diff --git a/compiler/cl-ast/src/ast.rs b/compiler/cl-ast/src/ast.rs
index 249f3ba..97c96f7 100644
--- a/compiler/cl-ast/src/ast.rs
+++ b/compiler/cl-ast/src/ast.rs
@@ -317,6 +317,8 @@ pub enum ExprKind {
     /// An empty expression: `(` `)`
     #[default]
     Empty,
+    /// A [Closure] expression: `|` [`Expr`] `|` ( -> [`Ty`])? [`Expr`]
+    Closure(Closure),
     /// A [Tuple] expression: `(` [`Expr`] (`,` [`Expr`])+ `)`
     Tuple(Tuple),
     /// A [Struct creation](Structor) expression: [Path] `{` ([Fielder] `,`)* [Fielder]? `}`
@@ -371,6 +373,13 @@ pub enum ExprKind {
     Continue,
 }
 
+/// A Closure [expression](Expr): `|` [`Expr`] `|` ( -> [`Ty`])? [`Expr`]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct Closure {
+    pub arg: Box<Pattern>,
+    pub body: Box<Expr>,
+}
+
 /// A [Tuple] expression: `(` [`Expr`] (`,` [`Expr`])+ `)`
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Tuple {
diff --git a/compiler/cl-ast/src/ast_impl/convert.rs b/compiler/cl-ast/src/ast_impl/convert.rs
index 5d72213..a48e51a 100644
--- a/compiler/cl-ast/src/ast_impl/convert.rs
+++ b/compiler/cl-ast/src/ast_impl/convert.rs
@@ -51,6 +51,7 @@ impl_from! {
     }
     impl From for ExprKind {
         Let => ExprKind::Let,
+        Closure => ExprKind::Closure,
         Quote => ExprKind::Quote,
         Match => ExprKind::Match,
         Assign => ExprKind::Assign,
diff --git a/compiler/cl-ast/src/ast_impl/display.rs b/compiler/cl-ast/src/ast_impl/display.rs
index 323c852..1f24d3c 100644
--- a/compiler/cl-ast/src/ast_impl/display.rs
+++ b/compiler/cl-ast/src/ast_impl/display.rs
@@ -402,6 +402,7 @@ impl Display for ExprKind {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             ExprKind::Empty => "()".fmt(f),
+            ExprKind::Closure(v) => v.fmt(f),
             ExprKind::Quote(v) => v.fmt(f),
             ExprKind::Let(v) => v.fmt(f),
             ExprKind::Match(v) => v.fmt(f),
@@ -431,6 +432,17 @@ impl Display for ExprKind {
     }
 }
 
+impl Display for Closure {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let Self { arg, body } = self;
+        match arg.as_ref() {
+            Pattern::Tuple(args) => separate(args, ", ")(f.delimit_with("|", "|")),
+            _ => arg.fmt(f),
+        }?;
+        write!(f, " {body}")
+    }
+}
+
 impl Display for Quote {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let Self { quote } = self;
diff --git a/compiler/cl-ast/src/ast_impl/weight_of.rs b/compiler/cl-ast/src/ast_impl/weight_of.rs
index f1630ba..46ec5d1 100644
--- a/compiler/cl-ast/src/ast_impl/weight_of.rs
+++ b/compiler/cl-ast/src/ast_impl/weight_of.rs
@@ -283,6 +283,7 @@ impl WeightOf for ExprKind {
     fn weight_of(&self) -> usize {
         match self {
             ExprKind::Empty => size_of_val(self),
+            ExprKind::Closure(v) => v.weight_of(),
             ExprKind::Quote(v) => v.weight_of(),
             ExprKind::Let(v) => v.weight_of(),
             ExprKind::Match(v) => v.weight_of(),
@@ -312,6 +313,13 @@ impl WeightOf for ExprKind {
     }
 }
 
