Re: [Haskell-cafe] Questions about the Functor class and it's use in "Data types à la carte"

Felipe Lessa felipe.lessa at
Fri Dec 14 21:09:34 EST 2007

On Dec 15, 2007 12:00 AM, David Menendez <dave at> wrote:
> These can (and, if Val is a newtype, will) be compiled to the same code, but
> they don't have the same type. In particular, there is no way to unify "a ->
> a" with "f a -> f b" for any f.

Thanks for noticing that! I hadn't seen before that the two Val constructors in

  fmap f (Val x) = Val x

were of different types.



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