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Writer

Writer dataclass

Bases: Generic[W, V]

Writer is a monad.

Source code in funclift/types/writer.py 
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@dataclass
class Writer(Generic[W, V]):
    """Writer is a monad."""

    value: V
    written: W

    # @staticmethod
    # def pure(a: E, m: Type[Monoid[T]]) -> Writer[T, E]:
    # def pure(a: E, m: Type[T]) -> Writer[T, E]:
    #   return Writer(a, m.empty())
    #   return Writer(a, cast(T, m.empty()))

    # @staticmethod
    # def pure(a: E) -> Writer[W, E]:
    #     w: W
    #     wt = type(w)
    #     return Writer(a, monoid_empty(wt))
    def pure(self, a: E) -> Writer[W, E]:
        if isinstance(self.written, int):
            return Writer(a, cast(W, 0))
        else:
            return Writer(a, cast(Monoid, self.written).empty())

    @staticmethod
    def pure2(a: E, m: T) -> Writer[T, E]:
        return Writer(a, m)

    def fmap(self, f: Callable[[V], B]) -> Writer[W, B]:
        return Writer(f(self.value), self.written)

    def flatmap(self, f: Callable[[V], Writer[W, B]]) -> Writer[W, B]:
        new_writer = f(self.value)
        new_written = cast(W, self.written + new_writer.written)
        return Writer(new_writer.value, new_written)

    def ap(self: Writer[W, Callable[[C], E]],
           other: Writer[W, C]) -> Writer[W, E]:
        return other.fmap(self.value)