+impl WeightOf for Closure {
+    fn weight_of(&self) -> usize {
+        let Self { arg, body } = self;
+        arg.weight_of() + body.weight_of()
+    }
+}
+
 impl WeightOf for Quote {
     fn weight_of(&self) -> usize {
         let Self { quote } = self;
diff --git a/compiler/cl-ast/src/ast_visitor/fold.rs b/compiler/cl-ast/src/ast_visitor/fold.rs
index 2327365..5dc77ea 100644
--- a/compiler/cl-ast/src/ast_visitor/fold.rs
+++ b/compiler/cl-ast/src/ast_visitor/fold.rs
@@ -236,6 +236,10 @@ pub trait Fold {
     fn fold_expr_kind(&mut self, kind: ExprKind) -> ExprKind {
         or_fold_expr_kind(self, kind)
     }
+    fn fold_closure(&mut self, value: Closure) -> Closure {
+        let Closure { arg, body } = value;
+        Closure { arg: Box::new(self.fold_pattern(*arg)), body: Box::new(self.fold_expr(*body)) }
+    }
     fn fold_let(&mut self, l: Let) -> Let {
         let Let { mutable, name, ty, init } = l;
         Let {
@@ -547,6 +551,7 @@ pub fn or_fold_stmt_kind<F: Fold + ?Sized>(folder: &mut F, kind: StmtKind) -> St
 pub fn or_fold_expr_kind<F: Fold + ?Sized>(folder: &mut F, kind: ExprKind) -> ExprKind {
     match kind {
         ExprKind::Empty => ExprKind::Empty,
+        ExprKind::Closure(c) => ExprKind::Closure(folder.fold_closure(c)),
         ExprKind::Quote(q) => ExprKind::Quote(q), // quoted expressions are left unmodified
         ExprKind::Let(l) => ExprKind::Let(folder.fold_let(l)),
         ExprKind::Match(m) => ExprKind::Match(folder.fold_match(m)),
diff --git a/compiler/cl-ast/src/ast_visitor/visit.rs b/compiler/cl-ast/src/ast_visitor/visit.rs
index 60fd453..4ff40d9 100644
--- a/compiler/cl-ast/src/ast_visitor/visit.rs
+++ b/compiler/cl-ast/src/ast_visitor/visit.rs
@@ -17,8 +17,9 @@ use super::walk::Walk;
 pub trait Visit<'a>: Sized {
     /// Visits a [Walker](Walk)
     #[inline]
-    fn visit<W: Walk>(&mut self, walker: &'a W) {
-        walker.visit_in(self)
+    fn visit<W: Walk>(&mut self, walker: &'a W) -> &mut Self {
+        walker.visit_in(self);
+        self
     }
     /// Visits the children of a [Walker](Walk)
     fn visit_children<W: Walk>(&mut self, walker: &'a W) {
@@ -160,6 +161,9 @@ pub trait Visit<'a>: Sized {
     fn visit_expr_kind(&mut self, value: &'a ExprKind) {
         value.children(self)
     }
+    fn visit_closure(&mut self, value: &'a Closure) {
+        value.children(self)
+    }
     fn visit_quote(&mut self, value: &'a Quote) {
         value.children(self)
     }
diff --git a/compiler/cl-ast/src/ast_visitor/walk.rs b/compiler/cl-ast/src/ast_visitor/walk.rs
index eae8480..bd3a487 100644
--- a/compiler/cl-ast/src/ast_visitor/walk.rs
+++ b/compiler/cl-ast/src/ast_visitor/walk.rs
@@ -504,25 +504,26 @@ impl Walk for ExprKind {
     fn children<'a, V: Visit<'a>>(&'a self, v: &mut V) {
         match self {
             ExprKind::Empty => {}
+            ExprKind::Closure(value) => value.visit_in(v),
+            ExprKind::Tuple(value) => value.visit_in(v),
+            ExprKind::Structor(value) => value.visit_in(v),
+            ExprKind::Array(value) => value.visit_in(v),
+            ExprKind::ArrayRep(value) => value.visit_in(v),
+            ExprKind::AddrOf(value) => value.visit_in(v),
             ExprKind::Quote(value) => value.visit_in(v),
-            ExprKind::Let(value) => value.visit_in(v),
-            ExprKind::Match(value) => value.visit_in(v),
+            ExprKind::Literal(value) => value.visit_in(v),
+            ExprKind::Group(value) => value.visit_in(v),
+            ExprKind::Block(value) => value.visit_in(v),
             ExprKind::Assign(value) => value.visit_in(v),
             ExprKind::Modify(value) => value.visit_in(v),
             ExprKind::Binary(value) => value.visit_in(v),
             ExprKind::Unary(value) => value.visit_in(v),
-            ExprKind::Cast(value) => value.visit_in(v),
             ExprKind::Member(value) => value.visit_in(v),
             ExprKind::Index(value) => value.visit_in(v),
-            ExprKind::Structor(value) => value.visit_in(v),
+            ExprKind::Cast(value) => value.visit_in(v),
             ExprKind::Path(value) => value.visit_in(v),
-            ExprKind::Literal(value) => value.visit_in(v),
-            ExprKind::Array(value) => value.visit_in(v),
-            ExprKind::ArrayRep(value) => value.visit_in(v),
-            ExprKind::AddrOf(value) => value.visit_in(v),
-            ExprKind::Block(value) => value.visit_in(v),
-            ExprKind::Group(value) => value.visit_in(v),
-            ExprKind::Tuple(value) => value.visit_in(v),
+            ExprKind::Let(value) => value.visit_in(v),
+            ExprKind::Match(value) => value.visit_in(v),
             ExprKind::While(value) => value.visit_in(v),
             ExprKind::If(value) => value.visit_in(v),
             ExprKind::For(value) => value.visit_in(v),
@@ -533,6 +534,18 @@ impl Walk for ExprKind {
     }
 }
 
