: TypeOf + 'b + Clone,
+        &'b T: TryFrom<&'b Self>,
+    {
+        <&T>::try_from(self).map_err(|_| Error::InvalidType {
+            actual: self.type_().to_owned(),
+            expected: <T as TypeOf>::type_of(),
+        })
+    }
+
+    pub fn as_function<'b>(&'b self, function_type: FunctionType) -> Result<&'b Function<'a>> {
+        match self {
+            Val::Function(f) if f.type_ == function_type => Ok(&f),
+            _ => Err(Error::InvalidType {
+                actual: self.type_().to_owned(),
+                expected: Type::Function(function_type.to_owned()),
+            }),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Clone, Deref)]
+pub struct Value<'a>(Rc<Val<'a>>);
+
+impl<'a> Display for Value<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl<'a, T> From<T> for Value<'a>
+where
+    Val<'a>: From<T>,
+{
+    fn from(x: T) -> Self {
+        Self(Rc::new(x.into()))
+    }
+}
+
+impl<'a> Neg for Value<'a> {
+    type Output = Result<Value<'a>>;
+
+    fn neg(self) -> Self::Output {
+        Ok((-self.as_type::<i64>()?).into())
+    }
+}
+
+impl<'a> Add for Value<'a> {
+    type Output = Result<Value<'a>>;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Ok((self.as_type::<i64>()? + rhs.as_type::<i64>()?).into())
+    }
+}
+
+impl<'a> Sub for Value<'a> {
+    type Output = Result<Value<'a>>;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        Ok((self.as_type::<i64>()? - rhs.as_type::<i64>()?).into())
+    }
+}
+
+impl<'a> Mul for Value<'a> {
+    type Output = Result<Value<'a>>;
+
+    fn mul(self, rhs: Self) -> Self::Output {
+        Ok((self.as_type::<i64>()? * rhs.as_type::<i64>()?).into())
+    }
+}
+
+impl<'a> Div for Value<'a> {
+    type Output = Result<Value<'a>>;
+
+    fn div(self, rhs: Self) -> Self::Output {
+        Ok((self.as_type::<f64>()? / rhs.as_type::<f64>()?).into())
+    }
+}
+
+pub trait TypeOf {
+    fn type_of() -> Type<'static>;
+}
+
+impl TypeOf for i64 {
+    fn type_of() -> Type<'static> {
+        Type::Int
+    }
+}
+
+impl TypeOf for bool {
+    fn type_of() -> Type<'static> {
+        Type::Bool
+    }
+}
+
+impl TypeOf for f64 {
+    fn type_of() -> Type<'static> {
+        Type::Float
+    }
+}
+
+impl TypeOf for String {
+    fn type_of() -> Type<'static> {
+        Type::CString
+    }
+}
diff --git a/users/glittershark/achilles/src/main.rs b/users/glittershark/achilles/src/main.rs
new file mode 100644
index 000000000000..4ba0aaf33e91
--- /dev/null
+++ b/users/glittershark/achilles/src/main.rs
@@ -0,0 +1,38 @@
+#![feature(str_split_once)]
+#![feature(or_insert_with_key)]
+
+use clap::Clap;
+
+pub mod ast;
+pub mod codegen;
+pub(crate) mod commands;
+pub(crate) mod common;
+pub mod compiler;
+pub mod interpreter;
+#[macro_use]
+pub mod parser;
+pub mod tc;
+
+pub use common::{Error, Result};
+
+#[derive(Clap)]
+struct Opts {
+    #[clap(subcommand)]
+    subcommand: Command,
+}
+
+#[derive(Clap)]
+enum Command {
+    Eval(commands::Eval),
+    Compile(commands::Compile),
+    Check(commands::Check),
+}
+
+fn main() -> anyhow::Result<()> {
+    let opts = Opts::parse();
+    match opts.subcommand {
+        Command::Eval(eval) => Ok(eval.run()?),
+        Command::Compile(compile) => Ok(compile.run()?),
+        Command::Check(check) => Ok(check.run()?),
+    }
+}
diff --git a/users/glittershark/achilles/src/parser/expr.rs b/users/glittershark/achilles/src/parser/expr.rs
new file mode 100644
index 000000000000..12c55df02b80
--- /dev/null
+++ b/users/glittershark/achilles/src/parser/expr.rs
@@ -0,0 +1,620 @@
+use std::borrow::Cow;
+
+use nom::alt;
+use nom::character::complete::{digit1, multispace0, multispace1};
+use nom::{
+    call, char, complete, delimited, do_parse, flat_map, many0, map, named, opt, parse_to,
+    preceded, separated_list0, separated_list1, tag, tuple,
+};
+use pratt::{Affix, Associativity, PrattParser, Precedence};
+
+use crate::ast::{BinaryOperator, Binding, Expr, Fun, Literal, UnaryOperator};
+use crate::parser::{arg, ident, type_};
+
+#[derive(Debug)]
+enum TokenTree<'a> {
+    Prefix(UnaryOperator),
+    // Postfix(char),
+    Infix(BinaryOperator),
+    Primary(Expr<'a>),
+    Group(Vec<TokenTree<'a>>),
+}
+
+named!(prefix(&str) -> TokenTree, map!(alt!(
+    complete!(char!('-')) => { |_| UnaryOperator::Neg } |
+    complete!(char!('!')) => { |_| UnaryOperator::Not }
+), TokenTree::Prefix));
+
+named!(infix(&str) -> TokenTree, map!(alt!(
+    complete!(tag!("==")) => { |_| BinaryOperator::Equ } |
+    complete!(tag!("!=")) => { |_| BinaryOperator::Neq } |
+    complete!(char!('+')) => { |_| BinaryOperator::Add } |
+    complete!(char!('-')) => { |_| BinaryOperator::Sub } |
+    complete!(char!('*')) => { |_| BinaryOperator::Mul } |
+    complete!(char!('/')) => { |_| BinaryOperator::Div } |
+    complete!(char!('^')) => { |_| BinaryOperator::Pow }
+), TokenTree::Infix));
+
+named!(primary(&str) -> TokenTree, alt!(
+    do_parse!(
+        multispace0 >>
+        char!('(') >>
+        multispace0 >>
+        group: group >>
+        multispace0 >>
+        char!(')') >>
+        multispace0 >>
+            (TokenTree::Group(group))
+    ) |
+    delimited!(multispace0, simple_expr, multispace0) => { |s| TokenTree::Primary(s) }
+));
+
+named!(
+    rest(&str) -> Vec<(TokenTree, Vec<TokenTree>, TokenTree)>,
+    many0!(tuple!(
+        infix,
+        delimited!(multispace0, many0!(prefix), multispace0),
+        primary
+        // many0!(postfix)
+    ))
+);
+
+named!(group(&str) -> Vec<TokenTree>, do_parse!(
+    prefix: many0!(prefix)
+        >> primary: primary
+        // >> postfix: many0!(postfix)
+        >> rest: rest
+        >> ({
+            let mut res = prefix;
+            res.push(primary);
+            // res.append(&mut postfix);
+            for (infix, mut prefix, primary/*, mut postfix*/) in rest {
+                res.push(infix);
+                res.append(&mut prefix);
+                res.push(primary);
+                // res.append(&mut postfix);
+            }
+            res
+        })
+));
+
+fn token_tree(i: &str) -> nom::IResult<&str, Vec<TokenTree>> {
+    group(i)
+}
+
+struct ExprParser;
+
+impl<'a, I> PrattParser<I> for ExprParser
+where
+    I: Iterator<Item = TokenTree<'a>>,
+{
+    type Error = pratt::NoError;
+    type Input = TokenTree<'a>;
+    type Output = Expr<'a>;
+
+    fn query(&mut self, input: &Self::Input) -> Result<Affix, Self::Error> {
+        use BinaryOperator::*;
+        use UnaryOperator::*;
+
+        Ok(match input {
+            TokenTree::Infix(Add) => Affix::Infix(Precedence(6), Associativity::Left),
+            TokenTree::Infix(Sub) => Affix::Infix(Precedence(6), Associativity::Left),
+            TokenTree::Infix(Mul) => Affix::Infix(Precedence(7), Associativity::Left),
+            TokenTree::Infix(Div) => Affix::Infix(Precedence(7), Associativity::Left),
