chore(users): grfn -> aspen r/7511

Change-Id: I6c6847fac56f0a9a1a2209792e00a3aec5e672b9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10809
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

1 files changed, 808 insertions, 0 deletions

diff --git a/users/aspen/achilles/src/tc/mod.rs b/users/aspen/achilles/src/tc/mod.rs
new file mode 100644
index 000000000000..5825bab1fbe9
--- /dev/null
+++ b/users/aspen/achilles/src/tc/mod.rs
@@ -0,0 +1,808 @@
+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, Pattern};
+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),
+    Tuple(Vec<Type>),
+    Unit,
+    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::Unit => Ok(ast::Type::Unit),
+            Type::Univ(_) => todo!(),
+            Type::Exist(_) => Err(value),
+            Type::Nullary(_) => todo!(),
+            Type::Prim(p) => Ok(p.into()),
+            Type::Tuple(members) => Ok(ast::Type::Tuple(
+                members.into_iter().map(|ty| ty.try_into()).try_collect()?,
+            )),
+            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),
+            Type::Tuple(members) => write!(f, "({})", members.iter().join(", ")),
+            Type::Unit => write!(f, "()"),
+        }
+    }
+}
+
+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(),
+        }
+    }
+
+    fn bind_pattern(
+        &mut self,
+        pat: Pattern<'ast>,
+        type_: Type,
+    ) -> Result<hir::Pattern<'ast, Type>> {
+        match pat {
+            Pattern::Id(ident) => {
+                self.env.set(ident.clone(), type_.clone());
+                Ok(hir::Pattern::Id(ident, type_))
+            }
+            Pattern::Tuple(members) => {
+                let mut tys = Vec::with_capacity(members.len());
+                let mut hir_members = Vec::with_capacity(members.len());
+                for pat in members {
+                    let ty = self.fresh_ex();
+                    hir_members.push(self.bind_pattern(pat, ty.clone())?);
+                    tys.push(ty);
+                }
+                let tuple_type = Type::Tuple(tys);
+                self.unify(&tuple_type, &type_)?;
+                Ok(hir::Pattern::Tuple(hir_members))
+            }
+        }
+    }
+
+    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),
+                    Literal::Unit => Type::Unit,
+                };
+                Ok(hir::Expr::Literal(lit.to_owned(), type_))
+            }
+            ast::Expr::Tuple(members) => {
+                let members = members
+                    .into_iter()
+                    .map(|expr| self.tc_expr(expr))
+                    .collect::<Result<Vec<_>>>()?;
+                let type_ = Type::Tuple(members.iter().map(|expr| expr.type_().clone()).collect());
+                Ok(hir::Expr::Tuple(members, 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 { pat, 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_)?;
+                            }
+                            let pat = self.bind_pattern(pat, body.type_().clone())?;
+                            Ok(hir::Binding { pat, 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_: Type::Fun {
+                        args: args.iter().map(|(_, ty)| ty.clone()).collect(),
+                        ret: Box::new(body.type_().clone()),
+                    },
+                    type_args: vec![], // TODO fill in once we do let generalization
+                    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()?;
+                let type_args = self.commit_instantiations();
+                Ok(hir::Expr::Call {
+                    fun: Box::new(fun),
+                    type_args,
+                    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())?,
+                    };
+                }
+
+                self.env.push();
+                let body = self.tc_expr(expr)?;
+                let type_ = body.type_().clone();
+                self.env.set(name.clone(), type_);
+                self.env.pop();
+                match body {
+                    hir::Expr::Fun {
+                        type_args,
+                        args,
+                        body,
+                        type_,
+                    } => Ok(Some(hir::Decl::Fun {
+                        name,
+                        type_args,
+                        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)
+            }
+            ast::Decl::Extern { name, type_ } => {
+                let type_ = self.type_from_ast_type(ast::Type::Function(type_));
+                self.env.set(name.clone(), type_.clone());
+                let (arg_types, ret_type) = match type_ {
+                    Type::Fun { args, ret } => (args, *ret),
+                    _ => unreachable!(),
+                };
+                Ok(Some(hir::Decl::Extern {
+                    name,
+                    arg_types,
+                    ret_type,
+                }))
+            }
+        }
+    }
+
+    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())
+    }
+
+    fn unify(&mut self, ty1: &Type, ty2: &Type) -> Result<Type> {
+        match (ty1, ty2) {
+            (Type::Unit, Type::Unit) => Ok(Type::Unit),
+            (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::Tuple(t1), Type::Tuple(t2)) if t1.len() == t2.len() => {
+                let ts = t1
+                    .iter()
+                    .zip(t2.iter())
