Generic typeclasses

You can define generic typeclasses, just like regular generic functions. You have to ways of doing this:

  • Via raw type variables like in def copy(instance: X) -> X

  • Via instances with type variables like in get_zero_item(instance: Sequence[X]) -> X

Using typevars

When defining typeclasses with generic instances, we will typecheck that all instance definitions match the shape of the typeclass itself:

>>> from typing import TypeVar
>>> from classes import typeclass

>>> X = TypeVar('X')

>>> @typeclass
... def copy(instance: X) -> X:
...     ...

This one will typecheck correctly:

>>> @copy.instance(int)
... def _copy_int(instance: int) -> int:
...     ...

But, this won’t:

def _copy_str(instance: str) -> bool:

# error: Instance callback is incompatible "def (first: builtins.str, second: builtins.float) -> builtins.bool"; expected "def (first: builtins.str, second: builtins.str*) -> builtins.bool"

Using instances

When using instances, you can define type restrictions to limit typeclass instances to only subtypes of a given restriction.

Here’s an example definition:

>>> from typing import Iterable, TypeVar, List
>>> from classes import typeclass

>>> X = TypeVar('X')

>>> @typeclass
... def get_item(instance: Iterable[X], index: int) -> X:
...    ...

This instance will match the definition:

>>> @get_item.instance(list)
... def _get_item_list(instance: List[X], index: int) -> X:
...     ...

reveal_type(get_item([1, 2, 3], 0))  # Revealed type is "*"
reveal_type(get_item(['a', 'b'], 0)) # Revealed type is "builtins.str*"

But, this won’t match and mypy will warn you:

def _get_item_int(instance: int, index: int) -> int:
# error: Instance callback is incompatible "def (instance:, index: ->"; expected "def [X] (instance:, index: -> X`-1"
# error: Instance "" does not match original type "typing.Iterable[X`-1]"

Generic Supports type

You can also use generic Supports type with generic AssociatedType.

To do so, you will need: 1. Declare AssociatedType with type arguments, just like regular Generic 2. Use correct type arguments to define a variable

Let’s get back to get_item example and use a generic Supports type:

>>> from typing import Iterable, List, TypeVar
>>> from classes import AssociatedType, Supports, typeclass

>>> X = TypeVar('X')

>>> class GetItem(AssociatedType[X]):
...     ...

>>> @typeclass(GetItem)
... def get_item(instance: Iterable[X], index: int) -> X:
...     ...

>>> numbers: Supports[GetItem[int]]
>>> strings: Supports[GetItem[str]]

reveal_type(get_item(numbers, 0))  # Revealed type is "*"
reveal_type(get_item(strings, 0))  # Revealed type is "builtins.str*"


We are limited in generics support. We support them, but without concrete type parameters.

  • We support: X, list, List, Dict, Mapping, Iterable, MyCustomGeneric

  • We also support: Iterable[Any], List[X], Dict[X, Y], etc

  • We don’t support List[int], Dict[str, str], etc

Why? Because we cannot tell the difference between List[int] and List[str] in runtime.

Python just does not have this information. It requires types to be inferred by some other tool. And that’s currently not supported.

So, this would not work:

>>> from typing import List
>>> from classes import typeclass

>>> @typeclass
... def generic_typeclass(instance) -> str:
...     """We use this example to demonstrate the typing limitation."""

>>> @generic_typeclass.instance(List[int])
... def _generic_typeclass_list_int(instance: List[int]):
...   ...
Traceback (most recent call last):
TypeError: ...