+            TokenTree::Infix(Pow) => Affix::Infix(Precedence(8), Associativity::Right),
+            TokenTree::Infix(Equ) => Affix::Infix(Precedence(4), Associativity::Right),
+            TokenTree::Infix(Neq) => Affix::Infix(Precedence(4), Associativity::Right),
+            TokenTree::Prefix(Neg) => Affix::Prefix(Precedence(6)),
+            TokenTree::Prefix(Not) => Affix::Prefix(Precedence(6)),
+            TokenTree::Primary(_) => Affix::Nilfix,
+            TokenTree::Group(_) => Affix::Nilfix,
+        })
+    }
+
+    fn primary(&mut self, input: Self::Input) -> Result<Self::Output, Self::Error> {
+        Ok(match input {
+            TokenTree::Primary(expr) => expr,
+            TokenTree::Group(group) => self.parse(&mut group.into_iter()).unwrap(),
+            _ => unreachable!(),
+        })
+    }
+
+    fn infix(
+        &mut self,
+        lhs: Self::Output,
+        op: Self::Input,
+        rhs: Self::Output,
+    ) -> Result<Self::Output, Self::Error> {
+        let op = match op {
+            TokenTree::Infix(op) => op,
+            _ => unreachable!(),
+        };
+        Ok(Expr::BinaryOp {
+            lhs: Box::new(lhs),
+            op,
+            rhs: Box::new(rhs),
+        })
+    }
+
+    fn prefix(&mut self, op: Self::Input, rhs: Self::Output) -> Result<Self::Output, Self::Error> {
+        let op = match op {
+            TokenTree::Prefix(op) => op,
+            _ => unreachable!(),
+        };
+
+        Ok(Expr::UnaryOp {
+            op,
+            rhs: Box::new(rhs),
+        })
+    }
+
+    fn postfix(
+        &mut self,
+        _lhs: Self::Output,
+        _op: Self::Input,
+    ) -> Result<Self::Output, Self::Error> {
+        unreachable!()
+    }
+}
+
+named!(int(&str) -> Literal, map!(flat_map!(digit1, parse_to!(u64)), Literal::Int));
+
+named!(bool_(&str) -> Literal, alt!(
+    complete!(tag!("true")) => { |_| Literal::Bool(true) } |
+    complete!(tag!("false")) => { |_| Literal::Bool(false) }
+));
+
+fn string_internal(i: &str) -> nom::IResult<&str, Cow<'_, str>, nom::error::Error<&str>> {
+    let (s, rem) = i
+        .split_once('"')
+        .ok_or_else(|| nom::Err::Error(nom::error::Error::new(i, nom::error::ErrorKind::Tag)))?;
+    Ok((rem, Cow::Borrowed(s)))
+}
+
+named!(string(&str) -> Literal, preceded!(
+    complete!(char!('"')),
+    map!(
+        string_internal,
+        |s| Literal::String(s)
+    )
+));
+
+named!(literal(&str) -> Literal, alt!(int | bool_ | string));
+
+named!(literal_expr(&str) -> Expr, map!(literal, Expr::Literal));
+
+named!(binding(&str) -> Binding, do_parse!(
+    multispace0
+        >> ident: ident
+        >> multispace0
+        >> type_: opt!(preceded!(tuple!(tag!(":"), multispace0), type_))
+        >> multispace0
+        >> char!('=')
+        >> multispace0
+        >> body: expr
+        >> (Binding {
+            ident,
+            type_,
+            body
+        })
+));
+
+named!(let_(&str) -> Expr, do_parse!(
+    tag!("let")
+        >> multispace0
+        >> bindings: separated_list1!(alt!(char!(';') | char!('\n')), binding)
+        >> multispace0
+        >> tag!("in")
+        >> multispace0
+        >> body: expr
+        >> (Expr::Let {
+            bindings,
+            body: Box::new(body)
+        })
+));
+
+named!(if_(&str) -> Expr, do_parse! (
+    tag!("if")
+        >> multispace0
+        >> condition: expr
+        >> multispace0
+        >> tag!("then")
+        >> multispace0
+        >> then: expr
+        >> multispace0
+        >> tag!("else")
+        >> multispace0
+        >> else_: expr
+        >> (Expr::If {
+            condition: Box::new(condition),
+            then: Box::new(then),
+            else_: Box::new(else_)
+        })
+));
+
+named!(ident_expr(&str) -> Expr, map!(ident, Expr::Ident));
+
+fn ascripted<'a>(
+    p: impl Fn(&'a str) -> nom::IResult<&'a str, Expr, nom::error::Error<&'a str>> + 'a,
+) -> impl Fn(&'a str) -> nom::IResult<&str, Expr, nom::error::Error<&'a str>> {
+    move |i| {
+        do_parse!(
+            i,
+            expr: p
+                >> multispace0
+                >> complete!(tag!(":"))
+                >> multispace0
+                >> type_: type_
+                >> (Expr::Ascription {
+                    expr: Box::new(expr),
+                    type_
+                })
+        )
+    }
+}
+
+named!(paren_expr(&str) -> Expr,
+       delimited!(complete!(tag!("(")), expr, complete!(tag!(")"))));
+
+named!(funcref(&str) -> Expr, alt!(
+    ident_expr |
+    paren_expr
+));
+
+named!(no_arg_call(&str) -> Expr, do_parse!(
+    fun: funcref
+        >> multispace0
+        >> complete!(tag!("()"))
+        >> (Expr::Call {
+            fun: Box::new(fun),
+            args: vec![],
+        })
+));
+
+named!(fun_expr(&str) -> Expr, do_parse!(
+    tag!("fn")
+        >> multispace1
+        >> args: separated_list0!(multispace1, arg)
+        >> multispace0
+        >> char!('=')
+        >> multispace0
+        >> body: expr
+        >> (Expr::Fun(Box::new(Fun {
+            args,
+            body
+        })))
+));
+
+named!(fn_arg(&str) -> Expr, alt!(
+    ident_expr |
+    literal_expr |
+    paren_expr
+));
+
+named!(call_with_args(&str) -> Expr, do_parse!(
+    fun: funcref
+        >> multispace1
+        >> args: separated_list1!(multispace1, fn_arg)
+        >> (Expr::Call {
+            fun: Box::new(fun),
+            args
+        })
+));
+
+named!(simple_expr_unascripted(&str) -> Expr, alt!(
+    let_ |
+    if_ |
+    fun_expr |
+    literal_expr |
+    ident_expr
+));
+
+named!(simple_expr(&str) -> Expr, alt!(
+    call!(ascripted(simple_expr_unascripted)) |
+    simple_expr_unascripted
+));
+
+named!(pub expr(&str) -> Expr, alt!(
+    no_arg_call |
+    call_with_args |
+    map!(token_tree, |tt| {
+        ExprParser.parse(&mut tt.into_iter()).unwrap()
+    }) |
+    simple_expr
+));
+
+#[cfg(test)]
+pub(crate) mod tests {
+    use super::*;
+    use crate::ast::{Arg, Ident, Type};
+    use std::convert::TryFrom;
+    use BinaryOperator::*;
+    use Expr::{BinaryOp, If, Let, UnaryOp};
+    use UnaryOperator::*;
+
+    pub(crate) fn ident_expr(s: &str) -> Box<Expr> {
+        Box::new(Expr::Ident(Ident::try_from(s).unwrap()))
+    }
+
+    mod operators {
+        use super::*;
+
+        #[test]
+        fn mul_plus() {
+            let (rem, res) = expr("x*y+z").unwrap();
+            assert!(rem.is_empty());
+            assert_eq!(
+                res,
+                BinaryOp {
+                    lhs: Box::new(BinaryOp {
+                        lhs: ident_expr("x"),
+                        op: Mul,
+                        rhs: ident_expr("y")
+                    }),
+                    op: Add,
+                    rhs: ident_expr("z")
+                }
+            )
+        }
+
+        #[test]
+        fn mul_plus_ws() {
+            let (rem, res) = expr("x * y    +    z").unwrap();
