feat(grfn/achilles): Implement tuples, and tuple patterns r/2522

Implement tuple expressions, types, and patterns, all the way through
the parser down to the typechecker. In LLVM, these are implemented as
anonymous structs, using an `extract` instruction when they're pattern
matched on to get out the individual fields.

Currently the only limitation here is patterns aren't supported in
function argument position, but you can still do something like

    fn xy = let (x, y) = xy in x + y

Change-Id: I357f17e9d4052e741eda8605b6662822f331efde
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3027
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

12 files changed, 413 insertions, 54 deletions

diff --git a/users/grfn/achilles/src/ast/hir.rs b/users/grfn/achilles/src/ast/hir.rs
index 0d145d620bef..cdfaef567d7a 100644
--- a/users/grfn/achilles/src/ast/hir.rs
+++ b/users/grfn/achilles/src/ast/hir.rs
@@ -5,9 +5,42 @@ use itertools::Itertools;
 use super::{BinaryOperator, Ident, Literal, UnaryOperator};
 
 #[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Pattern<'a, T> {
+    Id(Ident<'a>, T),
+    Tuple(Vec<Pattern<'a, T>>),
+}
+
+impl<'a, T> Pattern<'a, T> {
+    pub fn to_owned(&self) -> Pattern<'static, T>
+    where
+        T: Clone,
+    {
+        match self {
+            Pattern::Id(id, t) => Pattern::Id(id.to_owned(), t.clone()),
+            Pattern::Tuple(pats) => {
+                Pattern::Tuple(pats.into_iter().map(Pattern::to_owned).collect())
+            }
+        }
+    }
+
+    pub fn traverse_type<F, U, E>(self, f: F) -> Result<Pattern<'a, U>, E>
+    where
+        F: Fn(T) -> Result<U, E> + Clone,
+    {
+        match self {
+            Pattern::Id(id, t) => Ok(Pattern::Id(id, f(t)?)),
+            Pattern::Tuple(pats) => Ok(Pattern::Tuple(
+                pats.into_iter()
+                    .map(|pat| pat.traverse_type(f.clone()))
+                    .collect::<Result<Vec<_>, _>>()?,
+            )),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub struct Binding<'a, T> {
-    pub ident: Ident<'a>,
-    pub type_: T,
+    pub pat: Pattern<'a, T>,
     pub body: Expr<'a, T>,
 }
 
@@ -17,8 +50,7 @@ impl<'a, T> Binding<'a, T> {
         T: Clone,
     {
         Binding {
-            ident: self.ident.to_owned(),
-            type_: self.type_.clone(),
+            pat: self.pat.to_owned(),
             body: self.body.to_owned(),
         }
     }
@@ -30,6 +62,8 @@ pub enum Expr<'a, T> {
 
     Literal(Literal<'a>, T),
 
+    Tuple(Vec<Expr<'a, T>>, T),
+
     UnaryOp {
         op: UnaryOperator,
         rhs: Box<Expr<'a, T>>,
@@ -76,6 +110,7 @@ impl<'a, T> Expr<'a, T> {
         match self {
             Expr::Ident(_, t) => t,
             Expr::Literal(_, t) => t,
+            Expr::Tuple(_, t) => t,
             Expr::UnaryOp { type_, .. } => type_,
             Expr::BinaryOp { type_, .. } => type_,
             Expr::Let { type_, .. } => type_,
@@ -115,10 +150,9 @@ impl<'a, T> Expr<'a, T> {
             } => Ok(Expr::Let {
                 bindings: bindings
                     .into_iter()
-                    .map(|Binding { ident, type_, body }| {
+                    .map(|Binding { pat, body }| {
                         Ok(Binding {
-                            ident,
-                            type_: f(type_)?,
+                            pat: pat.traverse_type(f.clone())?,
                             body: body.traverse_type(f.clone())?,
                         })
                     })
@@ -168,6 +202,13 @@ impl<'a, T> Expr<'a, T> {
                     .collect::<Result<Vec<_>, E>>()?,
                 type_: f(type_)?,
             }),
+            Expr::Tuple(members, t) => Ok(Expr::Tuple(
+                members
+                    .into_iter()
+                    .map(|t| t.traverse_type(f.clone()))
+                    .try_collect()?,
+                f(t)?,
+            )),
         }
     }
 
