diff --git a/toy-impl/test_top_level.py b/toy-impl/test_top_level.py
new file mode 100644
index 0000000..c5b72f5
--- /dev/null
+++ b/toy-impl/test_top_level.py
@@ -0,0 +1,31 @@
+import ast
+from type_def import *
+from top_level import *
+
+test = """
+class A:
+    a: int
+    def foo(a: B) -> int:
+        pass
+
+class B(A):
+    a: str
+    def bar(a: list[list[virtual[A]]]) -> A:
+        pass
+"""
+
+variables = {'X': TypeVariable('X', []), 'Y': TypeVariable('Y', [])}
+types = {'int': PrimitiveType('int'), 'str': PrimitiveType('str')}
+ctx = Context(variables, types)
+
+ctx, functions, _ = parse_top_level(ctx, ast.parse(test))
+
+for name, t in ctx.types.items():
+    if isinstance(t, ClassType):
+        print(f"class {t.name}")
+        for name, ty in t.fields.items():
+            print(f"    {name}: {ty}")
+        for name, (args, result, _) in t.methods.items():
+            print(f"    {name}: ({', '.join([str(v) for v in args])}) -> {result}")
+
+
diff --git a/toy-impl/top_level.py b/toy-impl/top_level.py
new file mode 100644
index 0000000..0f92c77
--- /dev/null
+++ b/toy-impl/top_level.py
@@ -0,0 +1,141 @@
+import ast
+from type_def import *
+
+class CustomError(Exception):
+    def __init__(self, msg):
+        self.msg = msg
+
+class Context:
+    variables: dict[str, TypeVariable]
+    types: dict[Type]
+
+    def __init__(self, variables, types):
+        self.variables = variables
+        self.types = types
+
+
+def parse_type(ctx: Context, ty):
+    if ty is None:
+        return None
+    elif isinstance(ty, ast.Name):
+        if ty.id in ctx.types:
+            return ctx.types[ty.id], set()
+        elif ty.id in ctx.variables:
+            return ctx.variables[ty.id], {ty.id}
+        else:
+            raise CustomError(f"Unbounded Type {ty.id}")
+    elif isinstance(ty, ast.Subscript):
+        if isinstance(ty.value, ast.Name):
+            generic = ty.value.id
+        else:
+            raise CustomError(f"Unknown Generic Type {ty.value}")
+        if not isinstance(ty.slice, ast.Name) and not isinstance(ty.slice, ast.Tuple) \
+                and not isinstance(ty.slice, ast.Subscript):
+            raise CustomError(f"Generic Type of the form {ty.slice} is not supported")
+        if generic == 'tuple':
+            if not isinstance(ty.slice, ast.Tuple):
+                raise CustomError(f"Generic Type of the form {ty} is not supported")
+            param = []
+            var = set()
+            for t in ty.slice.elts:
+                p, v = parse_type(ctx, t)
+                param.append(p)
+                var |= v
+            return TupleType(param), var
+        elif generic == 'list':
+            param, var = parse_type(ctx, ty.slice)
+            return ListType(param), var
+        elif generic == 'virtual':
+            param, var = parse_type(ctx, ty.slice)
+            if not isinstance(param, ClassType):
+                raise CustomError(f"Parameter of virtual must be a class instead of {param}")
+            return VirtualClassType(param), var
+        else:
+            raise CustomError(f"Unknown Generic Type {ty.value}")
+
+
+def parse_function(ctx: Context, base, fn: ast.FunctionDef):
+    args = []
+    var = set()
+    for arg in fn.args.args:
+        name = arg.arg
+        ty, v = parse_type(ctx, arg.annotation)
+        var |= v
+        if name == 'self' and ty is None and base is not None:
+            ty = base
+        args.append(ty)
+    result, v = parse_type(ctx, fn.returns)
+    if len(v - var) > 0:
+        raise CustomError(f"Unbounded variable in return type of {fn.name}")
+    return args, result, var
+
+
+def parse_class(ctx, c: ast.ClassDef):
+    node = ctx.types[c.name]
+    functions = []
+
+    for base in c.bases:
+        if not isinstance(base, ast.Name):
+            raise CustomError(f"Base class of the form {base} is not supported")
+        name = base.id
+        if name not in ctx.types:
+            raise CustomError(f"Unbounded base class name {base}")
+        if not isinstance(ctx.types[name], ClassType):
