diff --git a/CHANGELOG.md b/CHANGELOG.md
index 84a1573e..7e53a03d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ All notable changes are documented in this file.
 
 
 ## 0.0.9 - unreleased
+### Breaking Changes
+- Syntax: Require argument names in function type declarations ([GH-308](https://github.com/bait-lang/bait/pull/308))
+
 ### New Language Features
 - Array slicing using range indexing: `arr[low..high]`
 
diff --git a/examples/function_types.bt b/examples/function_types.bt
index dbedd1f1..bcfdf7e8 100644
--- a/examples/function_types.bt
+++ b/examples/function_types.bt
@@ -1,4 +1,4 @@
-type CheckInt := fun (i32) bool
+type CheckInt := fun (i i32) bool
 
 // You can use the alias like any other type, for example as parameter
 fun check_nums(nums []i32, c CheckInt) {
diff --git a/lib/bait/parser/type.bt b/lib/bait/parser/type.bt
index 9cce46f9..9c9fdafc 100644
--- a/lib/bait/parser/type.bt
+++ b/lib/bait/parser/type.bt
@@ -110,11 +110,7 @@ fun (mut p Parser) parse_fun_type() !ast.Type {
 	p.check(.lpar)!
 	mut param_types := []ast.Type
 	for p.tok != .rpar{
-		if p.peek() != .rpar {
-			_ = p.check_name()! // Require argument name but ignore it for now
-		} else {
-			p.warn("expected argument name")
-		}
+		_ = p.check_name()! // Require argument name but ignore it for now
 		typ := p.parse_type()!
 		param_types.push(typ)
 		if p.tok != .rpar{
diff --git a/lib/builtin/array.js.bt b/lib/builtin/array.js.bt
index 4f82bfd1..5167c571 100644
--- a/lib/builtin/array.js.bt
+++ b/lib/builtin/array.js.bt
@@ -104,7 +104,7 @@ pub fun (a Array) reverse_in_place() {
 	a.data.reverse()
 }
 
-pub fun (a Array) filter(fn fun (any) bool) Array {
+pub fun (a Array) filter(fn fun (cond any) bool) Array {
 	return from_js_arr(a.data.filter(fn as #JS.Any))
 }
 
diff --git a/lib/cli/cli.bt b/lib/cli/cli.bt
index 98d36c13..d01ca74f 100644
--- a/lib/cli/cli.bt
+++ b/lib/cli/cli.bt
@@ -4,7 +4,7 @@ package cli
 
 import strings
 
-type Callback := fun (Command)
+type Callback := fun (cmd Command)
 
 pub struct Command {
 	name string
diff --git a/tests/method/struct_field_fun_test.bt b/tests/method/struct_field_fun_test.bt
index 6c0fdd6b..898f0e0c 100644
--- a/tests/method/struct_field_fun_test.bt
+++ b/tests/method/struct_field_fun_test.bt
@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: MIT
 
 struct Helpers{
-	num_to_str fun (i32) string
+	num_to_str fun (i i32) string
 }
 
 fun test_struct_field_method() {
diff --git a/tests/out/error/redef/redef.in.bt b/tests/out/error/redef/redef.in.bt
index 2b272f2e..82cf014f 100644
--- a/tests/out/error/redef/redef.in.bt
+++ b/tests/out/error/redef/redef.in.bt
@@ -17,7 +17,7 @@ fun foo[X](x X) {}
 
 static MyCapName := 1
 struct MyCapName{}
-type MyCapName := fun (string, f64) bool
+type MyCapName := fun (x string, y f64) bool
 type MyCapName := []u8
 type MyCapName := i8 | i16
 enum MyCapName {}
diff --git a/tests/type/fun_types_test.bt b/tests/type/fun_types_test.bt
index b020475d..025274be 100644
--- a/tests/type/fun_types_test.bt
+++ b/tests/type/fun_types_test.bt
@@ -3,10 +3,10 @@
 
 // This struct has to compile
 struct Foo {
-	fun_field fun (string) i32
+	fun_field fun (s string) i32
 }
 
-fun takes_callback(s string, cb fun (string) i32) i32 {
+fun takes_callback(s string, cb fun (s string) i32) i32 {
 	return cb(s)
 }