@@ -242,6 +283,9 @@ impl<'a, T> Expr<'a, T> {
                 args: args.iter().map(|e| e.to_owned()).collect(),
                 type_: type_.clone(),
             },
+            Expr::Tuple(members, t) => {
+                Expr::Tuple(members.into_iter().map(Expr::to_owned).collect(), t.clone())
+            }
         }
     }
 }
diff --git a/users/grfn/achilles/src/ast/mod.rs b/users/grfn/achilles/src/ast/mod.rs
index 7dc2de895709..5438d29d2cf7 100644
--- a/users/grfn/achilles/src/ast/mod.rs
+++ b/users/grfn/achilles/src/ast/mod.rs
@@ -128,8 +128,23 @@ impl<'a> Literal<'a> {
 }
 
 #[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Pattern<'a> {
+    Id(Ident<'a>),
+    Tuple(Vec<Pattern<'a>>),
+}
+
+impl<'a> Pattern<'a> {
+    pub fn to_owned(&self) -> Pattern<'static> {
+        match self {
+            Pattern::Id(id) => Pattern::Id(id.to_owned()),
+            Pattern::Tuple(pats) => Pattern::Tuple(pats.iter().map(Pattern::to_owned).collect()),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub struct Binding<'a> {
-    pub ident: Ident<'a>,
+    pub pat: Pattern<'a>,
     pub type_: Option<Type<'a>>,
     pub body: Expr<'a>,
 }
@@ -137,7 +152,7 @@ pub struct Binding<'a> {
 impl<'a> Binding<'a> {
     fn to_owned(&self) -> Binding<'static> {
         Binding {
-            ident: self.ident.to_owned(),
+            pat: self.pat.to_owned(),
             type_: self.type_.as_ref().map(|t| t.to_owned()),
             body: self.body.to_owned(),
         }
@@ -179,6 +194,8 @@ pub enum Expr<'a> {
         args: Vec<Expr<'a>>,
     },
 
+    Tuple(Vec<Expr<'a>>),
+
     Ascription {
         expr: Box<Expr<'a>>,
         type_: Type<'a>,
@@ -190,6 +207,9 @@ impl<'a> Expr<'a> {
         match self {
             Expr::Ident(ref id) => Expr::Ident(id.to_owned()),
             Expr::Literal(ref lit) => Expr::Literal(lit.to_owned()),
+            Expr::Tuple(ref members) => {
+                Expr::Tuple(members.into_iter().map(Expr::to_owned).collect())
+            }
             Expr::UnaryOp { op, rhs } => Expr::UnaryOp {
                 op: *op,
                 rhs: Box::new((**rhs).to_owned()),
@@ -312,6 +332,7 @@ pub enum Type<'a> {
     Bool,
     CString,
     Unit,
+    Tuple(Vec<Type<'a>>),
     Var(Ident<'a>),
     Function(FunctionType<'a>),
 }
@@ -326,6 +347,7 @@ impl<'a> Type<'a> {
             Type::Unit => Type::Unit,
             Type::Var(v) => Type::Var(v.to_owned()),
             Type::Function(f) => Type::Function(f.to_owned()),
+            Type::Tuple(members) => Type::Tuple(members.iter().map(Type::to_owned).collect()),
         }
     }
 
@@ -379,9 +401,23 @@ impl<'a> Type<'a> {
             Type::Float => Type::Float,
             Type::Bool => Type::Bool,
             Type::CString => Type::CString,
+            Type::Tuple(members) => Type::Tuple(
+                members
+                    .into_iter()
+                    .map(|t| t.traverse_type_vars(f.clone()))
+                    .collect(),
+            ),
             Type::Unit => Type::Unit,
         }
     }
+
+    pub fn as_tuple(&self) -> Option<&Vec<Type<'a>>> {
+        if let Self::Tuple(v) = self {
+            Some(v)
+        } else {
+            None
+        }
+    }
 }
 
 impl<'a> Display for Type<'a> {
@@ -394,6 +430,7 @@ impl<'a> Display for Type<'a> {
             Type::Unit => f.write_str("()"),
             Type::Var(v) => v.fmt(f),
             Type::Function(ft) => ft.fmt(f),
+            Type::Tuple(ms) => write!(f, "({})", ms.iter().join(", ")),
         }
     }
 }
diff --git a/users/grfn/achilles/src/codegen/llvm.rs b/users/grfn/achilles/src/codegen/llvm.rs
index 17dec58b5ff7..9a71ac954e00 100644
--- a/users/grfn/achilles/src/codegen/llvm.rs
+++ b/users/grfn/achilles/src/codegen/llvm.rs
@@ -7,12 +7,13 @@ 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::types::{BasicType, BasicTypeEnum, FunctionType, IntType, StructType};
+use inkwell::values::{AnyValueEnum, BasicValueEnum, FunctionValue, StructValue};
 use inkwell::{AddressSpace, IntPredicate};
+use itertools::Itertools;
 use thiserror::Error;
 
