feat(checker): implement function subtyping

the logic for checking function subtypes is a WIP and has not been fully tested, there may be some errors and unhandled edge cases
Claude helped lay out and verify the overall steps

Co-authored-by: Claude <noreply@anthropic.com>

midas/checker/builtins.py

@@ -0,0 +1,4 @@
+BUILTIN_SUBTYPES: dict[str, set[str]] = {
+    "float": {"int"},
+    "int": {"bool"},
+}

midas/checker/checker.py

@@ -6,10 +6,19 @@ from typing import Optional
 import midas.ast.midas as m
 import midas.ast.python as p
 from midas.ast.location import Location
+from midas.checker.builtins import BUILTIN_SUBTYPES
 from midas.checker.diagnostic import Diagnostic, DiagnosticType
 from midas.checker.environment import Environment
 from midas.checker.operators import COMPARATOR_METHODS, OPERATOR_METHODS
-from midas.checker.types import Function, Type, UnitType, UnknownType
+from midas.checker.types import (
+    AliasType,
+    BaseType,
+    ComplexType,
+    Function,
+    Type,
+    UnitType,
+    UnknownType,
+)
 from midas.lexer.midas import MidasLexer
 from midas.lexer.token import Token
 from midas.parser.midas import MidasParser
@@ -48,6 +57,7 @@ class Checker(
         self.env: Environment = self.global_env
         self.locals: dict[p.Expr, int] = locals
         self.diagnostics: list[Diagnostic] = []
+        self.judgements: list[tuple[p.Expr, Type]] = []
 
     def diagnostic(self, type: DiagnosticType, location: Location, message: str):
         self.diagnostics.append(
@@ -89,7 +99,9 @@ class Checker(
         Returns:
             Type: the type of the given expression
         """
-        return expr.accept(self)
+        type: Type = expr.accept(self)
+        self.judgements.append((expr, type))
+        return type
 
     def process_block(self, block: list[p.Stmt], env: Environment) -> bool:
         """Evaluate a sequence of statements
@@ -165,6 +177,158 @@ class Checker(
         stmts: list[m.Stmt] = parser.parse()
         self.ctx.resolve(stmts)
 