+impl Walk for Closure {
+    fn visit_in<'a, V: Visit<'a>>(&'a self, v: &mut V) {
+        v.visit_closure(self);
+    }
+
+    fn children<'a, V: Visit<'a>>(&'a self, v: &mut V) {
+        let Self { arg, body } = self;
+        v.visit_pattern(arg);
+        v.visit_expr(body);
+    }
+}
+
 impl Walk for Tuple {
     #[inline]
     fn visit_in<'a, V: Visit<'a>>(&'a self, v: &mut V) {
diff --git a/compiler/cl-interpret/src/closure.rs b/compiler/cl-interpret/src/closure.rs
new file mode 100644
index 0000000..7f4be74
--- /dev/null
+++ b/compiler/cl-interpret/src/closure.rs
@@ -0,0 +1,68 @@
+use crate::{
+    Callable,
+    convalue::ConValue,
+    env::{Environment, Place},
+    error::{Error, ErrorKind, IResult},
+    function::collect_upvars::CollectUpvars,
+    interpret::Interpret,
+    pattern,
+};
+use cl_ast::{Sym, ast_visitor::Visit};
+use std::{collections::HashMap, fmt::Display};
+
+/// Represents an ad-hoc anonymous function
+/// which captures surrounding state by reference.
+#[derive(Clone, Debug)]
+pub struct Closure {
+    decl: cl_ast::Closure,
+    lift: HashMap<Sym, Place>,
+}
+
+impl Closure {
+    const NAME: &'static str = "{closure}";
+}
+
+impl Closure {
+    pub fn new(env: &mut Environment, decl: &cl_ast::Closure) -> Self {
+        let lift = CollectUpvars::new(env).visit(decl).finish();
+        Self { decl: decl.clone(), lift }
+    }
+}
+
+impl Display for Closure {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let Self { decl, lift: _ } = self;
+        write!(f, "{decl}")
+    }
+}
+
+impl Callable for Closure {
+    fn call(&self, env: &mut Environment, args: &[ConValue]) -> IResult<ConValue> {
+        let Self { decl, lift } = self;
+        let mut env = env.frame(Self::NAME);
+
+        // place lifts in scope
+        for (name, place) in lift {
+            env.insert(*name, Some(ConValue::Ref(*place)));
+        }
+
+        let mut env = env.frame("args");
+
+        for (name, value) in pattern::substitution(&decl.arg, ConValue::Tuple(args.into()))? {
+            env.insert(*name, Some(value));
+        }
+
+        let res = decl.body.interpret(&mut env);
+        drop(env);
+
+        match res {
+            Err(Error { kind: ErrorKind::Return(value), .. }) => Ok(value),
+            Err(Error { kind: ErrorKind::Break(value), .. }) => Err(Error::BadBreak(value)),
+            other => other,
+        }
+    }
+
+    fn name(&self) -> cl_ast::Sym {
+        "{closure}".into()
+    }
+}
diff --git a/compiler/cl-interpret/src/convalue.rs b/compiler/cl-interpret/src/convalue.rs
index 2770941..74a39b3 100644
--- a/compiler/cl-interpret/src/convalue.rs
+++ b/compiler/cl-interpret/src/convalue.rs
@@ -3,7 +3,7 @@
 //! The most permanent fix is a temporary one.
 use cl_ast::{Expr, Sym, format::FmtAdapter};
 
