Please add or_else

[juxeii]

Hi,

could you please add or_else to the Maybe monad?

[chuckwondo]

@jasondelaat, thank you for this fantastically useful, yet unbloated, library!

I would like to see this for Either as well, but perhaps instead of defining new methods, the Maybe.maybe and Either.either methods can simply be modified (since they appear to simply be or_else named differently) such that the identity function is the default value for the second argument to those methods, since that is arguably the most common use case, and having to repeat lambda x: x (or identity, if that is defined somewhere) is tiresome and unnecessarily verbose.

For example:

class Maybe(pymonad.monad.Monad, Generic[T]):
    ...
    def maybe(self: 'Maybe[S]', default_value: T, extraction_function: Callable[[S], T] = lambda x: x) -> T:
        ...


class Either(pymonad.monad.Monad, Generic[M, T]):
    ...
    def either(
            self: 'Either[M, S]', left_function: Callable[[M], T], right_function: Callable[[S], T] = lambda x: x
    ) -> T:
        ...

[chuckwondo]

I'd be happy to submit a PR, if this is something you're agreeable to.