Correct Functor law hint

[MatthijsBlom]

<$> associates to the left, so f <$> g <$> x is equivalent to f . g <$> x not because of a Functor law, but because for functions (<$>) == (.).

The actual Functor law was missing, so I added it.

The original hint could perhaps be generalized to f <$> g ↦ f . g, but I could not figure out how to express that g should be a function.

[googleson78]

The original hint could perhaps be generalized to f <$> g ↦ f . g, but I could not figure out how to express that g should be a function.

You almost certainly can't in the general case, because hlint only does parsing, no typechecking.

[ndmitchell]

Thanks!