-use crate::env::Place;
+use crate::{closure::Closure, env::Place};
 
 use super::{
     Callable, Environment,
@@ -71,6 +71,8 @@ pub enum ConValue {
     Quote(Box<Expr>),
     /// A callable thing
     Function(Rc<Function>),
+    /// A closure, capturing by reference
+    Closure(Rc<Closure>),
     /// A built-in function
     Builtin(&'static Builtin),
 }
@@ -140,6 +142,7 @@ impl Callable for ConValue {
     fn name(&self) -> Sym {
         match self {
             ConValue::Function(func) => func.name(),
+            ConValue::Closure(func) => func.name(),
             ConValue::Builtin(func) => func.name(),
             _ => "".into(),
         }
@@ -147,6 +150,7 @@ impl Callable for ConValue {
     fn call(&self, interpreter: &mut Environment, args: &[ConValue]) -> IResult<ConValue> {
         match self {
             Self::Function(func) => func.call(interpreter, args),
+            Self::Closure(func) => func.call(interpreter, args),
             Self::Builtin(func) => func.call(interpreter, args),
             _ => Err(Error::NotCallable(self.clone())),
         }
@@ -368,6 +372,9 @@ impl std::fmt::Display for ConValue {
             ConValue::Function(func) => {
                 write!(f, "{}", func.decl())
             }
+            ConValue::Closure(func) => {
+                write!(f, "{}", func.as_ref())
+            }
             ConValue::Builtin(func) => {
                 write!(f, "{}", func.description())
             }
diff --git a/compiler/cl-interpret/src/env.rs b/compiler/cl-interpret/src/env.rs
index c901991..5010d56 100644
--- a/compiler/cl-interpret/src/env.rs
+++ b/compiler/cl-interpret/src/env.rs
@@ -159,30 +159,14 @@ impl Environment {
         }
     }
 
-    pub(crate) fn get_local(&self, name: Sym) -> IResult<ConValue> {
-        for EnvFrame { binds, .. } in self.frames.iter().skip(2).rev() {
-            if let Some(var) = binds.get(&name) {
-                if let Some(var) = self.values.get(*var) {
-                    return match var {
-                        Some(value) => Ok(value.clone()),
-                        None => Err(Error::NotInitialized(name)),
-                    };
-                }
-            }
-        }
-        Err(Error::NotDefined(name))
-    }
-
+    /// Resolves the [Place] associated with a [Sym]
     pub fn id_of(&self, name: Sym) -> IResult<Place> {
         for EnvFrame { binds, .. } in self.frames.iter().rev() {
             if let Some(id) = binds.get(&name).copied() {
                 return Ok(Place::Local(id));
             }
         }
-        self.global
-            .contains_key(&name)
-            .then_some(Place::Global(name))
-            .ok_or(Error::NotDefined(name))
+        Ok(Place::Global(name))
     }
 
     pub fn get_id(&self, at: Place) -> Option<&ConValue> {
diff --git a/compiler/cl-interpret/src/function/collect_upvars.rs b/compiler/cl-interpret/src/function/collect_upvars.rs
index c5cbae3..76a4778 100644
--- a/compiler/cl-interpret/src/function/collect_upvars.rs
+++ b/compiler/cl-interpret/src/function/collect_upvars.rs
@@ -1,5 +1,5 @@
 //! Collects the "Upvars" of a function at the point of its creation, allowing variable capture
-use crate::{convalue::ConValue, env::Environment};
+use crate::env::{Environment, Place};
 use cl_ast::{
     Function, Let, Path, PathPart, Pattern, Sym,
     ast_visitor::{visit::*, walk::Walk},
@@ -7,13 +7,18 @@ use cl_ast::{
 use std::collections::{HashMap, HashSet};
 
 pub fn collect_upvars(f: &Function, env: &Environment) -> super::Upvars {
-    CollectUpvars::new(env).get_upvars(f)
+    CollectUpvars::new(env)
+        .visit(f)
+        .finish()
+        .into_iter()
+        .map(|(k, v)| (k, env.get_id(v).cloned()))
+        .collect()
 }
 
 #[derive(Clone, Debug)]
 pub struct CollectUpvars<'env> {
     env: &'env Environment,
-    upvars: HashMap<Sym, Option<ConValue>>,
+    upvars: HashMap<Sym, Place>,
     blacklist: HashSet<Sym>,
 }
 