+            assert!(rem.is_empty(), "non-empty remainder: \"{}\"", rem);
+            assert_eq!(
+                res,
+                BinaryOp {
+                    lhs: Box::new(BinaryOp {
+                        lhs: ident_expr("x"),
+                        op: Mul,
+                        rhs: ident_expr("y")
+                    }),
+                    op: Add,
+                    rhs: ident_expr("z")
+                }
+            )
+        }
+
+        #[test]
+        fn unary() {
+            let (rem, res) = expr("x * -z").unwrap();
+            assert!(rem.is_empty(), "non-empty remainder: \"{}\"", rem);
+            assert_eq!(
+                res,
+                BinaryOp {
+                    lhs: ident_expr("x"),
+                    op: Mul,
+                    rhs: Box::new(UnaryOp {
+                        op: Neg,
+                        rhs: ident_expr("z"),
+                    })
+                }
+            )
+        }
+
+        #[test]
+        fn mul_literal() {
+            let (rem, res) = expr("x * 3").unwrap();
+            assert!(rem.is_empty());
+            assert_eq!(
+                res,
+                BinaryOp {
+                    lhs: ident_expr("x"),
+                    op: Mul,
+                    rhs: Box::new(Expr::Literal(Literal::Int(3))),
+                }
+            )
+        }
+
+        #[test]
+        fn equ() {
+            let res = test_parse!(expr, "x * 7 == 7");
+            assert_eq!(
+                res,
+                BinaryOp {
+                    lhs: Box::new(BinaryOp {
+                        lhs: ident_expr("x"),
+                        op: Mul,
+                        rhs: Box::new(Expr::Literal(Literal::Int(7)))
+                    }),
+                    op: Equ,
+                    rhs: Box::new(Expr::Literal(Literal::Int(7)))
+                }
+            )
+        }
+    }
+
+    #[test]
+    fn bools() {
+        assert_eq!(
+            test_parse!(expr, "true"),
+            Expr::Literal(Literal::Bool(true))
+        );
+        assert_eq!(
+            test_parse!(expr, "false"),
+            Expr::Literal(Literal::Bool(false))
+        );
+    }
+
+    #[test]
+    fn simple_string_lit() {
+        assert_eq!(
+            test_parse!(expr, "\"foobar\""),
+            Expr::Literal(Literal::String(Cow::Borrowed("foobar")))
+        )
+    }
+
+    #[test]
+    fn let_complex() {
+        let res = test_parse!(expr, "let x = 1; y = x * 7 in (x + y) * 4");
+        assert_eq!(
+            res,
+            Let {
+                bindings: vec![
+                    Binding {
+                        ident: Ident::try_from("x").unwrap(),
+                        type_: None,
+                        body: Expr::Literal(Literal::Int(1))
+                    },
+                    Binding {
+                        ident: Ident::try_from("y").unwrap(),
+                        type_: None,
+                        body: Expr::BinaryOp {
+                            lhs: ident_expr("x"),
+                            op: Mul,
+                            rhs: Box::new(Expr::Literal(Literal::Int(7)))
+                        }
+                    }
+                ],
+                body: Box::new(Expr::BinaryOp {
+                    lhs: Box::new(Expr::BinaryOp {
+                        lhs: ident_expr("x"),
+                        op: Add,
+                        rhs: ident_expr("y"),
+                    }),
+                    op: Mul,
+                    rhs: Box::new(Expr::Literal(Literal::Int(4))),
+                })
+            }
+        )
+    }
+
+    #[test]
+    fn if_simple() {
+        let res = test_parse!(expr, "if x == 8 then 9 else 20");
+        assert_eq!(
+            res,
+            If {
+                condition: Box::new(BinaryOp {
+                    lhs: ident_expr("x"),
+                    op: Equ,
+                    rhs: Box::new(Expr::Literal(Literal::Int(8))),
+                }),
+                then: Box::new(Expr::Literal(Literal::Int(9))),
+                else_: Box::new(Expr::Literal(Literal::Int(20)))
+            }
+        )
+    }
+
+    #[test]
+    fn no_arg_call() {
+        let res = test_parse!(expr, "f()");
+        assert_eq!(
+            res,
+            Expr::Call {
+                fun: ident_expr("f"),
+                args: vec![]
+            }
+        );
+    }
+
+    #[test]
+    fn call_with_args() {
+        let res = test_parse!(expr, "f x 1");
+        assert_eq!(
+            res,
+            Expr::Call {
+                fun: ident_expr("f"),
+                args: vec![*ident_expr("x"), Expr::Literal(Literal::Int(1))]
+            }
+        )
+    }
+
+    #[test]
+    fn call_funcref() {
+        let res = test_parse!(expr, "(let x = 1 in x) 2");
+        assert_eq!(
+            res,
+            Expr::Call {
+                fun: Box::new(Expr::Let {
+                    bindings: vec![Binding {
+                        ident: Ident::try_from("x").unwrap(),
+                        type_: None,
+                        body: Expr::Literal(Literal::Int(1))
+                    }],
+                    body: ident_expr("x")
+                }),
+                args: vec![Expr::Literal(Literal::Int(2))]
+            }
+        )
+    }
+
+    #[test]
+    fn anon_function() {
+        let res = test_parse!(expr, "let id = fn x = x in id 1");
+        assert_eq!(
+            res,
+            Expr::Let {
+                bindings: vec![Binding {
+                    ident: Ident::try_from("id").unwrap(),
+                    type_: None,
+                    body: Expr::Fun(Box::new(Fun {
+                        args: vec![Arg::try_from("x").unwrap()],
+                        body: *ident_expr("x")
+                    }))
+                }],
+                body: Box::new(Expr::Call {
+                    fun: ident_expr("id"),
+                    args: vec![Expr::Literal(Literal::Int(1))],
+                })
+            }
+        );
+    }
+
+    mod ascriptions {
+        use super::*;
+
+        #[test]
+        fn bare_ascription() {
+            let res = test_parse!(expr, "1: float");
+            assert_eq!(
+                res,
+                Expr::Ascription {
+                    expr: Box::new(Expr::Literal(Literal::Int(1))),
+                    type_: Type::Float
+                }
+            )
+        }
+
+        #[test]
+        fn fn_body_ascription() {
+            let res = test_parse!(expr, "let const_1 = fn x = 1: int in const_1 2");
+            assert_eq!(
+                res,
+                Expr::Let {
+                    bindings: vec![Binding {
+                        ident: Ident::try_from("const_1").unwrap(),
+                        type_: None,
+                        body: Expr::Fun(Box::new(Fun {