+                    .map(|(t1, t2)| self.unify(t1, t2))
+                    .try_collect()?;
+                Ok(Type::Tuple(ts))
+            }
+            (
+                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(
+        &mut self,
+        decl: hir::Decl<'ast, Type>,
+    ) -> Result<hir::Decl<'ast, ast::Type<'ast>>> {
+        let res = decl.traverse_type(|ty| self.finalize_type(ty))?;
+        if let Some(type_) = res.type_() {
+            let ty = self.type_from_ast_type(type_.clone());
+            self.env.set(res.name().clone(), ty);
+        }
+        Ok(res)
+    }
+
+    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::Unit => Ok(ast::Type::Unit),
+            Type::Nullary(_) => todo!(),
+            Type::Prim(pr) => Ok(pr.into()),
+            Type::Tuple(members) => Ok(ast::Type::Tuple(
+                members
+                    .into_iter()
+                    .map(|ty| self.finalize_type(ty))
+                    .try_collect()?,
+            )),
+            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) -> Result<Option<ast::Type<'static>>> {
+        let mut res = &Type::Exist(tv);
+        Ok(loop {
+            match res {
+                Type::Exist(tv) => {
+                    res = match self.ctx.get(tv) {
+                        Some(r) => r,
+                        None => return Ok(None),
+                    };
+                }
+                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::Unit => break Some(ast::Type::Unit),
+                Type::Fun { args, ret } => todo!(),
+                Type::Tuple(_) => break Some(self.finalize_type(res.clone())?),
+            }
+        })
+    }
+
+    fn type_from_ast_type(&mut self, ast_type: ast::Type<'ast>) -> Type {
+        match ast_type {
+            ast::Type::Unit => Type::Unit,
+            ast::Type::Int => INT,
+            ast::Type::Float => FLOAT,
+            ast::Type::Bool => BOOL,
+            ast::Type::CString => CSTRING,
+            ast::Type::Tuple(members) => Type::Tuple(
+                members
+                    .into_iter()
+                    .map(|ty| self.type_from_ast_type(ty))
+                    .collect(),
+            ),
+            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 = loop {
+                    let name = vars.1.make_name();
+                    if !vars.0.contains_left(&name) {
+                        break name;
+                    }
+                };
+                vars.0.insert_no_overwrite(name.clone(), tv).unwrap();
+                name
+            })
+    }
+
+    fn commit_instantiations(&mut self) -> HashMap<Ident<'ast>, Type> {
+        let mut res = HashMap::new();
+        let mut ctx = mem::take(&mut self.ctx);
+        for (_, v) in ctx.iter_mut() {
+            if let Type::Univ(tv) = v {
+                let tv_name = self.name_univ(*tv);
+                if let Some(concrete) = self.instantiations.resolve(&tv_name) {
+                    res.insert(tv_name, concrete.clone());
+                    *v = concrete.clone();
+                }
+            }
+        }
+        self.ctx = ctx;
+        self.instantiations.pop();
+        res
+    }
+
+    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::Unit, ast::Type::Unit) => true,
+            (Type::Unit, _) => false,
+            (Type::Nullary(_), _) => todo!(),
+            (Type::Prim(pr), ty) => ast::Type::from(*pr) == *ty,
+            (Type::Tuple(members), ast::Type::Tuple(members2)) => members
+                .iter()
+                .zip(members2.iter())
+                .all(|(t1, t2)| self.types_match(t1, t2)),
+            (Type::Tuple(members), _) => false,
+            (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_(),
+                $type
+            );
+        };
+
+        (toplevel($program: expr), $($decl: ident => $type: expr),+ $(,)?) => {{
+            use crate::parser::{toplevel, type_};
+            let program = test_parse!(toplevel, $program);
+            let res = typecheck_toplevel(program).unwrap_or_else(|e| panic!("{}", e));
+            $(
+            let parsed_type = test_parse!(type_, $type);
+            let ident = Ident::try_from(::std::stringify!($decl)).unwrap();
+            let decl = res.iter().find(|decl| {
+                matches!(decl, crate::ast::hir::Decl::Fun { name, .. } if name == &ident)
+            }).unwrap_or_else(|| panic!("Could not find declaration for {}", ident));
+            assert!(
+                decl.type_().unwrap().alpha_equiv(&parsed_type),
+                "inferred type {} for {}, but expected {}",
+                decl.type_().unwrap(),
+                ident,
+                $type
+            );
+            )+
+        }};
+    }
+
+    macro_rules! assert_type_error {
+        ($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 call_let_bound_generic() {
+        assert_type!("let id = fn x = x in id 1", "int");
+    }
+
+    #[test]
+    fn universal_ascripted_let() {
+        assert_type!("let id: fn a -> a = fn x = x in id 1", "int");
+    }
+
+    #[test]
+    fn call_generic_function_toplevel() {
+        assert_type!(
+            toplevel(
+                "ty id : fn a -> a
+                 fn id x = x
+
+                 fn main = id 0"
+            ),
+            main => "fn -> int",
+            id => "fn a -> a",
+        );
+    }
+
+    #[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");
+    }
+}