diff --git a/types/alias.go b/types/alias.go
index e68ca28..49f3346 100644
--- a/types/alias.go
+++ b/types/alias.go
@@ -12,7 +12,6 @@ func NewAlias(t Type, name string) *Alias {
 	if t == nil {
 		panic("alias to unknown must be used in built-in types only")
 	}
-
 	return &Alias{
 		base:   t,
 		actual: removeAlias(t),
@@ -41,9 +40,21 @@ func SkipAlias(t Type) Type {
 }
 
 func removeAlias(t0 Type) Type {
-	if a, ok := t0.(*Alias); ok {
-		return removeAlias(a.actual)
+	if t0 == nil {
+		return nil
+	}
+
+	t := t0
+	a, ok := t0.(*Alias)
+
+	for ok && a != nil {
+		t = a.actual
+		a, ok = t.(*Alias)
 	}
 
-	return t0
+	if t == nil {
+		panic("invalid alias")
+	}
+
+	return t
 }
diff --git a/types/array.go b/types/array.go
index e0dc544..db4099e 100644
--- a/types/array.go
+++ b/types/array.go
@@ -11,15 +11,13 @@ func NewArray(size int, t Type) *Array {
 	if size < 0 {
 		panic(fmt.Sprintf("invalid array size (%d)", size))
 	}
-
 	return &Array{size, t}
 }
 
 func (t *Array) Equals(other Type) bool {
-	if otherArray, ok := other.Underlying().(*Array); ok {
-		return (t.size == -1 || t.size == otherArray.size) && t.elem.Equals(otherArray.elem)
+	if t2 := AsArray(other); t2 != nil {
+		return t.size == t2.size && t.elem.Equals(t2.elem)
 	}
-
 	return false
 }
 
@@ -29,7 +27,6 @@ func (t *Array) String() string {
 	if t.size == -1 {
 		return "[_]" + t.elem.String()
 	}
-
 	return fmt.Sprintf("[%d]%s", t.size, t.elem)
 }
 
diff --git a/types/func.go b/types/func.go
index ad3df17..f0a3f8a 100644
--- a/types/func.go
+++ b/types/func.go
@@ -15,11 +15,9 @@ func NewFunc(result *Tuple, params *Tuple) *Func {
 	if result == nil {
 		result = Unit
 	}
-
 	if params == nil {
 		params = Unit
 	}
-
 	return &Func{
 		params: params,
 		result: result,
@@ -27,11 +25,9 @@ func NewFunc(result *Tuple, params *Tuple) *Func {
 }
 
 func (t *Func) Equals(other Type) bool {
-	if otherFunc, ok := other.(*Func); ok {
-		return t.result.Equals(otherFunc.result) &&
-			t.params.Equals(otherFunc.params)
+	if t2 := AsFunc(other); t2 != nil {
+		return t.result.Equals(t2.result) && t.params.Equals(t2.params)
 	}
-
 	return false
 }
 
diff --git a/types/primitive.go b/types/primitive.go
index da68adf..be21e08 100644
--- a/types/primitive.go
+++ b/types/primitive.go
@@ -6,9 +6,12 @@ type Primitive struct {
 	kind PrimitiveKind
 }
 
-func (p *Primitive) Equals(other Type) bool {
-	p2, _ := other.Underlying().(*Primitive)
-	return p.kind == KindAny || (p2 != nil && (p.kind == p2.kind || p.kind == SkipUntyped(p2).(*Primitive).kind))
+func (t *Primitive) Equals(other Type) bool {
+	if t2 := AsPrimitive(other); t2 != nil {
+		return t.kind == KindAny || (t2 != nil && (t.kind == t2.kind || t.kind == SkipUntyped(t2).(*Primitive).kind))
+	}
+
+	return false
 }
 
 func (p *Primitive) Underlying() Type { return p }
@@ -25,7 +28,6 @@ func AsPrimitive(t Type) *Primitive {
 	if primitive, _ := t.Underlying().(*Primitive); primitive != nil {
 		return primitive
 	}
-
 	return nil
 }
 
@@ -38,11 +40,10 @@ func SkipUntyped(t Type) Type {
 		case KindUntypedInt:
 			return I32
 
-			// case UntypedFloat, UntypedString:
-			// 	panic("not implemented")
+		// case UntypedFloat, UntypedString:
+		// 	panic("not implemented")
 		}
 	}
-
 	return t
 }
 
diff --git a/types/ref.go b/types/ref.go
index 20b629f..df9d965 100644
--- a/types/ref.go
+++ b/types/ref.go
@@ -8,15 +8,13 @@ func NewRef(t Type) *Ref {
 	if IsTypeDesc(t) {
 		panic("references to meta type is not allowed")
 	}
-
 	return &Ref{base: t}
 }
 
 func (t *Ref) Equals(other Type) bool {
-	if otherRef, ok := other.Underlying().(*Ref); ok {
-		return t.base.Equals(otherRef.base)
+	if t2 := AsRef(other); t2 != nil {
+		return t.base.Equals(t2.base)
 	}
-
 	return false
 }
 
diff --git a/types/struct.go b/types/struct.go
index aa3855e..614952c 100644
--- a/types/struct.go
+++ b/types/struct.go
@@ -16,18 +16,15 @@ func NewStruct(fields map[string]Type) *Struct {
 }
 
 func (t *Struct) Equals(other Type) bool {
-	if otherStruct, _ := other.Underlying().(*Struct); otherStruct != nil {
+	if t2 := AsStruct(other); t2 != nil {
 		for name, tField := range t.fields {
-			tOtherField, ok := otherStruct.fields[name]
-
-			if !ok || !tField.Equals(tOtherField) {
+			t2Field, ok := t2.fields[name]
+			if !ok || !tField.Equals(t2Field) {
 				return false
 			}
 		}
-
 		return true
 	}
-
 	return false
 }
 
diff --git a/types/tuple.go b/types/tuple.go
index b167569..a6f24c0 100644
--- a/types/tuple.go
+++ b/types/tuple.go
@@ -19,17 +19,16 @@ func NewTuple(types ...Type) *Tuple {
 //
 // NOTE: name of the elements are not required to be the same.
 func (t *Tuple) Equals(other Type) bool {
-	if otherTuple, ok := other.Underlying().(*Tuple); ok {
+	if t2 := AsTuple(other); t2 != nil {
 		return slices.EqualFunc(
 			t.types,
-			otherTuple.types,
+			t2.types,
 			func(a, b Type) bool { return a.Equals(b) },
 		)
 	} else if underlying := t.Underlying(); underlying != t {
 		// The tuple has 1 element and can be equals to the element type.
 		return underlying.Equals(other.Underlying())
 	}
-
 	return false
 }
 
diff --git a/types/typedesc.go b/types/typedesc.go
index e727b0d..f601a28 100644
--- a/types/typedesc.go
+++ b/types/typedesc.go
@@ -21,8 +21,10 @@ func NewTypeDesc(t Type) *TypeDesc {
 }
 
 func (t *TypeDesc) Equals(other Type) bool {
-	typedesc, _ := other.(*TypeDesc)
-	return typedesc != nil && t.base.Equals(typedesc.base)
+	if t2 := AsTypeDesc(other); t2 != nil {
+		return t.base.Equals(t2.base)
+	}
+	return false
 }
 
 func (t *TypeDesc) Underlying() Type { return t }