Add some instances

[joneshf]

I found myself wanting Functor Proxy. So I added that. Then I figured, what the heck, let's add everything we can.

This does add a dependency on prelude. I'm not sure if that's wanted, or if it's been avoided for a reason.

I'm fine with throwing this all away if it's not wanted.

[paf31]

So this one means we'd allow discarding values of type Proxy a in do notation. Discard is meant for types like Unit so I'm not sure that's what we want.

[paf31]

I mean, of course, it is a unit type, but somehow it seems to convey more information.

That said, we can't use that information in a pattern match, since we can't bind type variables, so actually I'm fine with this.

[joneshf]

Yeah, up to you whether we keep these Discard instances. I don't have enough experience with it to know if it's a good idea or a bad idea.

[paf31]

👍 I like it, thanks!

[garyb]

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

[paf31]

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

[garyb]

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!

[paf31]

Thanks!

[safareli]

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

[paf31]

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