+                            args: vec![Arg::try_from("x").unwrap()],
+                            body: Expr::Ascription {
+                                expr: Box::new(Expr::Literal(Literal::Int(1))),
+                                type_: Type::Int,
+                            }
+                        }))
+                    }],
+                    body: Box::new(Expr::Call {
+                        fun: ident_expr("const_1"),
+                        args: vec![Expr::Literal(Literal::Int(2))]
+                    })
+                }
+            )
+        }
+
+        #[test]
+        fn let_binding_ascripted() {
+            let res = test_parse!(expr, "let x: int = 1 in x");
+            assert_eq!(
+                res,
+                Expr::Let {
+                    bindings: vec![Binding {
+                        ident: Ident::try_from("x").unwrap(),
+                        type_: Some(Type::Int),
+                        body: Expr::Literal(Literal::Int(1))
+                    }],
+                    body: ident_expr("x")
+                }
+            )
+        }
+    }
+}
diff --git a/users/glittershark/achilles/src/parser/macros.rs b/users/glittershark/achilles/src/parser/macros.rs
new file mode 100644
index 000000000000..406e5c0e699e
--- /dev/null
+++ b/users/glittershark/achilles/src/parser/macros.rs
@@ -0,0 +1,16 @@
+#[cfg(test)]
+#[macro_use]
+macro_rules! test_parse {
+    ($parser: ident, $src: expr) => {{
+        let res = $parser($src);
+        nom_trace::print_trace!();
+        let (rem, res) = res.unwrap();
+        assert!(
+            rem.is_empty(),
+            "non-empty remainder: \"{}\", parsed: {:?}",
+            rem,
+            res
+        );
+        res
+    }};
+}
diff --git a/users/glittershark/achilles/src/parser/mod.rs b/users/glittershark/achilles/src/parser/mod.rs
new file mode 100644
index 000000000000..dd7874aff853
--- /dev/null
+++ b/users/glittershark/achilles/src/parser/mod.rs
@@ -0,0 +1,197 @@
+use nom::character::complete::{multispace0, multispace1};
+use nom::error::{ErrorKind, ParseError};
+use nom::{alt, char, complete, do_parse, many0, named, separated_list0, tag, terminated};
+
+#[macro_use]
+mod macros;
+mod expr;
+mod type_;
+
+use crate::ast::{Arg, Decl, Fun, Ident};
+pub use expr::expr;
+pub use type_::type_;
+
+pub type Error = nom::Err<nom::error::Error<String>>;
+
+pub(crate) fn is_reserved(s: &str) -> bool {
+    matches!(
+        s,
+        "if" | "then"
+            | "else"
+            | "let"
+            | "in"
+            | "fn"
+            | "int"
+            | "float"
+            | "bool"
+            | "true"
+            | "false"
+            | "cstring"
+    )
+}
+
+pub(crate) fn ident<'a, E>(i: &'a str) -> nom::IResult<&'a str, Ident, E>
+where
+    E: ParseError<&'a str>,
+{
+    let mut chars = i.chars();
+    if let Some(f) = chars.next() {
+        if f.is_alphabetic() || f == '_' {
+            let mut idx = 1;
+            for c in chars {
+                if !(c.is_alphanumeric() || c == '_') {
+                    break;
+                }
+                idx += 1;
+            }
+            let id = &i[..idx];
+            if is_reserved(id) {
+                Err(nom::Err::Error(E::from_error_kind(i, ErrorKind::Satisfy)))
+            } else {
+                Ok((&i[idx..], Ident::from_str_unchecked(id)))
+            }
+        } else {
+            Err(nom::Err::Error(E::from_error_kind(i, ErrorKind::Satisfy)))
+        }
+    } else {
+        Err(nom::Err::Error(E::from_error_kind(i, ErrorKind::Eof)))
+    }
+}
+
+named!(ascripted_arg(&str) -> Arg, do_parse!(
+    complete!(char!('(')) >>
+        multispace0 >>
+        ident: ident >>
+        multispace0 >>
+        complete!(char!(':')) >>
+        multispace0 >>
+        type_: type_ >>
+        multispace0 >>
+        complete!(char!(')')) >>
+        (Arg {
+            ident,
+            type_: Some(type_)
+        })
+));
+
+named!(arg(&str) -> Arg, alt!(
+    ident => { |ident| Arg {ident, type_: None}} |
+    ascripted_arg
+));
+
+named!(fun_decl(&str) -> Decl, do_parse!(
+    complete!(tag!("fn"))
+        >> multispace0
+        >> name: ident
+        >> multispace1
+        >> args: separated_list0!(multispace1, arg)
+        >> multispace0
+        >> char!('=')
+        >> multispace0
+        >> body: expr
+        >> (Decl::Fun {
+            name,
+            body: Fun {
+                args,
+                body
+            }
+        })
+));
+
+named!(ascription_decl(&str) -> Decl, do_parse!(
+    name: ident
+        >> multispace0
+        >> complete!(char!(':'))
+        >> multispace0
+        >> type_: type_
+        >> (Decl::Ascription {
+            name,
+            type_
+        })
+));
+
+named!(pub decl(&str) -> Decl, alt!(
+    ascription_decl |
+    fun_decl
+));
+
+named!(pub toplevel(&str) -> Vec<Decl>, terminated!(many0!(decl), multispace0));
+
+#[cfg(test)]
+mod tests {
+    use std::convert::TryInto;
+
+    use crate::ast::{BinaryOperator, Expr, FunctionType, Literal, Type};
+
+    use super::*;
+    use expr::tests::ident_expr;
+
+    #[test]
+    fn fn_decl() {
+        let res = test_parse!(decl, "fn id x = x");
+        assert_eq!(
+            res,
+            Decl::Fun {
+                name: "id".try_into().unwrap(),
+                body: Fun {
+                    args: vec!["x".try_into().unwrap()],
+                    body: *ident_expr("x"),
+                }
+            }
+        )
+    }
+
+    #[test]
+    fn ascripted_fn_args() {
+        test_parse!(ascripted_arg, "(x : int)");
+        let res = test_parse!(decl, "fn plus1 (x : int) = x + 1");
+        assert_eq!(
+            res,
+            Decl::Fun {
+                name: "plus1".try_into().unwrap(),
+                body: Fun {
+                    args: vec![Arg {
+                        ident: "x".try_into().unwrap(),
+                        type_: Some(Type::Int),
+                    }],
+                    body: Expr::BinaryOp {
+                        lhs: ident_expr("x"),
+                        op: BinaryOperator::Add,
+                        rhs: Box::new(Expr::Literal(Literal::Int(1))),
+                    }
+                }
+            }
+        );
+    }
+
+    #[test]
+    fn multiple_decls() {
+        let res = test_parse!(
+            toplevel,
+            "fn id x = x
+             fn plus x y = x + y
+             fn main = plus (id 2) 7"
+        );
+        assert_eq!(res.len(), 3);
+        let res = test_parse!(
+            toplevel,
+            "fn id x = x\nfn plus x y = x + y\nfn main = plus (id 2) 7\n"
+        );
+        assert_eq!(res.len(), 3);
+    }
+
+    #[test]
+    fn top_level_ascription() {
+        let res = test_parse!(toplevel, "id : fn a -> a");
+        assert_eq!(
+            res,
+            vec![Decl::Ascription {
