Python 3.10 Maybe
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Authored by vlorentz on Sep 13 2020, 2:48 PM.

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	import typing
	from typing import *

	@typing.sealed
	class Maybe(Generic[T]):
	"""An Option like it ought to be: an ADT, not like Python's Optional[T].
	"""

	# Ideally this would be named Option/None/Some for consistency with Rust,
	# but it clashes with Python names, so this uses Haskell names instead.

	__cache = {}

	def __class_getitem__(cls, type_param):
	"""Generates Just and Nothing variants for the non-generic class."""
	if type_param in cls.__cache:
	return cls.__cache[type_param]
	new_cls = type(f"Maybe[{type_param}]", (cls,), {})

	@dataclass
	class Just(new_cls):
	item: type_param

	@classmethod
	def from_ast(cls, ast):
	return Just(type_param.from_ast(ast))

	@dataclass
	class Nothing(new_cls):
	pass

	new_cls.Just = Just
	new_cls.Nothing = Nothing

	cls.__cache[type_param] = new_cls

	return new_cls







	T = typing.TypeVar("T")
	U = typing.TypeVar("U")

	def map_maybe(m: Maybe[T], f: typing.Callable[[T], U]) -> Maybe[U]:
	M = m.__class__

	match m:
	case M.Nothing():
	return M.Nothing()
	case M.Just(item):
	return M.Just(f(item))
	case _:
	print("oops", repr(m))

	print(map_maybe(Maybe[int].Nothing(), lambda x: x*2)) # Maybe.__class_getitem__.<locals>.Nothing()
	print(map_maybe(Maybe[int].Just(21), lambda x: x*2)) # Maybe.__class_getitem__.<locals>.Just(item=42)

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Python 3.10 MaybeActivePublicActions