Enable `Supports[]` to refer to multiple typeclasses

[eganjs]

class Foo:
    def __call__(self, instance) -> A:
        ...

foo = typeclass(Foo)

...

class Bar:
    def __call__(self, instance) -> B:
        ...

bar = typeclass(Bar)

...

def fn(instance: Supports[Foo, Bar]) -> C:
    ...

This is similar to #83 and python/typing#213, but offers the advantages of composability and conciseness.

I'd be willing to have a go at implementing this but I'm not very familiar with mypy plugins (where I suspect most of the work would be required).

[sobolevn]

@eganjs great idea! I will do this in several days, I am preparing a new release right now! 👍

[eganjs]

Awesome @sobolevn I look forward to using it in action!

[sobolevn]

Sneak peak (I am working on Generic typeclasses):

[sobolevn]

So! I'm on it!

Let's see what we can possibly do here.

1. Supports[] type can be variadic (we already have this for AssociatedType: https://github.com/dry-python/classes/blob/master/classes/contrib/mypy/classes_plugin.py#L56-L62) to receive any amount of associated types
2. Supports[] uses nominal inheritance with mro tweaks, probably we can use this feature to model intersenctions

Let's say that we have this code:

from classes import typeclass, AssociatedType, Supports


class A(AssociatedType):
    ...

@typeclass(A)
def a(instance) -> str:
    ...

@a.instance(int)
def _a_int(instance: int) -> str:
    ...

@a.instance(str)
def _a_str(instance: str) -> str:
    ...

# =====

class B(AssociatedType):
    ...

@typeclass(B)
def b(instance) -> str:
    ...

@b.instance(int)
def _b_int(instance: int) -> str:
    ...

@b.instance(list)
def _b_list(instance: lost) -> str:
    ...

Next thing, we want a type that Supports[A, B], in our case it is just int, because str and list supports just A and B respectively.

This must work:

number: int
a: Supports[A] = number
b: Supports[B] = number
ab: Supports[A, B] = number

a = ab
b = ab

And this should be an error, because a and b does not support all ab features:

ab = a
ab = b

So, it feels like this should work:

class A: ...

class B: ...

class AB(A, B): ...

But, then we have a problem. Because we would have to track all types and their subtypes and inject these new intersection types everywhere.

I will need to think about other approaches.

[sobolevn]

Looks like this might be a way to go:

from typing_extensions import Protocol

class A(Protocol):
    _classes_supports_A: 'A'


class B(Protocol):
    _classes_supports_B: 'B'


class AB(Protocol, A, B):
    ...


# This is an emulation of `@some.instance(Number)`
# Consider `Number` as the instance type we are going to work with.
class Number(object):
    _classes_supports_A: 'A'
    _classes_supports_B: 'B'

n: Number
a: A = n
b: B = n
ab: AB = n

a1: A = AB
# Incompatible types in assignment (expression has type "Type[AB]", variable has type "A")
b1: B = AB
# Incompatible types in assignment (expression has type "Type[AB]", variable has type "B")

[sobolevn]

Ok, this is hard. I am going to release it later, not in the closest version.

[sobolevn]

Here's my attempt:

from typing_extensions import Protocol
from typing import Union, TypeVar
from classes import AssociatedType

_T = TypeVar('_T')

class Supports(Protocol[_T]):
    __classes_supports: _T

class A(AssociatedType):
    ...

class B(AssociatedType):
    ...

class C(AssociatedType):
    ...

# Real data types:
# It will be used in `@some.instance(HasA)`

class HasA(object):
    __classes_supports: A

class HasB(object):
    __classes_supports: B

class HasAB(object):
    __classes_supports: Union[A, B]

class HasABC(object):
    __classes_supports: Union[A, B, C]


a: HasA
b: HasB
ab: HasAB

sup_a: Supports[A]
sup_b: Supports[B]
sup_ab: Supports[Union[A, B]]

sup_a = a  # ok
sup_b = b  # ok
sup_ab = ab  # ok

sup_ab = a  # fail
sup_ab = b  # fail

sup_abc: Supports[Union[A, B, C]]
sup_ab = sup_abc  # fails, is it correct?

The only problem for me right now:

sup_abc: Supports[Union[A, B, C]]
sup_ab = sup_abc  # fails, is it correct?

If some typeclass supports A, B and C, it should be assignable to Supports[A, B] type.

I think that to achieve this I need to fallback to individual props 😞

[sobolevn]

Ok, seems that this works:

from typing import Union, Generic, TypeVar

T = TypeVar('T', contravariant=True)

class Supports(Generic[T]):
    ...

class A:
    ...

class B:
    ...

def some_a(instance: Supports[A]):
    ...

def some_b(instance: Supports[B]):
    ...

def some_ab(instance: Supports[Union[A, B]]):
    ...

a: Supports[A]
b: Supports[B]
ab: Supports[Union[A, B]]

some_a(a)    # ok
some_a(b)    # incompatible type "Supports[B]"; expected "Supports[A]"
some_a(ab)   # ok

some_b(a)    # incompatible type "Supports[A]"; expected "Supports[B]"
some_b(b)    # ok
some_b(ab)   # ok

some_ab(a)   # incompatible type "Supports[A]"; expected "Supports[Union[A, B]]"
some_ab(b)   # incompatible type "Supports[B]"; expected "Supports[Union[A, B]]"
some_ab(ab)  # ok

a = ab       # ok
b = ab       # ok

ab = a       # error
ab = b       # error

My plan:

1. Transform Supports[A, B] into Supports[Union[A, B]]
2. When adding new Supports[] base type into mro, I will need to find existing instances and unify them into a single one
3. Test all the things

[sobolevn]

See #250