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Do we think the instances for Monoid and Contravariant should be defined in here, or would it be better to require Const in those packages and provide instances there instead?

I'm asking as if we move Identity out into its own package we'll have the same quandary there too. It seems to me like we'll end up having less problems later if Const and Identity have no dependencies but rather are required in a lot of places. It might end up similar to the situation with purecript-control where almost everything will end up requiring them transitively, but they're fairly fundamental things anyway - in fact they should never need to change again unless we alter the classes in Prelude.

Thoughts? (@paf31, @joneshf)

I was surprised to see this existed. It's not compatible with apply.

Currently, Const a has a Bind instance that is defined as:

instance bindConst :: Semigroup a => Bind (Const a) where
  bind (Const x) _ = Const x

This instance fulfills the associativity law of the Bind class trivially:

-- To show:
(x >>= f) >>= g = x >>= (\k -> f k >>= g)

(x >>= f) >>= g
  = x >>= g
  = x
  = x >>= (\k -> f k >>= g)

I have two questions:

• Is there a reason for the Semigroup a constraint? It is not used in the code and it is not required in order to fulfill the laws... Nevermind, it needs the Semigroup a constraint in order to be an Apply, of course...
• Is this instance useful in any sense? I see that it fulfills the associativity law of Bind, but it is not powerful enough to make Const a a Monad (even though Const is also an Applicative), because the following law can never be fulfilled:

pure x >>= f = f x