feat: add Expr::as_lambda (#6048)

# Description

## Problem

Part of #5668

## Summary

## Additional Contex


## Documentation

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/hir/comptime/interpreter/builtin.rs

@@ -83,6 +83,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "expr_as_if" => expr_as_if(interner, arguments, return_type, location),
             "expr_as_index" => expr_as_index(interner, arguments, return_type, location),
             "expr_as_integer" => expr_as_integer(interner, arguments, return_type, location),
+            "expr_as_lambda" => expr_as_lambda(interner, arguments, return_type, location),
             "expr_as_let" => expr_as_let(interner, arguments, return_type, location),
             "expr_as_member_access" => {
                 expr_as_member_access(interner, arguments, return_type, location)
@@ -1612,6 +1613,68 @@ fn expr_as_integer(
     })
 }
 
+// fn as_lambda(self) -> Option<([(Expr, Option<UnresolvedType>)], Option<UnresolvedType>, Expr)>
+fn expr_as_lambda(
+    interner: &NodeInterner,
+    arguments: Vec<(Value, Location)>,
+    return_type: Type,
+    location: Location,
+) -> IResult<Value> {
+    expr_as(interner, arguments, return_type.clone(), location, |expr| {
+        if let ExprValue::Expression(ExpressionKind::Lambda(lambda)) = expr {
+            // ([(Expr, Option<UnresolvedType>)], Option<UnresolvedType>, Expr)
+            let option_type = extract_option_generic_type(return_type);
+            let Type::Tuple(mut tuple_types) = option_type else {
+                panic!("Expected the return type option generic arg to be a tuple");
+            };
+            assert_eq!(tuple_types.len(), 3);
+
+            // Expr
+            tuple_types.pop().unwrap();
+
+            // Option<UnresolvedType>
+            let option_unresolved_type = tuple_types.pop().unwrap();
+
+            let parameters = lambda
+                .parameters
+                .into_iter()
+                .map(|(pattern, typ)| {
+                    let pattern = Value::pattern(pattern);
+                    let typ = if let UnresolvedTypeData::Unspecified = typ.typ {
+                        None
+                    } else {
+                        Some(Value::UnresolvedType(typ.typ))
+                    };
+                    let typ = option(option_unresolved_type.clone(), typ).unwrap();
+                    Value::Tuple(vec![pattern, typ])
+                })
+                .collect();
+            let parameters = Value::Slice(
+                parameters,
+                Type::Slice(Box::new(Type::Tuple(vec![
+                    Type::Quoted(QuotedType::Expr),
+                    Type::Quoted(QuotedType::UnresolvedType),
+                ]))),
+            );
+
+            let return_type = lambda.return_type.typ;
+            let return_type = if let UnresolvedTypeData::Unspecified = return_type {
+                None
+            } else {
+                Some(return_type)
+            };
+            let return_type = return_type.map(Value::UnresolvedType);
+            let return_type = option(option_unresolved_type, return_type).ok()?;
+
+            let body = Value::expression(lambda.body.kind);
+
+            Some(Value::Tuple(vec![parameters, return_type, body]))
+        } else {
+            None
+        }
+    })
+}
+
 // fn as_let(self) -> Option<(Expr, Option<UnresolvedType>, Expr)>
 fn expr_as_let(
     interner: &NodeInterner,

docs/docs/noir/standard_library/meta/expr.md

@@ -116,6 +116,12 @@ array and the index.
 If this expression is an integer literal, return the integer as a field
 as well as whether the integer is negative (true) or not (false).
 
+### as_lambda
+
+#include_code as_lambda noir_stdlib/src/meta/expr.nr rust
+
+If this expression is a lambda, returns the parameters, return type and body.
+
 ### as_let
 
 #include_code as_let noir_stdlib/src/meta/expr.nr rust

noir_stdlib/src/meta/expr.nr

@@ -115,6 +115,12 @@ impl Expr {
     comptime fn as_integer(self) -> Option<(Field, bool)> {}
     // docs:end:as_integer
 