+                name: "id".try_into().unwrap(),
+                type_: Type::Function(FunctionType {
+                    args: vec![Type::Var("a".try_into().unwrap())],
+                    ret: Box::new(Type::Var("a".try_into().unwrap()))
+                })
+            }]
+        )
+    }
+}
diff --git a/users/glittershark/achilles/src/parser/type_.rs b/users/glittershark/achilles/src/parser/type_.rs
new file mode 100644
index 000000000000..c90ceda4d72e
--- /dev/null
+++ b/users/glittershark/achilles/src/parser/type_.rs
@@ -0,0 +1,125 @@
+use nom::character::complete::{multispace0, multispace1};
+use nom::{alt, delimited, do_parse, map, named, opt, separated_list0, tag, terminated, tuple};
+
+use super::ident;
+use crate::ast::{FunctionType, Type};
+
+named!(function_type(&str) -> Type, do_parse!(
+    tag!("fn")
+        >> multispace1
+        >> args: map!(opt!(terminated!(separated_list0!(
+            tuple!(
+                multispace0,
+                tag!(","),
+                multispace0
+            ),
+            type_
+        ), multispace1)), |args| args.unwrap_or_default())
+        >> tag!("->")
+        >> multispace1
+        >> ret: type_
+        >> (Type::Function(FunctionType {
+            args,
+            ret: Box::new(ret)
+        }))
+));
+
+named!(pub type_(&str) -> Type, alt!(
+    tag!("int") => { |_| Type::Int } |
+    tag!("float") => { |_| Type::Float } |
+    tag!("bool") => { |_| Type::Bool } |
+    tag!("cstring") => { |_| Type::CString } |
+    function_type |
+    ident => { |id| Type::Var(id) } |
+    delimited!(
+        tuple!(tag!("("), multispace0),
+        type_,
+        tuple!(tag!(")"), multispace0)
+    )
+));
+
+#[cfg(test)]
+mod tests {
+    use std::convert::TryFrom;
+
+    use super::*;
+    use crate::ast::Ident;
+
+    #[test]
+    fn simple_types() {
+        assert_eq!(test_parse!(type_, "int"), Type::Int);
+        assert_eq!(test_parse!(type_, "float"), Type::Float);
+        assert_eq!(test_parse!(type_, "bool"), Type::Bool);
+        assert_eq!(test_parse!(type_, "cstring"), Type::CString);
+    }
+
+    #[test]
+    fn no_arg_fn_type() {
+        assert_eq!(
+            test_parse!(type_, "fn -> int"),
+            Type::Function(FunctionType {
+                args: vec![],
+                ret: Box::new(Type::Int)
+            })
+        );
+    }
+
+    #[test]
+    fn fn_type_with_args() {
+        assert_eq!(
+            test_parse!(type_, "fn int, bool -> int"),
+            Type::Function(FunctionType {
+                args: vec![Type::Int, Type::Bool],
+                ret: Box::new(Type::Int)
+            })
+        );
+    }
+
+    #[test]
+    fn fn_taking_fn() {
+        assert_eq!(
+            test_parse!(type_, "fn fn int, bool -> bool, float -> float"),
+            Type::Function(FunctionType {
+                args: vec![
+                    Type::Function(FunctionType {
+                        args: vec![Type::Int, Type::Bool],
+                        ret: Box::new(Type::Bool)
+                    }),
+                    Type::Float
+                ],
+                ret: Box::new(Type::Float)
+            })
+        )
+    }
+
+    #[test]
+    fn parenthesized() {
+        assert_eq!(
+            test_parse!(type_, "fn (fn int, bool -> bool), float -> float"),
+            Type::Function(FunctionType {
+                args: vec![
+                    Type::Function(FunctionType {
+                        args: vec![Type::Int, Type::Bool],
+                        ret: Box::new(Type::Bool)
+                    }),
+                    Type::Float
+                ],
+                ret: Box::new(Type::Float)
+            })
+        )
+    }
+
+    #[test]
+    fn type_vars() {
+        assert_eq!(
+            test_parse!(type_, "fn x, y -> x"),
+            Type::Function(FunctionType {
+                args: vec![
+                    Type::Var(Ident::try_from("x").unwrap()),
+                    Type::Var(Ident::try_from("y").unwrap()),
+                ],
+                ret: Box::new(Type::Var(Ident::try_from("x").unwrap())),
+            })
+        )
+    }
+}
diff --git a/users/glittershark/achilles/src/tc/mod.rs b/users/glittershark/achilles/src/tc/mod.rs
new file mode 100644
index 000000000000..559ac993cc9b
--- /dev/null
+++ b/users/glittershark/achilles/src/tc/mod.rs
@@ -0,0 +1,680 @@
+use bimap::BiMap;
+use derive_more::From;
+use itertools::Itertools;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::convert::{TryFrom, TryInto};
+use std::fmt::{self, Display};
+use std::{mem, result};
+use thiserror::Error;
+
+use crate::ast::{self, hir, Arg, BinaryOperator, Ident, Literal};
+use crate::common::env::Env;
+use crate::common::{Namer, NamerOf};
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("Undefined variable {0}")]
+    UndefinedVariable(Ident<'static>),
+
+    #[error("Mismatched types: expected {expected}, but got {actual}")]
+    TypeMismatch { expected: Type, actual: Type },
+
+    #[error("Mismatched types, expected numeric type, but got {0}")]
+    NonNumeric(Type),
+
+    #[error("Ambiguous type {0}")]
+    AmbiguousType(TyVar),
+}
+
+pub type Result<T> = result::Result<T, Error>;
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash)]
+pub struct TyVar(u64);
+
+impl Display for TyVar {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "t{}", self.0)
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Hash)]
+pub struct NullaryType(String);
+
+impl Display for NullaryType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.0)
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum PrimType {
+    Int,
+    Float,
+    Bool,
+    CString,
+}
+
+impl<'a> From<PrimType> for ast::Type<'a> {
+    fn from(pr: PrimType) -> Self {
+        match pr {
+            PrimType::Int => ast::Type::Int,
+            PrimType::Float => ast::Type::Float,
+            PrimType::Bool => ast::Type::Bool,
+            PrimType::CString => ast::Type::CString,
+        }
+    }
+}
+
+impl Display for PrimType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            PrimType::Int => f.write_str("int"),
+            PrimType::Float => f.write_str("float"),
+            PrimType::Bool => f.write_str("bool"),
+            PrimType::CString => f.write_str("cstring"),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, From)]
+pub enum Type {
+    #[from(ignore)]
+    Univ(TyVar),
+    #[from(ignore)]
+    Exist(TyVar),
+    Nullary(NullaryType),
+    Prim(PrimType),
+    Fun {
+        args: Vec<Type>,
+        ret: Box<Type>,
+    },
+}
+
+impl<'a> TryFrom<Type> for ast::Type<'a> {
+    type Error = Type;
+
+    fn try_from(value: Type) -> result::Result<Self, Self::Error> {
+        match value {
+            Type::Univ(_) => todo!(),
+            Type::Exist(_) => Err(value),
+            Type::Nullary(_) => todo!(),
+            Type::Prim(p) => Ok(p.into()),
