In this post, I explain what typeclasses are (an alternative to subtyping polymorphism) and how to use them. I give examples in 4 very different languages to show that this concept is universal. I also show that this idea is very pythonic by comparing our classes implementation with functools.singledispatch.

Check how easy it is to define a typeclass with classes:

from classes import AssociatedType, Supports, typeclass

class Greet(AssociatedType):
    """Special type to represent that some instance can `greet`."""

@typeclass(Greet)
def greet(instance) -> str:
    """No implementation needed."""

@greet.instance(str)
def _greet_str(instance: str) -> str:
    return 'Hello, {0}!'.format(instance)

def greet_and_print(instance: Supports[Greet]) -> None:
    print(greet(instance))

greet_and_print('world')
# Hello, world!

Check it out!