[Haskell] Functor ((,) a)
dan.doel at gmail.com
Wed Sep 19 04:12:28 EDT 2007
On Wednesday 19 September 2007, Janis Voigtlaender wrote:
> the Prelude docs found via
> claim that there is an instance
> Functor ((,) a)
> And yet, I get (in GHC, but similarly in Hugs):
> Prelude> fmap (+1) (undefined,2)
> No instance for (Functor ((,) a))
> arising from use of `fmap' at <interactive>:1:0-22
> Possible fix: add an instance declaration for (Functor ((,) a))
> In the expression: fmap ((+ 1)) (undefined, 2)
> In the definition of `it': it = fmap ((+ 1)) (undefined, 2)
> What do I have to import to get the Functor ((,) a) instance?
> (Of course, I can define it myself, but this is not the point.)
In GHC 6.6 and above, you'll need to import Control.Monad.Instances (a bit of
a weird place to put it, but I guess there's no Control.Functor and such).
If you have an earlier version than that, I'd say import Control.Monad.Writer
at a guess.
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