From 653612ee87c5679105fd41f425ed247d7f1451f2 Mon Sep 17 00:00:00 2001
From: LordBaryhobal <lordbaryhobal@gmail.com>
Date: Thu, 9 Jul 2026 00:02:25 +0200
Subject: [PATCH] feat(checker): add variance manager to improve recursive
 types

---
 midas/checker/midas.py    |  8 ++--
 midas/checker/variance.py | 79 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 78 insertions(+), 9 deletions(-)

diff --git a/midas/checker/midas.py b/midas/checker/midas.py
index 02eb547..cfa881d 100644
--- a/midas/checker/midas.py
+++ b/midas/checker/midas.py
@@ -24,7 +24,7 @@ from midas.checker.types import (
     TypeVar,
     UnknownType,
 )
-from midas.checker.variance import VarianceInferrer
+from midas.checker.variance import VarianceManager
 from midas.lexer.midas import MidasLexer
 from midas.lexer.token import Token, TokenType
 from midas.parser.midas import MidasParser
@@ -147,10 +147,8 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
         for stmt in stmts:
             stmt.accept(self)
 
-        for name, type in self.types._types.items():
-            if isinstance(type, GenericType):
-                inferrer = VarianceInferrer(self.types)
-                self.types._types[name] = inferrer.infer(type)
+        manager: VarianceManager = VarianceManager(self.types)
+        manager.infer_all()
 
     def assert_bool(self, expr: m.Expr):
         """Check that the given expression is a subtype of `bool` or report an error
diff --git a/midas/checker/variance.py b/midas/checker/variance.py
index bcbdc2b..5a7bfc6 100644
--- a/midas/checker/variance.py
+++ b/midas/checker/variance.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from typing import Literal, Optional, cast
 
 from midas.checker.registry import Member, TypesRegistry
@@ -73,10 +75,14 @@ class Tracker:
 class VarianceInferrer:
     """Helper class to compute type parameter variance"""
 
-    def __init__(self, types: TypesRegistry) -> None:
-        self.types: TypesRegistry = types
+    def __init__(self, manager: VarianceManager) -> None:
+        self.manager: VarianceManager = manager
         self.tracker: Tracker = Tracker([])
 
+    @property
+    def types(self) -> TypesRegistry:
+        return self.manager.types
+
     def infer(self, type: GenericType) -> GenericType:
         """Infer the variance of a generic type's parameters
 
@@ -150,11 +156,13 @@ class VarianceInferrer:
             # Get inferred variance of parameters and multiply with current
             # polarity to recurse through arguments
             case AppliedType(name=name, args=args):
-                # TODO: handle mutually recursive types
-                if name == base_name:
+                if self.manager.is_in_queue(name):
                     return
+
                 generic: Type = self.types.get_type(name)
                 assert isinstance(generic, GenericType)
+                generic = self.manager.infer(name, generic)
+
                 params: list[TypeVar] = generic.params
                 polarities: dict[Variance, Polarity] = {
                     Variance.INVARIANT: 0,
@@ -179,3 +187,66 @@ class VarianceInferrer:
             case TypeVar():
                 if type in self.tracker:
                     self.tracker.record(type, polarity)
+
+
+class VarianceManager:
+    """Coordinator for VarianceInferrer to handle recursive types"""
+
+    def __init__(self, types: TypesRegistry) -> None:
+        self.types: TypesRegistry = types
+        self._queue: list[str] = []
+        self._inferred: set[str] = set()
+
+    def infer_all(self):
+        """Infer variance on all generic types defined in the registry"""
+
+        for name, type in self.types._types.items():
+            if isinstance(type, GenericType):
+                self.infer(name, type)
+
+    def infer(self, name: str, type: GenericType) -> GenericType:
+        """Infer variance of parameters of the given type
+
+        Args:
+            name (str): the type's name
+            type (GenericType): the type
+
+        Returns:
+            GenericType: a new generic type with its parameters updated with
+                their inferred variance
+        """
+        if self.is_inferred(name):
+            return type
+
+        self._queue.append(name)
+
+        inferrer: VarianceInferrer = VarianceInferrer(self)
+        inferred: GenericType = inferrer.infer(type)
+        self.types._types[name] = inferred
+
+        self._queue.pop()
+        self._inferred.add(name)
+
+        return inferred
+
+    def is_in_queue(self, name: str) -> bool:
+        """Whether the given type's variance is currently being inferred
+
+        Args:
+            name (str): the type's name
+
+        Returns:
+            bool: whether the type is in the queue
+        """
+        return name in self._queue
+
+    def is_inferred(self, name: str) -> bool:
+        """Whether the given type's variance has already been inferred
+
+        Args:
+            name (str): the type's name
+
+        Returns:
+            bool: whether the type has been processed
+        """
+        return name in self._inferred