+            Type::Fun { ref args, ref ret } => Ok(ast::Type::Function(ast::FunctionType {
+                args: args
+                    .clone()
+                    .into_iter()
+                    .map(Self::try_from)
+                    .try_collect()
+                    .map_err(|_| value.clone())?,
+                ret: Box::new((*ret.clone()).try_into().map_err(|_| value.clone())?),
+            })),
+        }
+    }
+}
+
+const INT: Type = Type::Prim(PrimType::Int);
+const FLOAT: Type = Type::Prim(PrimType::Float);
+const BOOL: Type = Type::Prim(PrimType::Bool);
+const CSTRING: Type = Type::Prim(PrimType::CString);
+
+impl Display for Type {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Type::Nullary(nt) => nt.fmt(f),
+            Type::Prim(p) => p.fmt(f),
+            Type::Univ(TyVar(n)) => write!(f, "∀{}", n),
+            Type::Exist(TyVar(n)) => write!(f, "∃{}", n),
+            Type::Fun { args, ret } => write!(f, "fn {} -> {}", args.iter().join(", "), ret),
+        }
+    }
+}
+
+struct Typechecker<'ast> {
+    ty_var_namer: NamerOf<TyVar>,
+    ctx: HashMap<TyVar, Type>,
+    env: Env<Ident<'ast>, Type>,
+
+    /// AST type var -> type
+    instantiations: Env<Ident<'ast>, Type>,
+
+    /// AST type-var -> universal TyVar
+    type_vars: RefCell<(BiMap<Ident<'ast>, TyVar>, NamerOf<Ident<'static>>)>,
+}
+
+impl<'ast> Typechecker<'ast> {
+    fn new() -> Self {
+        Self {
+            ty_var_namer: Namer::new(TyVar).boxed(),
+            type_vars: RefCell::new((
+                Default::default(),
+                Namer::alphabetic().map(|n| Ident::try_from(n).unwrap()),
+            )),
+            ctx: Default::default(),
+            env: Default::default(),
+            instantiations: Default::default(),
+        }
+    }
+
+    pub(crate) fn tc_expr(&mut self, expr: ast::Expr<'ast>) -> Result<hir::Expr<'ast, Type>> {
+        match expr {
+            ast::Expr::Ident(ident) => {
+                let type_ = self
+                    .env
+                    .resolve(&ident)
+                    .ok_or_else(|| Error::UndefinedVariable(ident.to_owned()))?
+                    .clone();
+                Ok(hir::Expr::Ident(ident, type_))
+            }
+            ast::Expr::Literal(lit) => {
+                let type_ = match lit {
+                    Literal::Int(_) => Type::Prim(PrimType::Int),
+                    Literal::Bool(_) => Type::Prim(PrimType::Bool),
+                    Literal::String(_) => Type::Prim(PrimType::CString),
+                };
+                Ok(hir::Expr::Literal(lit.to_owned(), type_))
+            }
+            ast::Expr::UnaryOp { op, rhs } => todo!(),
+            ast::Expr::BinaryOp { lhs, op, rhs } => {
+                let lhs = self.tc_expr(*lhs)?;
+                let rhs = self.tc_expr(*rhs)?;
+                let type_ = match op {
+                    BinaryOperator::Equ | BinaryOperator::Neq => {
+                        self.unify(lhs.type_(), rhs.type_())?;
+                        Type::Prim(PrimType::Bool)
+                    }
+                    BinaryOperator::Add | BinaryOperator::Sub | BinaryOperator::Mul => {
+                        let ty = self.unify(lhs.type_(), rhs.type_())?;
+                        // if !matches!(ty, Type::Int | Type::Float) {
+                        //     return Err(Error::NonNumeric(ty));
+                        // }
+                        ty
+                    }
+                    BinaryOperator::Div => todo!(),
+                    BinaryOperator::Pow => todo!(),
+                };
+                Ok(hir::Expr::BinaryOp {
+                    lhs: Box::new(lhs),
+                    op,
+                    rhs: Box::new(rhs),
+                    type_,
+                })
+            }
+            ast::Expr::Let { bindings, body } => {
+                self.env.push();
+                let bindings = bindings
+                    .into_iter()
+                    .map(
+                        |ast::Binding { ident, type_, body }| -> Result<hir::Binding<Type>> {
+                            let body = self.tc_expr(body)?;
+                            if let Some(type_) = type_ {
+                                let type_ = self.type_from_ast_type(type_);
+                                self.unify(body.type_(), &type_)?;
+                            }
+                            self.env.set(ident.clone(), body.type_().clone());
+                            Ok(hir::Binding {
+                                ident,
+                                type_: body.type_().clone(),
+                                body,
+                            })
+                        },
+                    )
+                    .collect::<Result<Vec<hir::Binding<Type>>>>()?;
+                let body = self.tc_expr(*body)?;
+                self.env.pop();
+                Ok(hir::Expr::Let {
+                    bindings,
+                    type_: body.type_().clone(),
+                    body: Box::new(body),
+                })
+            }
+            ast::Expr::If {
+                condition,
+                then,
+                else_,
+            } => {
+                let condition = self.tc_expr(*condition)?;
+                self.unify(&Type::Prim(PrimType::Bool), condition.type_())?;
+                let then = self.tc_expr(*then)?;
+                let else_ = self.tc_expr(*else_)?;
+                let type_ = self.unify(then.type_(), else_.type_())?;
+                Ok(hir::Expr::If {
+                    condition: Box::new(condition),
+                    then: Box::new(then),
+                    else_: Box::new(else_),
+                    type_,
+                })
+            }
+            ast::Expr::Fun(f) => {
+                let ast::Fun { args, body } = *f;
+                self.env.push();
+                let args: Vec<_> = args
+                    .into_iter()
+                    .map(|Arg { ident, type_ }| {
+                        let ty = match type_ {
+                            Some(t) => self.type_from_ast_type(t),
+                            None => self.fresh_ex(),
+                        };
+                        self.env.set(ident.clone(), ty.clone());
+                        (ident, ty)
+                    })
+                    .collect();
+                let body = self.tc_expr(body)?;
+                self.env.pop();
+                Ok(hir::Expr::Fun {
+                    type_: self.universalize(
+                        args.iter().map(|(_, ty)| ty.clone()).collect(),
+                        body.type_().clone(),
+                    ),
+                    args,
+                    body: Box::new(body),
+                })
+            }
+            ast::Expr::Call { fun, args } => {
+                let ret_ty = self.fresh_ex();