-use crate::ast::hir::{Binding, Decl, Expr};
+use crate::ast::hir::{Binding, Decl, Expr, Pattern};
 use crate::ast::{BinaryOperator, Ident, Literal, Type, UnaryOperator};
 use crate::common::env::Env;
 
@@ -82,6 +83,25 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
             .append_basic_block(*self.function_stack.last().unwrap(), name)
     }
 
+    fn bind_pattern(&mut self, pat: &'ast Pattern<'ast, Type>, val: AnyValueEnum<'ctx>) {
+        match pat {
+            Pattern::Id(id, _) => self.env.set(id, val),
+            Pattern::Tuple(pats) => {
+                for (i, pat) in pats.iter().enumerate() {
+                    let member = self
+                        .builder
+                        .build_extract_value(
+                            StructValue::try_from(val).unwrap(),
+                            i as _,
+                            "pat_bind",
+                        )
+                        .unwrap();
+                    self.bind_pattern(pat, member.into());
+                }
+            }
+        }
+    }
+
     pub fn codegen_expr(
         &mut self,
         expr: &'ast Expr<'ast, Type>,
@@ -164,9 +184,9 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
             }
             Expr::Let { bindings, body, .. } => {
                 self.env.push();
-                for Binding { ident, body, .. } in bindings {
+                for Binding { pat, body, .. } in bindings {
                     if let Some(val) = self.codegen_expr(body)? {
-                        self.env.set(ident, val);
+                        self.bind_pattern(pat, val);
                     }
                 }
                 let res = self.codegen_expr(body);
@@ -244,6 +264,19 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
                 self.env.restore(env);
                 Ok(Some(function.into()))
             }
+            Expr::Tuple(members, ty) => {
+                let values = members
+                    .into_iter()
+                    .map(|expr| self.codegen_expr(expr))
+                    .collect::<Result<Vec<_>>>()?
+                    .into_iter()
+                    .filter_map(|x| x)
+                    .map(|x| x.try_into().unwrap())
+                    .collect_vec();
+                let field_types = ty.as_tuple().unwrap();
+                let tuple_type = self.codegen_tuple_type(field_types);
+                Ok(Some(tuple_type.const_named_struct(&values).into()))
+            }
         }
     }
 
@@ -341,9 +374,20 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
             Type::Function(_) => todo!(),
             Type::Var(_) => unreachable!(),
             Type::Unit => None,
+            Type::Tuple(ts) => Some(self.codegen_tuple_type(ts).into()),
         }
     }
 
+    fn codegen_tuple_type(&self, ts: &'ast [Type]) -> StructType<'ctx> {
+        self.context.struct_type(
+            ts.iter()
+                .filter_map(|t| self.codegen_type(t))
+                .collect_vec()
+                .as_slice(),
+            false,
+        )
+    }
+
     fn codegen_int_type(&self, type_: &'ast Type) -> IntType<'ctx> {
         // TODO
         self.context.i64_type()
@@ -433,4 +477,10 @@ mod tests {
         let res = jit_eval::<i64>("let id = fn x = x in id 1").unwrap();
         assert_eq!(res, 1);
     }
+
+    #[test]
+    fn bind_tuple_pattern() {
+        let res = jit_eval::<i64>("let (x, y) = (1, 2) in x + y").unwrap();
+        assert_eq!(res, 3);
+    }
 }
diff --git a/users/grfn/achilles/src/interpreter/mod.rs b/users/grfn/achilles/src/interpreter/mod.rs
index a8ba2dd3acdc..70df7a0724a5 100644
--- a/users/grfn/achilles/src/interpreter/mod.rs
+++ b/users/grfn/achilles/src/interpreter/mod.rs
@@ -1,9 +1,12 @@
 mod error;
 mod value;
 
+use itertools::Itertools;
+use value::Val;
+
 pub use self::error::{Error, Result};
 pub use self::value::{Function, Value};
-use crate::ast::hir::{Binding, Expr};
+use crate::ast::hir::{Binding, Expr, Pattern};
 use crate::ast::{BinaryOperator, FunctionType, Ident, Literal, Type, UnaryOperator};
 use crate::common::env::Env;
 
