👍 I like it, thanks!

A question tangentially related to this: what happens about instances with purescript/purescript/pull/2846? They'll just be impossible?

@garyb Do you think those instances would be useful? I'll have to think if the polykindedness of the instances would be an issue.

I don't know, it was just a thought. Maybe this is something that would need real PolyKinds. Here's what happens in GHC:

{-# LANGUAGE KindSignatures, PolyKinds #-}

data Proxy (a :: k) = Proxy

instance Functor Proxy where
  fmap _ _ = Proxy

data Const a b = Const a

bar :: Proxy Int
bar = Proxy

foo :: Proxy Const
foo = Proxy

barB = const True <$> bar

-- Only becomes an error here, everything before this is 👌
fooB = const True <$> foo

test.hs:18:23: error:
    • Couldn't match kind ‘* -> k -> *’ with ‘*’
      When matching the kind of ‘Proxy’
    • In the second argument of ‘(<$>)’, namely ‘foo’
      In the expression: const True <$> foo
      In an equation for ‘fooB’: fooB = const True <$> foo
    • Relevant bindings include
        fooB :: Proxy Bool (bound at test.hs:18:1)

Kind of interesting!

Thanks!

Any use case of let's say Bind instance on proxy? or any other instance?

Proxy is the terminal object in the category of monads, which might be useful. It's sort of useful in a weird way for designing DSLs, when you want to not bother implementing something:

http://hackage.haskell.org/package/acme-all-monad