+                let arg_tys = args.iter().map(|_| self.fresh_ex()).collect::<Vec<_>>();
+                let ft = Type::Fun {
+                    args: arg_tys.clone(),
+                    ret: Box::new(ret_ty.clone()),
+                };
+                let fun = self.tc_expr(*fun)?;
+                self.instantiations.push();
+                self.unify(&ft, fun.type_())?;
+                let args = args
+                    .into_iter()
+                    .zip(arg_tys)
+                    .map(|(arg, ty)| {
+                        let arg = self.tc_expr(arg)?;
+                        self.unify(&ty, arg.type_())?;
+                        Ok(arg)
+                    })
+                    .try_collect()?;
+                self.commit_instantiations();
+                Ok(hir::Expr::Call {
+                    fun: Box::new(fun),
+                    args,
+                    type_: ret_ty,
+                })
+            }
+            ast::Expr::Ascription { expr, type_ } => {
+                let expr = self.tc_expr(*expr)?;
+                let type_ = self.type_from_ast_type(type_);
+                self.unify(expr.type_(), &type_)?;
+                Ok(expr)
+            }
+        }
+    }
+
+    pub(crate) fn tc_decl(
+        &mut self,
+        decl: ast::Decl<'ast>,
+    ) -> Result<Option<hir::Decl<'ast, Type>>> {
+        match decl {
+            ast::Decl::Fun { name, body } => {
+                let mut expr = ast::Expr::Fun(Box::new(body));
+                if let Some(type_) = self.env.resolve(&name) {
+                    expr = ast::Expr::Ascription {
+                        expr: Box::new(expr),
+                        type_: self.finalize_type(type_.clone())?,
+                    };
+                }
+
+                let body = self.tc_expr(expr)?;
+                let type_ = body.type_().clone();
+                self.env.set(name.clone(), type_);
+                match body {
+                    hir::Expr::Fun { args, body, type_ } => Ok(Some(hir::Decl::Fun {
+                        name,
+                        args,
+                        body,
+                        type_,
+                    })),
+                    _ => unreachable!(),
+                }
+            }
+            ast::Decl::Ascription { name, type_ } => {
+                let type_ = self.type_from_ast_type(type_);
+                self.env.set(name.clone(), type_);
+                Ok(None)
+            }
+        }
+    }
+
+    fn fresh_tv(&mut self) -> TyVar {
+        self.ty_var_namer.make_name()
+    }
+
+    fn fresh_ex(&mut self) -> Type {
+        Type::Exist(self.fresh_tv())
+    }
+
+    fn fresh_univ(&mut self) -> Type {
+        Type::Univ(self.fresh_tv())
+    }
+
+    #[allow(clippy::redundant_closure)] // https://github.com/rust-lang/rust-clippy/issues/6903
+    fn universalize(&mut self, args: Vec<Type>, ret: Type) -> Type {
+        let mut vars = HashMap::new();
+        let mut universalize_type = move |ty| match ty {
+            Type::Exist(tv) if self.resolve_tv(tv).is_none() => vars
+                .entry(tv)
+                .or_insert_with_key(|tv| {
+                    let ty = self.fresh_univ();
+                    self.ctx.insert(*tv, ty.clone());
+                    ty
+                })
+                .clone(),
+            _ => ty,
+        };
+
+        Type::Fun {
+            args: args.into_iter().map(|t| universalize_type(t)).collect(),
+            ret: Box::new(universalize_type(ret)),
+        }
+    }
+
+    fn unify(&mut self, ty1: &Type, ty2: &Type) -> Result<Type> {
+        match (ty1, ty2) {
+            (Type::Exist(tv), ty) | (ty, Type::Exist(tv)) => match self.resolve_tv(*tv) {
+                Some(existing_ty) if self.types_match(ty, &existing_ty) => Ok(ty.clone()),
+                Some(var @ ast::Type::Var(_)) => {
+                    let var = self.type_from_ast_type(var);
+                    self.unify(&var, ty)
+                }
+                Some(existing_ty) => match ty {
+                    Type::Exist(_) => {
+                        let rhs = self.type_from_ast_type(existing_ty);
+                        self.unify(ty, &rhs)
+                    }
+                    _ => Err(Error::TypeMismatch {
+                        expected: ty.clone(),
+                        actual: self.type_from_ast_type(existing_ty),
+                    }),
+                },
+                None => match self.ctx.insert(*tv, ty.clone()) {
+                    Some(existing) => self.unify(&existing, ty),
+                    None => Ok(ty.clone()),
+                },
+            },
+            (Type::Univ(u1), Type::Univ(u2)) if u1 == u2 => Ok(ty2.clone()),
+            (Type::Univ(u), ty) | (ty, Type::Univ(u)) => {
+                let ident = self.name_univ(*u);
+                match self.instantiations.resolve(&ident) {
+                    Some(existing_ty) if ty == existing_ty => Ok(ty.clone()),
+                    Some(existing_ty) => Err(Error::TypeMismatch {
+                        expected: ty.clone(),
+                        actual: existing_ty.clone(),
+                    }),
+                    None => {
+                        self.instantiations.set(ident, ty.clone());
+                        Ok(ty.clone())
+                    }
+                }
+            }
+            (Type::Prim(p1), Type::Prim(p2)) if p1 == p2 => Ok(ty2.clone()),
+            (
+                Type::Fun {
+                    args: args1,
+                    ret: ret1,
+                },
+                Type::Fun {
+                    args: args2,
+                    ret: ret2,
+                },
+            ) => {
+                let args = args1
+                    .iter()
+                    .zip(args2)
+                    .map(|(t1, t2)| self.unify(t1, t2))
+                    .try_collect()?;
+                let ret = self.unify(ret1, ret2)?;
+                Ok(Type::Fun {
+                    args,
+                    ret: Box::new(ret),
+                })
+            }
+            (Type::Nullary(_), _) | (_, Type::Nullary(_)) => todo!(),
+            _ => Err(Error::TypeMismatch {
+                expected: ty1.clone(),
+                actual: ty2.clone(),
+            }),
+        }
+    }
+
+    fn finalize_expr(
+        &self,
+        expr: hir::Expr<'ast, Type>,
+    ) -> Result<hir::Expr<'ast, ast::Type<'ast>>> {
+        expr.traverse_type(|ty| self.finalize_type(ty))
+    }
+
+    fn finalize_decl(
+        &self,
+        decl: hir::Decl<'ast, Type>,
+    ) -> Result<hir::Decl<'ast, ast::Type<'ast>>> {
+        decl.traverse_type(|ty| self.finalize_type(ty))
+    }
+
+    fn finalize_type(&self, ty: Type) -> Result<ast::Type<'static>> {
+        let ret = match ty {
+            Type::Exist(tv) => self.resolve_tv(tv).ok_or(Error::AmbiguousType(tv)),
