> data X a b = X a b> instance Functor (X a) where > fmap f (X a b) = X a (f b)
Yeah, that works just fine. On Fri, Jul 17, 2009 at 1:14 PM, Petr Pudlak <d...@pudlak.name> wrote: > Hi, I have probably a very simple question, but I wasn't able to figure it > out > myself. > > Consider a two-parameter data type: > > > data X a b = X a b > > If I want to make it a functor in the last type variable (b), I can just > define > > > instance Functor (X a) where > > fmap f (X a b) = X a (f b) > > But how do I write it if I want X to be a functor in its first type > variable? > Is that possible at all? > Something like: > > > instance Functor ??? where > > fmap f (X a b) = X (f a) b > > Thanks in advance, > Petr > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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