Higher Kinded Types in Python

hkt_in_python.py

import abc
import dataclasses
from typing import Callable, Generic, TypeVar

# TODO: pip install returns
# TODO: add `returns.contrib.mypy.returns_plugin` to mypy plugins
# TODO: read the docs at https://github.com/dry-python/returns
from returns.primitives.hkt import Kind1, SupportsKind1, kinded

_ValueType = TypeVar('_ValueType')
_NewValueType = TypeVar('_NewValueType')
_InstanceKind = TypeVar('_InstanceKind', bound='HasValue')


class HasValue(Generic[_ValueType]):
    @abc.abstractmethod
    @property
    def value(self) -> _ValueType:
        """Returns a value property."""

    @abc.abstractmethod
    def with_value(
        self: _InstanceKind,
        new_value: _NewValueType,
    ) -> Kind1[_InstanceKind, _NewValueType]:
        """Creates a new instance with a changed value."""


@dataclasses.dataclass
class Box(SupportsKind1['Box', _ValueType], HasValue[_ValueType]):
    value: _ValueType
    length: int
    width: int
    height: int

    def with_value(self, new_value: _NewValueType) -> 'Box[_NewValueType]':
        return Box(new_value, self.length, self.width, self.height)


@dataclasses.dataclass
class Bag(SupportsKind1['Bag', _ValueType], HasValue[_ValueType]):
    value: _ValueType
    brand: str
    model: str

    def with_value(self, new_value: _NewValueType) -> 'Bag[_NewValueType]':
        return Bag(new_value, self.brand, self.model)


@kinded
def apply_function(
    instance: Kind1[_InstanceKind, _ValueType],
    callback: Callable[[_ValueType], _NewValueType],
) -> Kind1[_InstanceKind, _NewValueType]:
    new_value = callback(instance.value)
    return instance.with_value(new_value)


box = Box(value=10, length=1, width=2, height=3)
bag = Bag(value=5, brand='Fancy', model='Baggy')

reveal_type(apply_function(box, str))
reveal_type(apply_function(bag, str))
# ex.py:58: note: Revealed type is 'ex.Box[builtins.str]'
# ex.py:59: note: Revealed type is 'ex.Bag[builtins.str]'