from abc import abstractmethod
from typing import (
Callable,
ClassVar,
Generic,
NoReturn,
Sequence,
TypeVar,
final,
)
from returns.functions import compose, identity
from returns.primitives.asserts import assert_equal
from returns.primitives.hkt import KindN
from returns.primitives.laws import (
Law,
Law1,
Law3,
Lawful,
LawSpecDef,
law_definition,
)
_FirstType = TypeVar('_FirstType')
_SecondType = TypeVar('_SecondType')
_ThirdType = TypeVar('_ThirdType')
_UpdatedType = TypeVar('_UpdatedType')
_MappableType = TypeVar('_MappableType', bound='MappableN')
# Used in laws:
_NewType1 = TypeVar('_NewType1')
_NewType2 = TypeVar('_NewType2')
[docs]@final
class _LawSpec(LawSpecDef):
"""
Mappable or functor laws.
https://en.wikibooks.org/wiki/Haskell/The_Functor_class#The_functor_laws
"""
__slots__ = ()
[docs] @law_definition
def identity_law(
mappable: 'MappableN[_FirstType, _SecondType, _ThirdType]',
) -> None:
"""Mapping identity over a value must return the value unchanged."""
assert_equal(mappable.map(identity), mappable)
[docs] @law_definition
def associative_law(
mappable: 'MappableN[_FirstType, _SecondType, _ThirdType]',
first: Callable[[_FirstType], _NewType1],
second: Callable[[_NewType1], _NewType2],
) -> None:
"""Mapping twice or mapping a composition is the same thing."""
assert_equal(
mappable.map(first).map(second),
mappable.map(compose(first, second)),
)
[docs]class MappableN(
Generic[_FirstType, _SecondType, _ThirdType],
Lawful['MappableN[_FirstType, _SecondType, _ThirdType]'],
):
"""
Allows to chain wrapped values in containers with regular functions.
Behaves like a functor.
See also:
- https://en.wikipedia.org/wiki/Functor
"""
__slots__ = ()
_laws: ClassVar[Sequence[Law]] = (
Law1(_LawSpec.identity_law),
Law3(_LawSpec.associative_law),
)
[docs] @abstractmethod # noqa: WPS125
def map(
self: _MappableType,
function: Callable[[_FirstType], _UpdatedType],
) -> KindN[_MappableType, _UpdatedType, _SecondType, _ThirdType]:
"""Allows to run a pure function over a container."""
#: Type alias for kinds with one type argument.
Mappable1 = MappableN[_FirstType, NoReturn, NoReturn]
#: Type alias for kinds with two type arguments.
Mappable2 = MappableN[_FirstType, _SecondType, NoReturn]
#: Type alias for kinds with three type arguments.
Mappable3 = MappableN[_FirstType, _SecondType, _ThirdType]