@@ -24,6 +27,17 @@ impl<'a> Interpreter<'a> {
             .ok_or_else(|| Error::UndefinedVariable(var.to_owned()))
     }
 
+    fn bind_pattern(&mut self, pattern: &'a Pattern<'a, Type>, value: Value<'a>) {
+        match pattern {
+            Pattern::Id(id, _) => self.env.set(id, value),
+            Pattern::Tuple(pats) => {
+                for (pat, val) in pats.iter().zip(value.as_tuple().unwrap().clone()) {
+                    self.bind_pattern(pat, val);
+                }
+            }
+        }
+    }
+
     pub fn eval(&mut self, expr: &'a Expr<'a, Type>) -> Result<Value<'a>> {
         let res = match expr {
             Expr::Ident(id, _) => self.resolve(id),
@@ -53,9 +67,9 @@ impl<'a> Interpreter<'a> {
             }
             Expr::Let { bindings, body, .. } => {
                 self.env.push();
-                for Binding { ident, body, .. } in bindings {
+                for Binding { pat, body, .. } in bindings {
                     let val = self.eval(body)?;
-                    self.env.set(ident, val);
+                    self.bind_pattern(pat, val);
                 }
                 let res = self.eval(body)?;
                 self.env.pop();
@@ -115,6 +129,13 @@ impl<'a> Interpreter<'a> {
                     body: (**body).to_owned(),
                 }))
             }
+            Expr::Tuple(members, _) => Ok(Val::Tuple(
+                members
+                    .into_iter()
+                    .map(|expr| self.eval(expr))
+                    .try_collect()?,
+            )
+            .into()),
         }?;
         debug_assert_eq!(&res.type_(), expr.type_());
         Ok(res)
diff --git a/users/grfn/achilles/src/interpreter/value.rs b/users/grfn/achilles/src/interpreter/value.rs
index 55ba42f9de58..272d1167a33c 100644
--- a/users/grfn/achilles/src/interpreter/value.rs
+++ b/users/grfn/achilles/src/interpreter/value.rs
@@ -6,6 +6,7 @@ use std::rc::Rc;
 use std::result;
 
 use derive_more::{Deref, From, TryInto};
+use itertools::Itertools;
 
 use super::{Error, Result};
 use crate::ast::hir::Expr;
@@ -25,6 +26,7 @@ pub enum Val<'a> {
     Float(f64),
     Bool(bool),
     String(Cow<'a, str>),
+    Tuple(Vec<Value<'a>>),
     Function(Function<'a>),
 }
 
@@ -49,6 +51,7 @@ impl<'a> fmt::Debug for Val<'a> {
             Val::Function(Function { type_, .. }) => {
                 f.debug_struct("Function").field("type_", type_).finish()
             }
+            Val::Tuple(members) => f.debug_tuple("Tuple").field(members).finish(),
         }
     }
 }
@@ -79,6 +82,7 @@ impl<'a> Display for Val<'a> {
             Val::Bool(x) => x.fmt(f),
             Val::String(s) => write!(f, "{:?}", s),
             Val::Function(Function { type_, .. }) => write!(f, "<{}>", type_),
+            Val::Tuple(members) => write!(f, "({})", members.iter().join(", ")),
         }
     }
 }
@@ -91,6 +95,7 @@ impl<'a> Val<'a> {
             Val::Bool(_) => Type::Bool,
             Val::String(_) => Type::CString,
             Val::Function(Function { type_, .. }) => Type::Function(type_.clone()),
+            Val::Tuple(members) => Type::Tuple(members.iter().map(|expr| expr.type_()).collect()),
         }
     }
 
@@ -114,6 +119,22 @@ impl<'a> Val<'a> {
             }),
         }
     }
+
+    pub fn as_tuple(&self) -> Option<&Vec<Value<'a>>> {
+        if let Self::Tuple(v) = self {
+            Some(v)
+        } else {
+            None
+        }
+    }
+
+    pub fn try_into_tuple(self) -> result::Result<Vec<Value<'a>>, Self> {
+        if let Self::Tuple(v) = self {
+            Ok(v)
+        } else {
+            Err(self)
+        }
+    }
 }
 
 #[derive(Debug, PartialEq, Clone, Deref)]
diff --git a/users/grfn/achilles/src/parser/expr.rs b/users/grfn/achilles/src/parser/expr.rs
index 8a28d00984c9..f596b18970aa 100644
--- a/users/grfn/achilles/src/parser/expr.rs
+++ b/users/grfn/achilles/src/parser/expr.rs
@@ -8,7 +8,8 @@ use nom::{
 };
 use pratt::{Affix, Associativity, PrattParser, Precedence};
 
