Example of typeclasses in python

typeclasses.py

import sys
import typing

import classes


Paper = typing.NewType("Paper", str)
# RecycledPaper = typing.NewType("RecycledPaper", str)


# typeclasses works with real types, protocols and generics only
class RecycledPaper(str):
    ...


@typing.runtime_checkable
class SupportConvertToPaper(typing.Protocol):
    def convert_to_paper(self) -> Paper:
        ...


class Wood:
    def convert_to_paper(self) -> Paper:
        return Paper("converted from Wood")


@classes.typeclass
def paper_factory(material) -> Paper:
    ...


@paper_factory.instance(protocol=SupportConvertToPaper)
def _paper_factory(material: SupportConvertToPaper) -> Paper:
    return material.convert_to_paper()


# @paper_factory.instance(RecycledPaper)
# def _paper_factory(material: RecycledPaper) -> Paper:
#     return RecycledPaper("converted from recycled paper")


@paper_factory.instance(RecycledPaper)
def _paper_factory(material: RecycledPaper) -> Paper:
    return RecycledPaper("converted from recycled paper")


def test_impl():
    print(paper_factory(Wood()), file=sys.stderr)
    # print(paper_factory(RecycledPaper("")))
    print(paper_factory(RecycledPaper("")), file=sys.stderr)