type guard

toy-impl/README.md

@@ -19,9 +19,7 @@ for simplicity reasons:
 * `with`, `try except`, etc.
 * const indexing with tuple.
 * method override check modulo variable renaming.
-
+* more complicated type guard
-These features are currently not implemented, and would be added in due course:
-* Type guards.
 
 
 ## Type Check Implementation

toy-impl/examples/a.py

@@ -1,14 +1,14 @@
-I = TypeVar('I', int32, int64)
+I = TypeVar('I', int32, Vec)
 
 class Vec:
     v: list[int32]
     def __init__(self, v: list[int32]):
         self.v = v
 
-    def __add__(self, other: int32) -> Vec:
+    def __add__(self, other: I) -> Vec:
+        if other is int32:
             return Vec([v + other for v in self.v])
+        else:
+            return Vec([self.v[i] + other.v[i] for i in range(len(self.v))])
 
 
-def addI(a: I, b: I) -> I:
-    return a + b
-

toy-impl/inference.py

@@ -43,7 +43,7 @@ def find_subst(ctx: dict[str, Type],
             return sub
 
     if isinstance(a, TypeVariable):
-        if a == b:
+        if len(a.constraints) == 1 and a.constraints[0] == b:
             return sub
         else:
             return f"{a} can take values other than {b}"

toy-impl/parse_expr.py

@@ -107,6 +107,19 @@ def parse_attribute(ctx: Context,
                     sym_table: dict[str, Type],
                     node):
     obj = parse_expr(ctx, sym_table, node.value)
+
+    if isinstance(obj, TypeVariable) and len(obj.constraints) > 0:
+        if node.attr not in obj.constraints[0].fields:
+            raise CustomError(f'unknown field {node.attr} in {obj}', node)
+        ty = obj.constraints[0].fields[node.attr]
+        for v in obj.constraints[1:]:
+            if node.attr not in v.fields:
+                raise CustomError(f'unknown field {node.attr} in {obj}', node)
+            if v.fields[node.attr] != ty:
+                raise CustomError(
+                    f'unknown field {node.attr} in {obj} (type mismatch)', node)
+        return ty
+
     if node.attr in obj.fields:
         return obj.fields[node.attr]
     raise CustomError(f'unknown field {node.attr} in {obj}', node)

toy-impl/parse_stmt.py

@@ -3,6 +3,7 @@ import copy
 from helper import *
 from type_def import *
 from inference import *
+from top_level import parse_type
 from parse_expr import parse_expr, parse_simple_binding
 
 def parse_stmts(ctx: Context,
@@ -113,6 +114,36 @@ def parse_if_stmt(ctx: Context,
                   used_sym_table: dict[str, Type],
                   return_ty: Type,
                   node):
+    if isinstance(node.test, ast.Compare) and \
+            len(node.test.ops) == 1 and \
+            (isinstance(node.test.ops[0], ast.Is) or\
+             isinstance(node.test.ops[0], ast.IsNot)):
+        if not isinstance(node.test.left, ast.Name):
+            raise CustomError(
+                'type guard only support testing variables',
+                node.test)
+        t = parse_expr(ctx, sym_table, node.test.left)
+        if not isinstance(t, TypeVariable) or len(t.constraints) < 2:
+            raise CustomError(
+                'type guard only support basic type variables with constraints',
+                node.test)
+        t1, _ = parse_type(ctx, node.test.comparators[0])
+        if t1 not in t.constraints:
+            raise CustomError(
+                f'{t1} is not in constraints of {t}',
+                node.test)
+        t2 = [v for v in t.constraints if v != t1]
+        try:
+            t.constraints = [t1]
+            a, b, r = parse_stmts(ctx, sym_table, used_sym_table, return_ty, node.body)
+            used_sym_table |= b
+            t.constraints = t2
+            a1, b1, r1 = parse_stmts(ctx, sym_table, used_sym_table, return_ty, node.orelse)
+            defined = {k: a[k] for k in a if k in a1}
+            return defined, b | b1, r and r1
+        finally:
+            t.constraints = t
+    else:
         test = parse_expr(ctx, sym_table, node.test)
         if test != ctx.types['bool']:
             raise CustomError(f'condition must be bool instead of {test}', node.test)
@@ -169,6 +200,10 @@ def parse_return_stmt(ctx: Context,
         return {}, {}, True
     ty = parse_expr(ctx, sym_table, node.value)
     if ty != return_ty:
+        if isinstance(return_ty, TypeVariable):
+            if len(return_ty.constraints) == 1 and \
+                    return_ty.constraints[0] == ty:
+                return {}, {}, True
         raise CustomError(f'expected returning {return_ty} but got {ty}', node)
     return {}, {}, True
 

toy-impl/top_level.py

@@ -121,7 +121,7 @@ def parse_type_var(ctx: Context, node):
                 not isinstance(node.value.args[0].value, str):
             raise CustomError('call to TypeVar must at least have a name',
                               node.value)
-        name = node.value.args[0]
+        name = node.targets[0].id
         if name in ctx.variables:
             raise CustomError('redefining type variable is not allowed', node)
         constraints = []
@@ -137,7 +137,7 @@ def parse_type_var(ctx: Context, node):
             if value not in ctx.types:
                 raise CustomError(f'unbounded type {value}', node)
             constraints.append(ctx.types[value])
-        ctx.variables[node.targets[0].id] = TypeVariable(name, constraints)
+        ctx.variables[name] = TypeVariable(node.value.args[0].value, constraints)
 
 
 def parse_top_level(ctx: Context, module: ast.Module):