[Haskell-cafe] A "commutative diagram" conjecture about applicative functors

[Twan van Laarhoven]

Isaac Dupree wrote:
> Unfortunately, I get puzzling type errors if I annotate either one of 
> them with their type (e.g.
> (Applicative f) => f (a -> b) -> f a -> f (Int, b)
> ) in an expression.  The very answer doesn't seem to typecheck.
> 
>  > :t \f x -> fmap ((,) (0::Int)) (f <*> x) :: (Applicative f) => f (a1 
> -> a) -> f a1 -> f (Int, a)

Here the type annotation applies to the *body* of the lambda abstraction, 
adding parentheses around the whole thing solve your problem.

  > :t (\f x -> fmap ((,) (0::Int)) (f <*> x)) :: (Applicative f) => f (a1 
-> a) -> f a1 -> f (Int, a)

Aside from the fact that ghci has some trouble formating the output.

Twan