-use crate::ast::{BinaryOperator, Binding, Expr, Fun, Literal, UnaryOperator};
+use super::util::comma;
+use crate::ast::{BinaryOperator, Binding, Expr, Fun, Literal, Pattern, UnaryOperator};
 use crate::parser::{arg, ident, type_};
 
 #[derive(Debug)]
@@ -192,9 +193,45 @@ named!(literal(&str) -> Literal, alt!(int | bool_ | string | unit));
 
 named!(literal_expr(&str) -> Expr, map!(literal, Expr::Literal));
 
+named!(tuple(&str) -> Expr, do_parse!(
+    complete!(tag!("("))
+        >> multispace0
+        >> fst: expr
+        >> comma
+        >> rest: separated_list0!(
+            comma,
+            expr
+        )
+        >> multispace0
+        >> tag!(")")
+        >> ({
+            let mut members = Vec::with_capacity(rest.len() + 1);
+            members.push(fst);
+            members.append(&mut rest.clone());
+            Expr::Tuple(members)
+        })
+));
+
+named!(tuple_pattern(&str) -> Pattern, do_parse!(
+    complete!(tag!("("))
+        >> multispace0
+        >> pats: separated_list0!(
+            comma,
+            pattern
+        )
+        >> multispace0
+        >> tag!(")")
+        >> (Pattern::Tuple(pats))
+));
+
+named!(pattern(&str) -> Pattern, alt!(
+    ident => { |id| Pattern::Id(id) } |
+    tuple_pattern
+));
+
 named!(binding(&str) -> Binding, do_parse!(
     multispace0
-        >> ident: ident
+        >> pat: pattern
         >> multispace0
         >> type_: opt!(preceded!(tuple!(tag!(":"), multispace0), type_))
         >> multispace0
@@ -202,7 +239,7 @@ named!(binding(&str) -> Binding, do_parse!(
         >> multispace0
         >> body: expr
         >> (Binding {
-            ident,
+            pat,
             type_,
             body
         })
@@ -267,6 +304,7 @@ named!(paren_expr(&str) -> Expr,
 
 named!(funcref(&str) -> Expr, alt!(
     ident_expr |
+    tuple |
     paren_expr
 ));
 
@@ -296,6 +334,7 @@ named!(fun_expr(&str) -> Expr, do_parse!(
 named!(fn_arg(&str) -> Expr, alt!(
     ident_expr |
     literal_expr |
+    tuple |
     paren_expr
 ));
 
@@ -314,7 +353,8 @@ named!(simple_expr_unascripted(&str) -> Expr, alt!(
     if_ |
     fun_expr |
     literal_expr |
-    ident_expr
+    ident_expr |
+    tuple
 ));
 
 named!(simple_expr(&str) -> Expr, alt!(
@@ -334,7 +374,7 @@ named!(pub expr(&str) -> Expr, alt!(
 #[cfg(test)]
 pub(crate) mod tests {
     use super::*;
-    use crate::ast::{Arg, Ident, Type};
+    use crate::ast::{Arg, Ident, Pattern, Type};
     use std::convert::TryFrom;
     use BinaryOperator::*;
     use Expr::{BinaryOp, If, Let, UnaryOp};
@@ -450,6 +490,17 @@ pub(crate) mod tests {
     }
 
     #[test]
+    fn tuple() {
+        assert_eq!(
+            test_parse!(expr, "(1, \"seven\")"),
+            Expr::Tuple(vec![
+                Expr::Literal(Literal::Int(1)),
+                Expr::Literal(Literal::String(Cow::Borrowed("seven")))
+            ])
+        )
+    }
+
+    #[test]
     fn simple_string_lit() {
         assert_eq!(
             test_parse!(expr, "\"foobar\""),
@@ -465,12 +516,12 @@ pub(crate) mod tests {
             Let {
                 bindings: vec![
                     Binding {
-                        ident: Ident::try_from("x").unwrap(),
+                        pat: Pattern::Id(Ident::try_from("x").unwrap()),
                         type_: None,
                         body: Expr::Literal(Literal::Int(1))
                     },
                     Binding {
-                        ident: Ident::try_from("y").unwrap(),
+                        pat: Pattern::Id(Ident::try_from("y").unwrap()),
                         type_: None,
                         body: Expr::BinaryOp {
                             lhs: ident_expr("x"),
@@ -553,7 +604,7 @@ pub(crate) mod tests {
             Expr::Call {
                 fun: Box::new(Expr::Let {
                     bindings: vec![Binding {
-                        ident: Ident::try_from("x").unwrap(),
+                        pat: Pattern::Id(Ident::try_from("x").unwrap()),
                         type_: None,
                         body: Expr::Literal(Literal::Int(1))
                     }],
@@ -571,7 +622,7 @@ pub(crate) mod tests {
             res,
             Expr::Let {
                 bindings: vec![Binding {
-                    ident: Ident::try_from("id").unwrap(),
+                    pat: Pattern::Id(Ident::try_from("id").unwrap()),
                     type_: None,
                     body: Expr::Fun(Box::new(Fun {
                         args: vec![Arg::try_from("x").unwrap()],
@@ -586,6 +637,28 @@ pub(crate) mod tests {
         );
     }
 