+            Type::Univ(tv) => Ok(ast::Type::Var(self.name_univ(tv))),
+            Type::Nullary(_) => todo!(),
+            Type::Prim(pr) => Ok(pr.into()),
+            Type::Fun { args, ret } => Ok(ast::Type::Function(ast::FunctionType {
+                args: args
+                    .into_iter()
+                    .map(|ty| self.finalize_type(ty))
+                    .try_collect()?,
+                ret: Box::new(self.finalize_type(*ret)?),
+            })),
+        };
+        ret
+    }
+
+    fn resolve_tv(&self, tv: TyVar) -> Option<ast::Type<'static>> {
+        let mut res = &Type::Exist(tv);
+        loop {
+            match res {
+                Type::Exist(tv) => {
+                    res = self.ctx.get(tv)?;
+                }
+                Type::Univ(tv) => {
+                    let ident = self.name_univ(*tv);
+                    if let Some(r) = self.instantiations.resolve(&ident) {
+                        res = r;
+                    } else {
+                        break Some(ast::Type::Var(ident));
+                    }
+                }
+                Type::Nullary(_) => todo!(),
+                Type::Prim(pr) => break Some((*pr).into()),
+                Type::Fun { args, ret } => todo!(),
+            }
+        }
+    }
+
+    fn type_from_ast_type(&mut self, ast_type: ast::Type<'ast>) -> Type {
+        match ast_type {
+            ast::Type::Int => INT,
+            ast::Type::Float => FLOAT,
+            ast::Type::Bool => BOOL,
+            ast::Type::CString => CSTRING,
+            ast::Type::Function(ast::FunctionType { args, ret }) => Type::Fun {
+                args: args
+                    .into_iter()
+                    .map(|t| self.type_from_ast_type(t))
+                    .collect(),
+                ret: Box::new(self.type_from_ast_type(*ret)),
+            },
+            ast::Type::Var(id) => Type::Univ({
+                let opt_tv = { self.type_vars.borrow_mut().0.get_by_left(&id).copied() };
+                opt_tv.unwrap_or_else(|| {
+                    let tv = self.fresh_tv();
+                    self.type_vars
+                        .borrow_mut()
+                        .0
+                        .insert_no_overwrite(id, tv)
+                        .unwrap();
+                    tv
+                })
+            }),
+        }
+    }
+
+    fn name_univ(&self, tv: TyVar) -> Ident<'static> {
+        let mut vars = self.type_vars.borrow_mut();
+        vars.0
+            .get_by_right(&tv)
+            .map(Ident::to_owned)
+            .unwrap_or_else(|| {
+                let name = vars.1.make_name();
+                vars.0.insert_no_overwrite(name.clone(), tv).unwrap();
+                name
+            })
+    }
+
+    fn commit_instantiations(&mut self) {
+        let mut ctx = mem::take(&mut self.ctx);
+        for (_, v) in ctx.iter_mut() {
+            if let Type::Univ(tv) = v {
+                if let Some(concrete) = self.instantiations.resolve(&self.name_univ(*tv)) {
+                    *v = concrete.clone();
+                }
+            }
+        }
+        self.ctx = ctx;
+        self.instantiations.pop();
+    }
+
+    fn types_match(&self, type_: &Type, ast_type: &ast::Type<'ast>) -> bool {
+        match (type_, ast_type) {
+            (Type::Univ(u), ast::Type::Var(v)) => {
+                Some(u) == self.type_vars.borrow().0.get_by_left(v)
+            }
+            (Type::Univ(_), _) => false,
+            (Type::Exist(_), _) => false,
+            (Type::Nullary(_), _) => todo!(),
+            (Type::Prim(pr), ty) => ast::Type::from(*pr) == *ty,
+            (Type::Fun { args, ret }, ast::Type::Function(ft)) => {
+                args.len() == ft.args.len()
+                    && args
+                        .iter()
+                        .zip(&ft.args)
+                        .all(|(a1, a2)| self.types_match(a1, &a2))
+                    && self.types_match(&*ret, &*ft.ret)
+            }
+            (Type::Fun { .. }, _) => false,
+        }
+    }
+}
+
+pub fn typecheck_expr(expr: ast::Expr) -> Result<hir::Expr<ast::Type>> {
+    let mut typechecker = Typechecker::new();
+    let typechecked = typechecker.tc_expr(expr)?;
+    typechecker.finalize_expr(typechecked)
+}
+
+pub fn typecheck_toplevel(decls: Vec<ast::Decl>) -> Result<Vec<hir::Decl<ast::Type>>> {
+    let mut typechecker = Typechecker::new();
+    let mut res = Vec::with_capacity(decls.len());
+    for decl in decls {
+        if let Some(hir_decl) = typechecker.tc_decl(decl)? {
+            let hir_decl = typechecker.finalize_decl(hir_decl)?;
+            res.push(hir_decl);
+        }
+        typechecker.ctx.clear();
+    }
+    Ok(res)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    macro_rules! assert_type {
+        ($expr: expr, $type: expr) => {
+            use crate::parser::{expr, type_};
+            let parsed_expr = test_parse!(expr, $expr);
+            let parsed_type = test_parse!(type_, $type);
+            let res = typecheck_expr(parsed_expr).unwrap_or_else(|e| panic!("{}", e));
+            assert!(
+                res.type_().alpha_equiv(&parsed_type),
+                "{} inferred type {}, but expected {}",
+                $expr,
+                res.type_(),
+                $type
+            );
+        };
+    }
+
+    macro_rules! assert_type_error {
+        ($expr: expr) => {
+            use crate::parser::expr;
+            let parsed_expr = test_parse!(expr, $expr);
+            let res = typecheck_expr(parsed_expr);
+            assert!(
+                res.is_err(),
+                "Expected type error, but got type: {}",
+                res.unwrap().type_()
+            );
+        };
+    }
+
+    #[test]
+    fn literal_int() {
+        assert_type!("1", "int");
+    }
+
+    #[test]
+    fn conditional() {
+        assert_type!("if 1 == 2 then 3 else 4", "int");
+    }
+
+    #[test]
+    #[ignore]
+    fn add_bools() {
+        assert_type_error!("true + false");
+    }
+
+    #[test]
+    fn call_generic_function() {
+        assert_type!("(fn x = x) 1", "int");
+    }
+
+    #[test]
+    fn generic_function() {
+        assert_type!("fn x = x", "fn x -> x");
+    }
+
+    #[test]
+    #[ignore]
+    fn let_generalization() {
+        assert_type!("let id = fn x = x in if id true then id 1 else 2", "int");
+    }
+
+    #[test]
+    fn concrete_function() {
+        assert_type!("fn x = x + 1", "fn int -> int");
+    }
+
+    #[test]
+    fn arg_ascriptions() {
+        assert_type!("fn (x: int) = x", "fn int -> int");
+    }
+
+    #[test]
+    fn call_concrete_function() {
+        assert_type!("(fn x = x + 1) 2", "int");
+    }
+
+    #[test]
+    fn conditional_non_bool() {
+        assert_type_error!("if 3 then true else false");
+    }
+
+    #[test]
+    fn let_int() {
+        assert_type!("let x = 1 in x", "int");
+    }
+}