@@ -21,9 +26,9 @@ impl<'env> CollectUpvars<'env> {
     pub fn new(env: &'env Environment) -> Self {
         Self { upvars: HashMap::new(), blacklist: HashSet::new(), env }
     }
-    pub fn get_upvars(mut self, f: &cl_ast::Function) -> HashMap<Sym, Option<ConValue>> {
-        self.visit_function(f);
-        self.upvars
+
+    pub fn finish(&mut self) -> HashMap<Sym, Place> {
+        std::mem::take(&mut self.upvars)
     }
 
     pub fn add_upvar(&mut self, name: &Sym) {
@@ -31,8 +36,8 @@ impl<'env> CollectUpvars<'env> {
         if blacklist.contains(name) || upvars.contains_key(name) {
             return;
         }
-        if let Ok(upvar) = env.get_local(*name) {
-            upvars.insert(*name, Some(upvar));
+        if let Ok(place) = env.id_of(*name) {
+            upvars.insert(*name, place);
         }
     }
 
diff --git a/compiler/cl-interpret/src/interpret.rs b/compiler/cl-interpret/src/interpret.rs
index b2e9d3e..67df8c8 100644
--- a/compiler/cl-interpret/src/interpret.rs
+++ b/compiler/cl-interpret/src/interpret.rs
@@ -295,6 +295,7 @@ impl Interpret for ExprKind {
     fn interpret(&self, env: &mut Environment) -> IResult<ConValue> {
         match self {
             ExprKind::Empty => Ok(ConValue::Empty),
+            ExprKind::Closure(v) => v.interpret(env),
             ExprKind::Quote(q) => q.interpret(env),
             ExprKind::Let(v) => v.interpret(env),
             ExprKind::Match(v) => v.interpret(env),
@@ -324,6 +325,14 @@ impl Interpret for ExprKind {
     }
 }
 
+impl Interpret for Closure {
+    fn interpret(&self, env: &mut Environment) -> IResult<ConValue> {
+        Ok(ConValue::Closure(
+            crate::closure::Closure::new(env, self).into(),
+        ))
+    }
+}
+
 impl Interpret for Quote {
     fn interpret(&self, _env: &mut Environment) -> IResult<ConValue> {
         // TODO: squoosh down into a ConValue?
diff --git a/compiler/cl-interpret/src/lib.rs b/compiler/cl-interpret/src/lib.rs
index 17598c4..5fdd71a 100644
--- a/compiler/cl-interpret/src/lib.rs
+++ b/compiler/cl-interpret/src/lib.rs
@@ -23,6 +23,8 @@ pub mod interpret;
 
 pub mod function;
 
+pub mod closure;
+
 pub mod builtin;
 
 pub mod pattern;
diff --git a/compiler/cl-parser/src/error.rs b/compiler/cl-parser/src/error.rs
index 11e04b8..9bd72ed 100644
--- a/compiler/cl-parser/src/error.rs
+++ b/compiler/cl-parser/src/error.rs
@@ -97,6 +97,7 @@ pub enum Parsing {
 
     Expr,
     ExprKind,
+    Closure,
     Assign,
     AssignKind,
     Binary,
@@ -214,6 +215,7 @@ impl Display for Parsing {
 
             Parsing::Expr => "an expression",
             Parsing::ExprKind => "an expression",
+            Parsing::Closure => "an anonymous function",
             Parsing::Assign => "an assignment",
             Parsing::AssignKind => "an assignment operator",
             Parsing::Binary => "a binary expression",
diff --git a/compiler/cl-parser/src/parser.rs b/compiler/cl-parser/src/parser.rs
index 8332211..097ef18 100644
--- a/compiler/cl-parser/src/parser.rs
+++ b/compiler/cl-parser/src/parser.rs
@@ -884,6 +884,31 @@ impl Parse<'_> for Expr {
     }
 }
 
