mirror of
https://github.com/correl/typesafe-monads.git
synced 2024-11-14 19:19:32 +00:00
112 lines
2.6 KiB
Python
112 lines
2.6 KiB
Python
from __future__ import annotations
|
|
from typing import Any, Callable, Generic, List, Optional, TypeVar
|
|
from .monad import Monad
|
|
from .monoid import Monoid
|
|
|
|
T = TypeVar("T")
|
|
S = TypeVar("S")
|
|
E = TypeVar("E")
|
|
|
|
|
|
class Maybe(Monad[T]):
|
|
def __init__(self) -> None:
|
|
raise NotImplementedError
|
|
|
|
@classmethod
|
|
def pure(cls, value: T) -> Maybe[T]:
|
|
return Just(value)
|
|
|
|
def bind(self, function: Callable[[T], Maybe[S]]) -> Maybe[S]:
|
|
if isinstance(self, Just):
|
|
return function(self.value)
|
|
else:
|
|
new: Maybe[S] = Nothing()
|
|
return new
|
|
|
|
def map(self, function: Callable[[T], S]) -> Maybe[S]:
|
|
if isinstance(self, Just):
|
|
return Just(function(self.value))
|
|
else:
|
|
new: Maybe[S] = Nothing()
|
|
return new
|
|
|
|
def withDefault(self, default: T) -> T:
|
|
if isinstance(self, Just):
|
|
return self.value
|
|
else:
|
|
return default
|
|
|
|
__rshift__ = bind
|
|
__mul__ = __rmul__ = map
|
|
|
|
|
|
class Just(Maybe[T]):
|
|
def __init__(self, value: T) -> None:
|
|
self.value = value
|
|
|
|
def __eq__(self, other: object):
|
|
return isinstance(other, Just) and self.value == other.value
|
|
|
|
def __repr__(self) -> str:
|
|
return f"<Just {self.value}>"
|
|
|
|
|
|
class Nothing(Maybe[T]):
|
|
def __init__(self) -> None:
|
|
...
|
|
|
|
def __eq__(self, other: object):
|
|
return isinstance(other, Nothing)
|
|
|
|
def __repr__(self) -> str:
|
|
return "<Nothing>"
|
|
|
|
|
|
def maybe(value: T, predicate: Optional[Callable[[T], bool]] = None) -> Maybe[T]:
|
|
predicate = predicate or (lambda x: x is not None)
|
|
if predicate(value):
|
|
return Just(value)
|
|
else:
|
|
return Nothing()
|
|
|
|
|
|
class First(Monoid[Maybe[T]]):
|
|
@classmethod
|
|
def mzero(cls) -> First:
|
|
return First(Nothing())
|
|
|
|
def mappend(self, other: First):
|
|
if isinstance(self.value, Just):
|
|
return self
|
|
else:
|
|
return other
|
|
|
|
def __repr__(self) -> str:
|
|
return f"<First {self.value}>"
|
|
|
|
__add__ = mappend
|
|
|
|
|
|
def first(xs: List[Maybe[T]]) -> Maybe[T]:
|
|
return First.mconcat(map(lambda x: First(x), xs)).value
|
|
|
|
|
|
class Last(Monoid[Maybe[T]]):
|
|
@classmethod
|
|
def mzero(cls) -> Last:
|
|
return Last(Nothing())
|
|
|
|
def mappend(self, other: Last):
|
|
if isinstance(other.value, Just):
|
|
return other
|
|
else:
|
|
return self
|
|
|
|
def __repr__(self) -> str:
|
|
return f"<Last {self.value}>"
|
|
|
|
__add__ = mappend
|
|
|
|
|
|
def last(xs: List[Maybe[T]]) -> Maybe[T]:
|
|
return Last.mconcat(map(lambda x: Last(x), xs)).value
|