feat(aiken-lang): anonymous functions

@MartinSchere noticed a weird error
where an unknown variable wasn't being reported
the type checker was incorrectly scoping
arguments for anonymous function definitions.
Luckily his compilation failed due to a FreeUnique
error during code gen which is good. But this may
have been the source of other mysterious FreeUnique
errors.

I also noticed that anonymous function allowed
arguments with the same name to be defined.

`fn(arg, arg)`

This now returns an error.

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## [next] - 2023-MM-DD
+
+### Fixed
+
+- **aiken-lang**: incorrect scoping for anonymous functions
+- **aiken-lang**: duplicate arguments were allowed in anonymous functions
+
 ## v1.0.0-alpha - 2023-04-13
 
 ### Added

crates/aiken-lang/src/tests/check.rs

@@ -154,6 +154,56 @@ fn multi_validator_warning() {
     ))
 }
 
+#[test]
+fn anonymous_function_scoping() {
+    let source_code = r#"
+        fn reduce(list, f, i) {
+          todo
+        }
+
+        pub fn foo() {
+          let sum =
+            reduce(
+              [1, 2, 3],
+              fn(acc: Int, n: Int) { acc + n },
+              0,
+            )
+
+          sum + acc
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::UnknownVariable { name, .. })) if name == "acc"
+    ))
+}
+
+#[test]
+fn anonymous_function_dupicate_args() {
+    let source_code = r#"
+        fn reduce(list, f, i) {
+          todo
+        }
+
+        pub fn foo() {
+          let sum =
+            reduce(
+              [1, 2, 3],
+              fn(acc: Int, acc: Int) { acc + acc },
+              0,
+            )
+
+          sum
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::DuplicateArgument { label, .. })) if label == "acc"
+    ))
+}
+
 #[test]
 fn if_scoping() {
     let source_code = r#"

crates/aiken-lang/src/tipo/expr.rs

@@ -1480,32 +1480,45 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
     ) -> Result<(Vec<TypedArg>, TypedExpr), Error> {
         self.assert_no_assignment(&body)?;
 
-        for arg in &args {
-            match &arg.arg_name {
-                ArgName::Named {
-                    name,
-                    is_validator_param,
-                    ..
-                } if !is_validator_param => {
-                    self.environment.insert_variable(
-                        name.to_string(),
-                        ValueConstructorVariant::LocalVariable {
-                            location: arg.location,
-                        },
-                        arg.tipo.clone(),
-                    );
+        let (body_rigid_names, body_infer) = self.in_new_scope(|body_typer| {
+            let mut argument_names = HashMap::with_capacity(args.len());
 
-                    self.environment.init_usage(
-                        name.to_string(),
-                        EntityKind::Variable,
-                        arg.location,
-                    );
-                }
-                ArgName::Named { .. } | ArgName::Discarded { .. } => (),
-            };
-        }
+            for arg in &args {
+                match &arg.arg_name {
+                    ArgName::Named {
+                        name,
+                        is_validator_param,
+                        location,
+                        ..
+                    } if !is_validator_param => {
+                        if let Some(duplicate_location) = argument_names.insert(name, location) {
+                            return Err(Error::DuplicateArgument {
+                                location: *location,
+                                duplicate_location: *duplicate_location,
+                                label: name.to_string(),
+                            });
+                        }
 
-        let (body_rigid_names, body_infer) = (self.hydrator.rigid_names(), self.infer(body));
+                        body_typer.environment.insert_variable(
+                            name.to_string(),
+                            ValueConstructorVariant::LocalVariable {
+                                location: arg.location,
+                            },
+                            arg.tipo.clone(),
+                        );
+
+                        body_typer.environment.init_usage(
+                            name.to_string(),
+                            EntityKind::Variable,
+                            arg.location,
+                        );
+                    }
+                    ArgName::Named { .. } | ArgName::Discarded { .. } => (),
+                };
+            }
+
+            Ok((body_typer.hydrator.rigid_names(), body_typer.infer(body)))
+        })?;
 
         let body = body_infer.map_err(|e| e.with_unify_error_rigid_names(&body_rigid_names))?;