+impl Parse<'_> for Closure {
+    fn parse(p: &mut Parser<'_>) -> PResult<Self> {
+        let args = sep(
+            Pattern::parse,
+            TokenKind::Comma,
+            TokenKind::Bar,
+            Parsing::Closure,
+        );
+
+        let arg = match p.peek_kind(Parsing::Closure)? {
+            TokenKind::BarBar => {
+                p.consume_peeked();
+                Box::new(Pattern::Tuple(vec![]))
+            }
+            _ => Box::new(delim(
+                |p| args(p).map(Pattern::Tuple),
+                (TokenKind::Bar, TokenKind::Bar),
+                Parsing::Closure,
+            )(p)?),
+        };
+        let body = p.parse()?;
+        Ok(Closure { arg, body })
+    }
+}
+
 impl Parse<'_> for Quote {
     fn parse(p: &mut Parser<'_>) -> PResult<Self> {
         let quote = delim(
@@ -1104,6 +1129,12 @@ impl Parse<'_> for MatchArm {
 fn ret_body(p: &mut Parser, while_parsing: Parsing) -> PResult<Option<Box<Expr>>> {
     Ok(match p.peek_kind(while_parsing)? {
         TokenKind::Semi => None,
-        _ => Some(Expr::parse(p)?.into()),
+        _ => Some(p.parse()?),
     })
 }
+
+impl<'t, P: Parse<'t>> Parse<'t> for Box<P> {
+    fn parse(p: &mut Parser<'t>) -> PResult<Self> {
+        p.parse().map(Box::new)
+    }
+}
diff --git a/compiler/cl-parser/src/parser/prec.rs b/compiler/cl-parser/src/parser/prec.rs
index 6efc267..fed343e 100644
--- a/compiler/cl-parser/src/parser/prec.rs
+++ b/compiler/cl-parser/src/parser/prec.rs
@@ -17,6 +17,7 @@ pub fn expr(p: &mut Parser, power: u8) -> PResult<Expr> {
             literal_like!() => Literal::parse(p)?.into(),
             path_like!() => exprkind_pathlike(p)?,
             TokenKind::Amp | TokenKind::AmpAmp => AddrOf::parse(p)?.into(),
+            TokenKind::Bar | TokenKind::BarBar => Closure::parse(p)?.into(),
             TokenKind::Grave => Quote::parse(p)?.into(),
             TokenKind::LCurly => Block::parse(p)?.into(),
             TokenKind::LBrack => exprkind_arraylike(p)?,
diff --git a/compiler/cl-repl/examples/to_c.rs b/compiler/cl-repl/examples/to_c.rs
index 242c09c..26ab218 100644
--- a/compiler/cl-repl/examples/to_c.rs
+++ b/compiler/cl-repl/examples/to_c.rs
@@ -428,6 +428,7 @@ pub mod clangify {
     impl CLangify for ExprKind {
         fn print(&self, y: &mut CLangifier) {
             match self {
+                ExprKind::Closure(k) => todo!("Downgrade {k}"),
                 ExprKind::Quote(k) => k.print(y),
                 ExprKind::Let(k) => k.print(y),
                 ExprKind::Match(k) => k.print(y),
diff --git a/compiler/cl-repl/examples/yaml.rs b/compiler/cl-repl/examples/yaml.rs
index a66bb08..12e31b7 100644
--- a/compiler/cl-repl/examples/yaml.rs
+++ b/compiler/cl-repl/examples/yaml.rs
@@ -370,6 +370,7 @@ pub mod yamlify {
     impl Yamlify for ExprKind {
         fn yaml(&self, y: &mut Yamler) {
             match self {
+                ExprKind::Closure(k) => k.yaml(y),
                 ExprKind::Quote(k) => k.yaml(y),
                 ExprKind::Let(k) => k.yaml(y),
                 ExprKind::Match(k) => k.yaml(y),
@@ -401,6 +402,12 @@ pub mod yamlify {
             }
         }
     }
+    impl Yamlify for Closure {
+        fn yaml(&self, y: &mut Yamler) {
+            let Self { arg, body } = self;
+            y.key("Closure").pair("arg", arg).pair("body", body);
+        }
+    }
     impl Yamlify for Quote {
         fn yaml(&self, y: &mut Yamler) {
             y.key("Quote").value(self);
diff --git a/compiler/cl-typeck/src/stage/infer/inference.rs b/compiler/cl-typeck/src/stage/infer/inference.rs
index 0b32174..ab76adc 100644
--- a/compiler/cl-typeck/src/stage/infer/inference.rs
+++ b/compiler/cl-typeck/src/stage/infer/inference.rs
@@ -30,6 +30,7 @@ impl<'a> Inference<'a> for cl_ast::ExprKind {
     fn infer(&'a self, e: &mut InferenceEngine<'_, 'a>) -> Result<Handle, InferenceError> {
         match self {
             ExprKind::Empty => Ok(e.empty()),
+            ExprKind::Closure(_) => todo!("Infer the type of a closure"),
             ExprKind::Tuple(tuple) => tuple.infer(e),
             ExprKind::Structor(structor) => structor.infer(e),
             ExprKind::Array(array) => array.infer(e),