+            raise CustomError(f"Base class must be a class instead of {base}")
+        node.parents.append(ctx.types[name])
+
+    for stmt in c.body:
+        if isinstance(stmt, ast.AnnAssign):
+            if not isinstance(stmt.target, ast.Name):
+                raise CustomError(f"Assignment of the form {stmt.target} is not supported")
+            field = stmt.target.id
+            if field in node.fields:
+                raise CustomError(f"Duplicated fields {field} in {c.name}")
+            ty, var = parse_type(ctx, stmt.annotation)
+            if len(var) > 0:
+                raise CustomError(f"Type variable is not allowed in class fields")
+            if ty == None:
+                raise CustomError(f"{field} of {c.name} is not annotated")
+            node.fields[field] = ty
+        elif isinstance(stmt, ast.FunctionDef):
+            name = stmt.name
+            if name in node.methods:
+                raise CustomError(f"Duplicated method {name} in {c.name}")
+            args, result, var = parse_function(ctx, node, stmt)
+            node.methods[name] = (args, result, var)
+            functions.append((c.name, name, stmt))
+        else:
+            raise CustomError(f"{stmt} is not supported")
+    return functions
+
+
+def parse_top_level(ctx: Context, module: ast.Module):
+    to_be_processed = []
+    # first pass, obtain all type names
+    for element in module.body:
+        if isinstance(element, ast.ClassDef):
+            name = element.name
+            if name in ctx.types or name in ctx.variables:
+                raise CustomError(f"Duplicated class name: {name}")
+            ctx.types[name] = ClassType(name)
+            to_be_processed.append(element)
+        elif isinstance(element, ast.FunctionDef):
+            to_be_processed.append(element)
+
+    # second pass, obtain all function types
+    functions = {}
+    function_stmts = []
+    for element in to_be_processed:
+        if isinstance(element, ast.ClassDef):
+            function_stmts += parse_class(ctx, element)
+        elif isinstance(element, ast.FunctionDef):
+            name = element.name
+            if name in functions:
+                raise CustomError(f"Duplicated function name {name}")
+            args, result, var = parse_function(ctx, None, element)
+            functions[name] = (args, result, var)
+            function_stmts += element
+
+    return ctx, functions, function_stmts
+
+
diff --git a/toy-impl/type_def.py b/toy-impl/type_def.py
new file mode 100644
index 0000000..21c981e
--- /dev/null
+++ b/toy-impl/type_def.py
@@ -0,0 +1,74 @@
+class Type:
+    pass
+
+
+class BotType:
+    pass
+
+
+class PrimitiveType(Type):
+    name: str
+
+    def __init__(self, name: str):
+        self.name = name
+
+    def __str__(self):
+        return self.name
+
+
+class TypeVariable(Type):
+    name: str
+    constraints: list[Type]
+
+    def __init__(self, name: str, constraints: list[Type]):
+        self.name = name
+        self.constraints = constraints
+
+    def __str__(self):
+        return self.name
+
+
+class ClassType(Type):
+    name: str
+    parents: list['ClassType']
+    methods: dict[str, tuple[list[Type], Type, set[str]]]
+    fields: dict[str, Type]
+
+    def __init__(self, name: str):
+        self.name    = name
+        self.parents = []
+        self.methods = {}
+        self.fields  = {}
+
+    def __str__(self):
+        return self.name
+
+
+class VirtualClassType(Type):
+    base: ClassType
+
+    def __init__(self, base: ClassType):
+        self.base = base
+
+    def __str__(self):
+        return f"virtual[{self.base}]"
+
+class ListType(Type):
+    elements: Type
+
+    def __init__(self, elements: Type):
+        self.elements = elements
+
+    def __str__(self):
+        return f"list[{self.elements}]"
+
+class TupleType(Type):
+    elements: list[Type]
+
+    def __init__(self, elements: Type):
+        self.elements = elements
+
+    def __str__(self):
+        return f"tuple[{', '.join([str(v) for v in self.elements])}]"
+
+