Shared via mypy Playground

main.py

from typing import *

T = TypeVar("T")
V = TypeVar("V")

class Input(Protocol[T]):
    def send(self, value: T, /) -> None:
        ...
        
        
class Connectable(Protocol[T]):
    def connect(self, other: V := InputLike[T], /) -> V:
        ...
    
    
class Broadcaster(Input[T], Connectable[T]):
    def __init__(self) -> None:
        self._connections = List[InputLike[T]] = []

    def send(self, arg: T, /) -> None:
        for c in self._connections:
            c.send(arg)

    def connect(self, other: V := InputLike[T], /) -> V:
        self._connections.append(other)
        return other
        

class Function(Input[T], Broadcaster[U]):
    def __init__(self, func: Callable[[T], U]) -> None:
        self._func = func
        super().__init__()
        
    def send(self, arg: T, /) -> None:
        return super().send(self._func(arg))