`filter` type signature, and other typing ideas

[james-pcdr]

This discussion started in PR #35.

Summary:

• Goal 1: In the signature of the filter function, indicate the type of the first argument. I started work on this in PR 35; I'm planning to fix and submit a new PR.
  Benefit: With this change, an editor would be able to autocomplete (and then type-check) the following: flu(["foo", "bar"]).filter(lambda x: x.startswi
• Goal 1b: Make a similar change in map, which currently has a # type: ignore comment in its impl.
• Goal 2: Look at the test suite to see which flu()... expressions have type warnings; improve.

[olirice]

flu(["foo", "bar"]).filter(lambda x: x.startswi 

this is great example. thanks, all of these would be welcome improvements

[james-pcdr]

Goal 1: Here's the update. It's basically the same as what I had in the original PR, but with proper contra|co-variance based on Pyright's errors (and based on my admittedly shallow understanding of variance concepts 😄 )

from typing import TypeVar, Protocol, Generic

T_contra = TypeVar("T_contra", contravariant=True)
T_co = TypeVar("T_co", covariant=True)
T = TypeVar("T")

class FluentFunc(Protocol[T_contra, T_co]):
    def __call__(self, x: T_contra, *args, **kwargs) -> T_co: ...


class Fluent(Generic[T]):
    
    def filter(self, func: FluentFunc[T, bool], *args, **kwargs) -> "Fluent[T]":
        # implementation here
        ...

Less abridged example here.

[olirice]

looks great

if you submit a PR I'll check it out!