diff --git a/tests/serializer/midas.py b/tests/serializer/midas.py
index 0991334..6da0dec 100644
--- a/tests/serializer/midas.py
+++ b/tests/serializer/midas.py
@@ -2,56 +2,61 @@ from typing import Optional, Sequence
 
 from midas.ast.midas import (
     BinaryExpr,
-    ComplexTypeStmt,
+    ComplexType,
+    ConstraintType,
     Expr,
     ExtendStmt,
+    GenericType,
     GetExpr,
     GroupingExpr,
     LiteralExpr,
     LogicalExpr,
+    NamedType,
     OpStmt,
     PredicateStmt,
     PropertyStmt,
-    SimpleTypeExpr,
-    SimpleTypeStmt,
     Stmt,
-    TemplateExpr,
-    TypeExpr,
+    Type,
+    TypeStmt,
     UnaryExpr,
+    UnionType,
     VariableExpr,
     WildcardExpr,
 )
 
 
-class MidasAstJsonSerializer(Stmt.Visitor[dict], Expr.Visitor[dict]):
+class MidasAstJsonSerializer(
+    Stmt.Visitor[dict], Expr.Visitor[dict], Type.Visitor[dict]
+):
     """An AST serializer which produces a JSON-compatible structure"""
 
     def serialize(self, stmts: list[Stmt]) -> list[dict]:
         return [stmt.accept(self) for stmt in stmts]
 
-    def _serialize_optional(self, element: Optional[Stmt | Expr]) -> Optional[dict]:
+    def _serialize_optional(
+        self, element: Optional[Stmt | Expr | Type]
+    ) -> Optional[dict]:
         if element is None:
             return None
         return element.accept(self)
 
-    def _serialize_list(self, elements: Sequence[Stmt | Expr]) -> list[dict]:
+    def _serialize_list(self, elements: Sequence[Stmt | Expr | Type]) -> list[dict]:
         return [element.accept(self) for element in elements]
 
-    def visit_simple_type_stmt(self, stmt: SimpleTypeStmt) -> dict:
+    def visit_type_stmt(self, stmt: TypeStmt) -> dict:
         return {
-            "_type": "SimpleTypeStmt",
+            "_type": "TypeStmt",
             "name": stmt.name.lexeme,
-            "template": self._serialize_optional(stmt.template),
-            "base": stmt.base.accept(self),
-            "constraint": self._serialize_optional(stmt.constraint),
+            "params": [
+                self._serialize_type_stmt_template_param(param) for param in stmt.params
+            ],
+            "type": stmt.type.accept(self),
         }
 
-    def visit_complex_type_stmt(self, stmt: ComplexTypeStmt) -> dict:
+    def _serialize_type_stmt_template_param(self, param: TypeStmt.Param) -> dict:
         return {
-            "_type": "ComplexTypeStmt",
-            "name": stmt.name.lexeme,
-            "template": self._serialize_optional(stmt.template),
-            "properties": self._serialize_list(stmt.properties),
+            "name": param.name.lexeme,
+            "bound": self._serialize_optional(param.bound),
         }
 
     def visit_property_stmt(self, stmt: PropertyStmt) -> dict:
@@ -59,7 +64,6 @@ class MidasAstJsonSerializer(Stmt.Visitor[dict], Expr.Visitor[dict]):
             "_type": "PropertyStmt",
             "name": stmt.name.lexeme,
             "type": stmt.type.accept(self),
-            "constraint": self._serialize_optional(stmt.constraint),
         }
 
     def visit_extend_stmt(self, stmt: ExtendStmt) -> dict:
@@ -86,13 +90,6 @@ class MidasAstJsonSerializer(Stmt.Visitor[dict], Expr.Visitor[dict]):
             "condition": stmt.condition.accept(self),
         }
 
-    def visit_simple_type_expr(self, expr: SimpleTypeExpr) -> dict:
-        return {
-            "_type": "SimpleTypeExpr",
-            "name": expr.name.lexeme,
-            "optional": expr.optional,
-        }
-
     def visit_logical_expr(self, expr: LogicalExpr) -> dict:
         return {
             "_type": "LogicalExpr",
@@ -144,16 +141,34 @@ class MidasAstJsonSerializer(Stmt.Visitor[dict], Expr.Visitor[dict]):
     def visit_wildcard_expr(self, expr: WildcardExpr) -> dict:
         return {"_type": "WildcardExpr"}
 
-    def visit_template_expr(self, expr: TemplateExpr) -> dict:
+    def visit_named_type(self, type: NamedType) -> dict:
         return {
-            "_type": "TemplateExpr",
-            "type": expr.type.accept(self),
+            "_type": "NamedType",
+            "name": type.name.lexeme,
         }
 
-    def visit_type_expr(self, expr: TypeExpr) -> dict:
+    def visit_generic_type(self, type: GenericType) -> dict:
         return {
-            "_type": "TypeExpr",
-            "name": expr.name.lexeme,
-            "template": self._serialize_optional(expr.template),
-            "optional": expr.optional,
+            "_type": "GenericType",
+            "type": type.type.accept(self),
+            "params": self._serialize_list(type.params),
+        }
+
+    def visit_constraint_type(self, type: ConstraintType) -> dict:
+        return {
+            "_type": "ConstraintType",
+            "type": type.type.accept(self),
+            "constraint": type.constraint.accept(self),
+        }
+
+    def visit_union_type(self, type: UnionType) -> dict:
+        return {
+            "_type": "UnionType",
+            "types": self._serialize_list(type.types),
+        }
+
+    def visit_complex_type(self, type: ComplexType) -> dict:
+        return {
+            "_type": "ComplexType",
+            "properties": self._serialize_list(type.properties),
         }