+    /// If this expression is a lambda, returns the parameters, return type and body.
+    #[builtin(expr_as_lambda)]
+    // docs:start:as_lambda
+    comptime fn as_lambda(self) -> Option<([(Expr, Option<UnresolvedType>)], Option<UnresolvedType>, Expr)> {}
+    // docs:end:as_lambda
+
     /// If this expression is a let statement, returns the let pattern as an `Expr`,
     /// the optional type annotation, and the assigned expression.
     #[builtin(expr_as_let)]
@@ -234,6 +240,7 @@ impl Expr {
         let result = result.or_else(|| modify_index(self, f));
         let result = result.or_else(|| modify_for(self, f));
         let result = result.or_else(|| modify_for_range(self, f));
+        let result = result.or_else(|| modify_lambda(self, f));
         let result = result.or_else(|| modify_let(self, f));
         let result = result.or_else(|| modify_function_call(self, f));
         let result = result.or_else(|| modify_member_access(self, f));
@@ -427,6 +434,17 @@ comptime fn modify_for_range<Env>(expr: Expr, f: fn[Env](Expr) -> Option<Expr>)
     )
 }
 
+comptime fn modify_lambda<Env>(expr: Expr, f: fn[Env](Expr) -> Option<Expr>) -> Option<Expr> {
+    expr.as_lambda().map(
+        |expr: ([(Expr, Option<UnresolvedType>)], Option<UnresolvedType>, Expr)| {
+            let (params, return_type, body) = expr;
+            let params = params.map(|param: (Expr, Option<UnresolvedType>)| (param.0.modify(f), param.1));
+            let body = body.modify(f);
+            new_lambda(params, return_type, body)
+    }
+    )
+}
+
 comptime fn modify_let<Env>(expr: Expr, f: fn[Env](Expr) -> Option<Expr>) -> Option<Expr> {
     expr.as_let().map(
         |expr: (Expr, Option<UnresolvedType>, Expr)| {
@@ -599,6 +617,31 @@ comptime fn new_index(object: Expr, index: Expr) -> Expr {
     quote { $object[$index] }.as_expr().unwrap()
 }
 
+comptime fn new_lambda(
+    params: [(Expr, Option<UnresolvedType>)],
+    return_type: Option<UnresolvedType>,
+    body: Expr
+) -> Expr {
+    let params = params.map(
+        |param: (Expr, Option<UnresolvedType>)| {
+        let (name, typ) = param;
+        if typ.is_some() {
+            let typ = typ.unwrap();
+            quote { $name: $typ }
+        } else {
+            quote { $name }
+        }
+    }
+    ).join(quote { , });
+
+    if return_type.is_some() {
+        let return_type = return_type.unwrap();
+        quote { |$params| -> $return_type { $body } }.as_expr().unwrap()
+    } else {
+        quote { |$params| { $body } }.as_expr().unwrap()
+    }
+}
+
 comptime fn new_let(pattern: Expr, typ: Option<UnresolvedType>, expr: Expr) -> Expr {
     if typ.is_some() {
         let typ = typ.unwrap();

test_programs/noir_test_success/comptime_expr/src/main.nr

@@ -635,6 +635,48 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_expr_as_lambda() {
+        comptime
+        {
+            let expr = quote { |x: Field| -> Field { 1 } }.as_expr().unwrap();
+            let (params, return_type, body) = expr.as_lambda().unwrap();
+            assert_eq(params.len(), 1);
+            assert(params[0].1.unwrap().is_field());
+            assert(return_type.unwrap().is_field());
+            assert_eq(body.as_block().unwrap()[0].as_integer().unwrap(), (1, false));
+
+            let expr = quote { |x| { 1 } }.as_expr().unwrap();
+            let (params, return_type, body) = expr.as_lambda().unwrap();
+            assert_eq(params.len(), 1);
+            assert(params[0].1.is_none());
+            assert(return_type.is_none());
+            assert_eq(body.as_block().unwrap()[0].as_integer().unwrap(), (1, false));
+        }
+    }
+
+    #[test]
+    fn test_expr_modify_lambda() {
+        comptime
+        {
+            let expr = quote { |x: Field| -> Field { 1 } }.as_expr().unwrap();
+            let expr = expr.modify(times_two);
+            let (params, return_type, body) = expr.as_lambda().unwrap();
+            assert_eq(params.len(), 1);
+            assert(params[0].1.unwrap().is_field());
+            assert(return_type.unwrap().is_field());
+            assert_eq(body.as_block().unwrap()[0].as_block().unwrap()[0].as_integer().unwrap(), (2, false));
+
+            let expr = quote { |x| { 1 } }.as_expr().unwrap();
+            let expr = expr.modify(times_two);
+            let (params, return_type, body) = expr.as_lambda().unwrap();
+            assert_eq(params.len(), 1);
+            assert(params[0].1.is_none());
+            assert(return_type.is_none());
+            assert_eq(body.as_block().unwrap()[0].as_block().unwrap()[0].as_integer().unwrap(), (2, false));
+        }
+    }
+
     #[test]
     fn test_expr_as_let() {
         comptime