[Haskell-cafe] A "commutative diagram" conjecture
about applicative functors
Isaac Dupree
isaacdupree at charter.net
Mon Dec 31 09:43:09 EST 2007
Twan van Laarhoven wrote:
> 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.
thank you, oops, how annoying. I wonder if GHCi should output
parentheses to make its :type result be a valid expression...
Isaac
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