+    #[test]
+    fn tuple_binding() {
+        let res = test_parse!(expr, "let (x, y) = (1, 2) in x");
+        assert_eq!(
+            res,
+            Expr::Let {
+                bindings: vec![Binding {
+                    pat: Pattern::Tuple(vec![
+                        Pattern::Id(Ident::from_str_unchecked("x")),
+                        Pattern::Id(Ident::from_str_unchecked("y"))
+                    ]),
+                    body: Expr::Tuple(vec![
+                        Expr::Literal(Literal::Int(1)),
+                        Expr::Literal(Literal::Int(2))
+                    ]),
+                    type_: None
+                }],
+                body: Box::new(Expr::Ident(Ident::from_str_unchecked("x")))
+            }
+        )
+    }
+
     mod ascriptions {
         use super::*;
 
@@ -608,7 +681,7 @@ pub(crate) mod tests {
                 res,
                 Expr::Let {
                     bindings: vec![Binding {
-                        ident: Ident::try_from("const_1").unwrap(),
+                        pat: Pattern::Id(Ident::try_from("const_1").unwrap()),
                         type_: None,
                         body: Expr::Fun(Box::new(Fun {
                             args: vec![Arg::try_from("x").unwrap()],
@@ -633,7 +706,7 @@ pub(crate) mod tests {
                 res,
                 Expr::Let {
                     bindings: vec![Binding {
-                        ident: Ident::try_from("x").unwrap(),
+                        pat: Pattern::Id(Ident::try_from("x").unwrap()),
                         type_: Some(Type::Int),
                         body: Expr::Literal(Literal::Int(1))
                     }],
diff --git a/users/grfn/achilles/src/parser/mod.rs b/users/grfn/achilles/src/parser/mod.rs
index 3e0081bd391d..e088cbca10a5 100644
--- a/users/grfn/achilles/src/parser/mod.rs
+++ b/users/grfn/achilles/src/parser/mod.rs
@@ -6,6 +6,7 @@ use nom::{alt, char, complete, do_parse, eof, many0, named, separated_list0, tag
 pub(crate) mod macros;
 mod expr;
 mod type_;
+mod util;
 
 use crate::ast::{Arg, Decl, Fun, Ident};
 pub use expr::expr;
diff --git a/users/grfn/achilles/src/parser/type_.rs b/users/grfn/achilles/src/parser/type_.rs
index 8a1081e2521f..b80f0e0860a1 100644
--- a/users/grfn/achilles/src/parser/type_.rs
+++ b/users/grfn/achilles/src/parser/type_.rs
@@ -2,17 +2,14 @@ use nom::character::complete::{multispace0, multispace1};
 use nom::{alt, delimited, do_parse, map, named, opt, separated_list0, tag, terminated, tuple};
 
 use super::ident;
+use super::util::comma;
 use crate::ast::{FunctionType, Type};
 
 named!(pub function_type(&str) -> FunctionType, do_parse!(
     tag!("fn")
         >> multispace1
         >> args: map!(opt!(terminated!(separated_list0!(
-            tuple!(
-                multispace0,
-                tag!(","),
-                multispace0
-            ),
+            comma,
             type_
         ), multispace1)), |args| args.unwrap_or_default())
         >> tag!("->")
@@ -24,12 +21,32 @@ named!(pub function_type(&str) -> FunctionType, do_parse!(
         })
 ));
 