+    def unfold_type(self, type: Type) -> Type:
+        match type:
+            case AliasType(type=ref_type):
+                return self.unfold_type(ref_type)
+            case _:
+                return type
+
+    def is_subtype(self, type1: Type, type2: Type) -> bool:
+        """Check whether `type1` is a subtype of `type2`
+
+        For more details on the rules checked here, see TAPL Chap. 15-16-17
+
+        Args:
+            type1 (Type): the potential subtype
+            type2 (Type): the potential supertype
+
+        Returns:
+            bool: whether `type1` is a subtype of `type2`
+        """
+
+        type1 = self.unfold_type(type1)
+        type2 = self.unfold_type(type2)
+
+        if type1 == type2:
+            return True
+
+        match (type1, type2):
+            case (BaseType(name=name1), BaseType(name=name2)):
+                return name1 in BUILTIN_SUBTYPES.get(name2, set())
+
+            case (ComplexType(properties=props1), ComplexType(properties=props2)):
+                for k, t in props2.items():
+                    if k not in props1:
+                        return False
+                    if self.is_subtype(props1[k], t):
+                        return False
+                return True
+
+            case (Function(returns=return1), Function(returns=return2)):
+                if not self.is_func_subtype(type1, type2):
+                    return False
+                if not self.is_subtype(return1, return2):
+                    return False
+                return True
+
+        return False
+
+    # TODO: verify the logic in here
+    def is_func_subtype(self, func1: Function, func2: Function) -> bool:
+        """Check whether a function is a subtype of another
+
+        Args:
+            func1 (Function): the potential function subtype
+            func2 (Function): the potential function supertype
+
+        Returns:
+            bool: whether `func1` is a subtype of `func2`
+        """
+        if not self.is_subtype(func1.returns, func2.returns):
+            return False
+
+        pos1: list[Function.Argument] = func1.pos_args
+        mixed1: list[Function.Argument] = func1.args
+        kw1: dict[str, Function.Argument] = {a.name: a for a in func1.kw_args}
+        pos2: list[Function.Argument] = func2.pos_args
+        mixed2: list[Function.Argument] = func2.args
+        kw2: dict[str, Function.Argument] = {a.name: a for a in func2.kw_args}
+
+        mixed_by_pos: dict[int, Function.Argument] = {arg.pos: arg for arg in mixed2}
+        mixed_by_name: dict[str, Function.Argument] = {arg.name: arg for arg in mixed2}
+
+        def is_arg_subtype(sub: Function.Argument, sup: Function.Argument) -> bool:
+            if not self.is_subtype(sub.type, sup.type):
+                return False
+            if not sup.required and sub.required:
+                return False
+            return True
+
+        for arg1 in pos1:
+            arg2: Function.Argument
+            if arg1.pos < len(pos2):
+                arg2 = pos2[arg1.pos]
+            elif arg1.pos in mixed_by_pos:
+                arg2 = mixed_by_pos[arg1.pos]
+            elif not arg1.required:
+                continue
+            else:
+                return False
+            if not is_arg_subtype(arg2, arg1):
+                return False
+
+        for name, arg1 in kw1.items():
+            arg2: Function.Argument
+            if name in kw2:
+                arg2 = kw2[name]
+            elif name in mixed_by_name:
+                arg2 = mixed_by_name[name]
+            elif not arg1.required:
+                continue
+            else:
+                return False
+            if not is_arg_subtype(arg2, arg1):
+                return False
+
+        for arg1 in mixed1:
+            pos_arg2: Optional[Function.Argument] = None
+            kw_arg2: Optional[Function.Argument] = None
+            if arg1.name in kw2:
+                kw_arg2 = kw2[arg1.name]
+            elif arg1.name in mixed_by_name:
+                kw_arg2 = mixed_by_name[arg1.name]
+            if arg1.pos < len(pos2):
+                pos_arg2 = pos2[arg1.pos]
+            elif arg1.pos in mixed_by_pos:
+                pos_arg2 = mixed_by_pos[arg1.pos]
+
+            # No match in func2 and arg is required
+            if pos_arg2 is None and kw_arg2 is None and arg1.required:
+                return False
+
+            # Matching keyword argument
+            if kw_arg2 is not None and not is_arg_subtype(kw_arg2, arg1):
+                return False
+
+            # Matching positional argument
+            if pos_arg2 is not None and not is_arg_subtype(pos_arg2, arg1):
+                return False
+
+        mixed_positions: set[int] = {a.pos for a in mixed1}
+        mixed_names: set[str] = {a.name for a in mixed1}
+        for arg2 in pos2:
+            if not arg2.required:
+                continue
+            if arg2.pos >= len(pos1) and arg2.pos not in mixed_positions:
+                return False
+
+        for name, arg2 in kw2.items():
+            if not arg2.required:
+                continue
+            if name not in kw1 and name not in mixed_names:
+                return False
+
+        for arg2 in mixed2:
+            if arg2.required:
+                continue
+            pos_match: bool = arg2.pos < len(pos1) or arg2.pos in mixed_positions
+            kw_match: bool = arg2.name in kw1 or arg2.name in mixed_names
+            if not pos_match or not kw_match:
+                return False
+
+        return True
+
     def visit_expression_stmt(self, stmt: p.ExpressionStmt) -> None:
         self.type_of(stmt.expr)
 
@@ -181,30 +345,37 @@ class Checker(
                 return arg.default.accept(self)
             return UnknownType()
 
+        pos: int = 0
         for arg in stmt.posonlyargs:
             pos_args.append(
                 Function.Argument(
+                    pos=pos,
                     name=arg.name,
                     type=eval_arg_type(arg),
                     required=arg.default is None,
                 )
             )
+            pos += 1
         for arg in stmt.args:
             args.append(
                 Function.Argument(
+                    pos=pos,
                     name=arg.name,
                     type=eval_arg_type(arg),
                     required=arg.default is None,
                 )
             )
+            pos += 1
         for arg in stmt.kwonlyargs:
             kw_args.append(
                 Function.Argument(
+                    pos=pos,  # not relevant
                     name=arg.name,
                     type=eval_arg_type(arg),
                     required=arg.default is None,
                 )
             )
+            pos += 1
 
         for arg in pos_args + args + kw_args:
             env.define(arg.name, arg.type)
@@ -263,7 +434,7 @@ class Checker(
         self.env.define(stmt.name, type)
 
     def visit_assign_stmt(self, stmt: p.AssignStmt) -> None:
-        value: Type = self.type_of(stmt.value)
+        value_type: Type = self.type_of(stmt.value)
         for target in stmt.targets:
             if not isinstance(target, p.VariableExpr):
                 self.logger.warning(f"Unsupported assignment to {target}")
@@ -273,13 +444,15 @@ class Checker(
             var_type: Optional[Type] = self.look_up_variable(name, target)
 
