From 58a5cbd7f86144ec776fc0f3cb710fd3d74b47c2 Mon Sep 17 00:00:00 2001
From: KotlinIsland <65446343+kotlinisland@users.noreply.github.com>
Date: Thu, 4 Jan 2024 16:09:05 +1000
Subject: [PATCH] `In`/`Out`

---
 basedtyping/__init__.py | 45 +++++++++++++++++++++++++++++++++++++++++
 tests/test_in_out.py    |  1 +
 2 files changed, 46 insertions(+)
 create mode 100644 tests/test_in_out.py

diff --git a/basedtyping/__init__.py b/basedtyping/__init__.py
index 1ec8a0a..a5d8431 100644
--- a/basedtyping/__init__.py
+++ b/basedtyping/__init__.py
@@ -51,6 +51,8 @@
     "Untyped",
     "Intersection",
     "TypeForm",
+    "In",
+    "Out",
 )
 
 if TYPE_CHECKING:
@@ -556,3 +558,46 @@ def f[T](t: TypeForm[T]) -> T: ...
                             
                              reveal_type(f(int | str))  # int | str
                          """)
+
+
+class _InForm(_BasedSpecialForm, _root=True):  # type: ignore[misc]
+    def __init__(self, doc: str):
+        self._name = "In"
+        self._doc = self.__doc__ = doc
+
+    def __getitem__(self, parameters: object | tuple[object]) -> _BasedGenericAlias:
+        if not isinstance(parameters, tuple):
+            parameters = (parameters,)
+
+        return _BasedGenericAlias(self, parameters)  # type: ignore[arg-type]
+
+
+In = _InForm(doc="""\
+                 A type that can be used to represent a contravariant form of a type.
+                 For example:
+                
+                     list_of_objects: list[object]
+                     list_of_ints: list[In[int]] = list_of_objects  # valid
+                 """)
+
+
+class _OutForm(_BasedSpecialForm, _root=True):  # type: ignore[misc]
+    def __init__(self, doc: str):
+        self._name = "Out"
+        self._doc = self.__doc__ = doc
+
+    def __getitem__(self, parameters: object | tuple[object]) -> _BasedGenericAlias:
+        if not isinstance(parameters, tuple):
+            parameters = (parameters,)
+
+        return _BasedGenericAlias(self, parameters)  # type: ignore[arg-type]
+
+
+Out = _OutForm(doc="""\
+                 An annotation that can be used to represent a covariant form of a type.
+                 For example:
+                
+                     list_of_ints: list[int]
+                     list_of_objects: list[Out[object]] = a  # valid
+                 """)
+
diff --git a/tests/test_in_out.py b/tests/test_in_out.py
new file mode 100644
index 0000000..a623ce9
--- /dev/null
+++ b/tests/test_in_out.py
@@ -0,0 +1 @@
+raise NotImplementedError
\ No newline at end of file