+named!(tuple_type(&str) -> Type, do_parse!(
+    tag!("(")
+        >> multispace0
+        >> fst: type_
+        >> comma
+        >> rest: separated_list0!(
+            comma,
+            type_
+        )
+        >> multispace0
+        >> tag!(")")
+        >> ({
+            let mut members = Vec::with_capacity(rest.len() + 1);
+            members.push(fst);
+            members.append(&mut rest.clone());
+            Type::Tuple(members)
+        })
+));
+
 named!(pub type_(&str) -> Type, alt!(
     tag!("int") => { |_| Type::Int } |
     tag!("float") => { |_| Type::Float } |
     tag!("bool") => { |_| Type::Bool } |
     tag!("cstring") => { |_| Type::CString } |
     tag!("()") => { |_| Type::Unit } |
+    tuple_type |
     function_type => { |ft| Type::Function(ft) }|
     ident => { |id| Type::Var(id) } |
     delimited!(
@@ -112,6 +129,14 @@ mod tests {
     }
 
     #[test]
+    fn tuple() {
+        assert_eq!(
+            test_parse!(type_, "(int, int)"),
+            Type::Tuple(vec![Type::Int, Type::Int])
+        )
+    }
+
+    #[test]
     fn type_vars() {
         assert_eq!(
             test_parse!(type_, "fn x, y -> x"),
diff --git a/users/grfn/achilles/src/parser/util.rs b/users/grfn/achilles/src/parser/util.rs
new file mode 100644
index 000000000000..bb53fb7fff50
--- /dev/null
+++ b/users/grfn/achilles/src/parser/util.rs
@@ -0,0 +1,8 @@
+use nom::character::complete::multispace0;
+use nom::{complete, map, named, tag, tuple};
+
+named!(pub(crate) comma(&str) -> (), map!(tuple!(
+    multispace0,
+    complete!(tag!(",")),
+    multispace0
+) ,|_| ()));
diff --git a/users/grfn/achilles/src/passes/hir/mod.rs b/users/grfn/achilles/src/passes/hir/mod.rs
index 845bfcb7ab6a..872c449eb020 100644
--- a/users/grfn/achilles/src/passes/hir/mod.rs
+++ b/users/grfn/achilles/src/passes/hir/mod.rs
@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 
-use crate::ast::hir::{Binding, Decl, Expr};
+use crate::ast::hir::{Binding, Decl, Expr, Pattern};
 use crate::ast::{BinaryOperator, Ident, Literal, UnaryOperator};
 
 pub(crate) mod monomorphize;
@@ -29,9 +29,12 @@ pub(crate) trait Visitor<'a, 'ast, T: 'ast>: Sized + 'a {
         Ok(())
     }
 
+    fn visit_pattern(&mut self, _pat: &mut Pattern<'ast, T>) -> Result<(), Self::Error> {
+        Ok(())
+    }
+
     fn visit_binding(&mut self, binding: &mut Binding<'ast, T>) -> Result<(), Self::Error> {
-        self.visit_ident(&mut binding.ident)?;
-        self.visit_type(&mut binding.type_)?;
+        self.visit_pattern(&mut binding.pat)?;
         self.visit_expr(&mut binding.body)?;
         Ok(())
     }
@@ -54,6 +57,13 @@ pub(crate) trait Visitor<'a, 'ast, T: 'ast>: Sized + 'a {
         Ok(())
     }
 
+    fn visit_tuple(&mut self, members: &mut Vec<Expr<'ast, T>>) -> Result<(), Self::Error> {
+        for expr in members {
+            self.visit_expr(expr)?;
+        }
+        Ok(())
+    }
+
     fn pre_visit_expr(&mut self, _expr: &mut Expr<'ast, T>) -> Result<(), Self::Error> {
         Ok(())
     }
@@ -137,12 +147,16 @@ pub(crate) trait Visitor<'a, 'ast, T: 'ast>: Sized + 'a {
                 self.visit_type(type_)?;
                 self.post_visit_call(fun, type_args, args)?;
             }
+            Expr::Tuple(tup, type_) => {
+                self.visit_tuple(tup)?;
+                self.visit_type(type_)?;
+            }
         }
 
         Ok(())
     }
 
-    fn post_visit_decl(&mut self, decl: &'a Decl<'ast, T>) -> Result<(), Self::Error> {
+    fn post_visit_decl(&mut self, _decl: &'a Decl<'ast, T>) -> Result<(), Self::Error> {
         Ok(())
     }
 
diff --git a/users/grfn/achilles/src/passes/hir/strip_positive_units.rs b/users/grfn/achilles/src/passes/hir/strip_positive_units.rs
index 91b56551c82d..85ee1cce4859 100644
--- a/users/grfn/achilles/src/passes/hir/strip_positive_units.rs
+++ b/users/grfn/achilles/src/passes/hir/strip_positive_units.rs
@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::mem;
 
