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fantasyland / fantasy-land

Specification for interoperability of common algebraic structures in JavaScript
MIT License
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Require that Profunctor implement the Contravariant spec #252

Open gabejohnson opened 7 years ago

gabejohnson commented 7 years ago

Values which implement Bifunctor or Profunctor must also implement Functor.

Profunctor is covariant in the second argument and Bifunctor in both.

Since Profunctor is contravariant in the first argument, shouldn't a value implementing it also be required to implement Contravariant?

gabejohnson commented 7 years ago

I realize the graph looks funky now. I'll try to fix that.

davidchambers commented 7 years ago

This sounds completely reasonable to me. I'll defer to those with better understandings. :)

:warning: This is a breaking change, so will warrant incrementing the major version number if merged.

gabejohnson commented 7 years ago

@davidchambers I'm no longer sure that this is valid. I think (based on a comment by @puffnfresh on Gitter) that this reasoning works for Bifunctor and Profunctor WRT Functor because fixing the first argument results in a type that has a functor.

:k Bifunctor
Bifunctor :: (* -> * -> *) -> Constraint
 :k Functor
Functor :: (* -> *) -> Constraint

Now this isn't the case with Profunctor because it would be required to fix the second argument. Though honestly my type foo isn't sufficient to be certain.

rjmk commented 7 years ago

It doesn't lead to inconsistency (though it breaks type inference), to allow type level lambdas like instance Contravariant (Λt -> t -> a) where ... (here the idea is to have the analogue to (->) a). It would quite a big change to the way Fantasy Land does things though

davidchambers commented 5 years ago

What's the status of this pull request, @gabejohnson?