             if var_type is None:
-                self.env.define(name, value)
+                self.env.define(name, value_type)
             else:
-                # TODO: implement real comparison method
-                if var_type != value:
+                # S <: T
+                # Γ, x: T   v: S
+                # x = v
+                if not self.is_subtype(value_type, var_type):
                     self.error(
                         stmt.location,
-                        f"Cannot assign {value} to {name} of type {var_type}",
+                        f"Cannot assign {value_type} to {name} of type {var_type}",
                     )
 
     def visit_return_stmt(self, stmt: p.ReturnStmt) -> None:
@@ -354,7 +527,7 @@ class Checker(
         function: Function = callee
         mapped: list[MappedArgument] = self.map_call_arguments(function, expr)
         for arg in mapped:
-            if arg.type != arg.argument.type:
+            if not self.is_subtype(arg.type, arg.argument.type):
                 self.error(
                     arg.expr.location,
                     f"Wrong type for argument '{arg.argument.name}', expected {arg.argument.type}, got {arg.type}",
@@ -383,13 +556,17 @@ class Checker(
     def visit_logical_expr(self, expr: p.LogicalExpr) -> Type:
         left: Type = expr.left.accept(self)
         right: Type = expr.right.accept(self)
-        # TODO: union type
-        if left != right:
-            self.error(
-                expr.location,
-                f"Operands must be of the same type, left={left} != right={right}",
-            )
-        return left
+
+        if self.is_subtype(left, right):
+            return right
+        if self.is_subtype(right, left):
+            return left
+
+        self.error(
+            expr.location,
+            f"Incompatible operand types, {left=} and {right=}",
+        )
+        return UnknownType()
 
     def visit_set_expr(self, expr: p.SetExpr) -> Type: ...
 
@@ -407,13 +584,16 @@ class Checker(
 
         true_type: Type = expr.if_true.accept(self)
         false_type: Type = expr.if_false.accept(self)
-        if true_type != false_type:
-            self.error(
-                expr.location,
-                f"Type mismatch in ternary if branches: true={true_type} != false={false_type}",
-            )
-            return UnknownType()
-        return true_type
+        if self.is_subtype(true_type, false_type):
+            return false_type
+        if self.is_subtype(false_type, true_type):
+            return true_type
+
+        self.error(
+            expr.location,
+            f"Incompatible types in ternary if branches: true={true_type} and false={false_type}",
+        )
+        return UnknownType()
 
     def visit_base_type(self, node: p.BaseType) -> Type:
         return self.ctx.get_type(node.base)

midas/checker/types.py

@@ -34,6 +34,7 @@ class Function:
 
     @dataclass(frozen=True, kw_only=True)
     class Argument:
+        pos: int
         name: str
         type: Type
         required: bool

midas/cli/main.py

@@ -35,9 +35,15 @@ def midas():
 @midas.command()
 @click.option("-l", "--highlight", type=click.File("w"))
 @click.option("-t", "--types", type=click.File("r"), multiple=True)
+@click.option("-v", "--verbose", is_flag=True)
 @click.argument("file", type=click.File("r"))
-def compile(highlight: Optional[TextIO], file: TextIO, types: tuple[TextIO]):
-    logging.basicConfig(level=logging.DEBUG)
+def compile(
+    highlight: Optional[TextIO],
+    types: tuple[TextIO],
+    verbose: bool,
+    file: TextIO,
+):
+    logging.basicConfig(level=logging.DEBUG if verbose else logging.WARN)
     source: str = file.read()
     tree: ast.Module = ast.parse(source, filename=file.name)
     parser = PythonParser()
@@ -54,16 +60,17 @@ def compile(highlight: Optional[TextIO], file: TextIO, types: tuple[TextIO]):
     for diagnostic in diagnostics:
         print(diagnostic)
 
-    print(
-        json.dumps(
-            UniversalJSONDumper.dump(
-                checker.global_env,
-                [("Environment", "_children")],
-                lambda obj: isinstance(obj, get_args(Type)),
-            ),
-            indent=4,
+    if verbose:
+        print(
+            json.dumps(
+                UniversalJSONDumper.dump(
+                    checker.global_env,
+                    [("Environment", "_children")],
+                    lambda obj: isinstance(obj, get_args(Type)),
+                ),
+                indent=4,
+            )
         )
-    )
     if highlight is not None:
         highlighter = DiagnosticsHighlighter(source)
         highlighter.highlight(diagnostics)