-use ast::hir::Binding;
+use ast::hir::{Binding, Pattern};
 use ast::Literal;
 use void::{ResultVoidExt, Void};
 
@@ -42,8 +42,10 @@ impl<'a, 'ast> Visitor<'a, 'ast, ast::Type<'ast>> for StripPositiveUnits {
                 bindings: extracted
                     .into_iter()
                     .map(|expr| Binding {
-                        ident: Ident::from_str_unchecked("___discarded"),
-                        type_: expr.type_().clone(),
+                        pat: Pattern::Id(
+                            Ident::from_str_unchecked("___discarded"),
+                            expr.type_().clone(),
+                        ),
                         body: expr,
                     })
                     .collect(),
diff --git a/users/grfn/achilles/src/tc/mod.rs b/users/grfn/achilles/src/tc/mod.rs
index d27c45075e97..5825bab1fbe9 100644
--- a/users/grfn/achilles/src/tc/mod.rs
+++ b/users/grfn/achilles/src/tc/mod.rs
@@ -8,7 +8,7 @@ use std::fmt::{self, Display};
 use std::{mem, result};
 use thiserror::Error;
 
-use crate::ast::{self, hir, Arg, BinaryOperator, Ident, Literal};
+use crate::ast::{self, hir, Arg, BinaryOperator, Ident, Literal, Pattern};
 use crate::common::env::Env;
 use crate::common::{Namer, NamerOf};
 
@@ -85,6 +85,7 @@ pub enum Type {
     Exist(TyVar),
     Nullary(NullaryType),
     Prim(PrimType),
+    Tuple(Vec<Type>),
     Unit,
     Fun {
         args: Vec<Type>,
@@ -102,6 +103,9 @@ impl<'a> TryFrom<Type> for ast::Type<'a> {
             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()
@@ -128,6 +132,7 @@ impl Display for Type {
             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, "()"),
         }
     }
@@ -159,6 +164,31 @@ impl<'ast> Typechecker<'ast> {
         }
     }
 
+    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) => {
@@ -178,6 +208,14 @@ impl<'ast> Typechecker<'ast> {
                 };
                 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)?;
@@ -209,18 +247,14 @@ impl<'ast> Typechecker<'ast> {
                 let bindings = bindings
                     .into_iter()
                     .map(
-                        |ast::Binding { ident, type_, body }| -> Result<hir::Binding<Type>> {
+                        |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_)?;
                             }
-                            self.env.set(ident.clone(), body.type_().clone());
-                            Ok(hir::Binding {
-                                ident,
-                                type_: body.type_().clone(),
-                                body,
-                            })
+                            let pat = self.bind_pattern(pat, body.type_().clone())?;
+                            Ok(hir::Binding { pat, body })
                         },
                     )
                     .collect::<Result<Vec<hir::Binding<Type>>>>()?;
@@ -382,7 +416,7 @@ impl<'ast> Typechecker<'ast> {
     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) {
+            (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);
@@ -419,6 +453,14 @@ impl<'ast> Typechecker<'ast> {
                 }
             }
             (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,
@@ -469,11 +511,17 @@ impl<'ast> Typechecker<'ast> {
 
     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::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()
@@ -485,12 +533,15 @@ impl<'ast> Typechecker<'ast> {
         ret
     }
 
-    fn resolve_tv(&self, tv: TyVar) -> Option<ast::Type<'static>> {
+    fn resolve_tv(&self, tv: TyVar) -> Result<Option<ast::Type<'static>>> {
         let mut res = &Type::Exist(tv);
-        loop {
+        Ok(loop {
             match res {
                 Type::Exist(tv) => {
-                    res = self.ctx.get(tv)?;
+                    res = match self.ctx.get(tv) {
+                        Some(r) => r,
+                        None => return Ok(None),
+                    };
                 }
                 Type::Univ(tv) => {
                     let ident = self.name_univ(*tv);
@@ -504,8 +555,9 @@ impl<'ast> Typechecker<'ast> {
                 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 {
@@ -515,6 +567,12 @@ impl<'ast> Typechecker<'ast> {
             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()
@@ -582,6 +640,11 @@ impl<'ast> Typechecker<'ast> {
             (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