[Haskell-cafe] Functor and Haskell

Kim-Ee Yeoh a.biurvOir4 at asuhan.com
Wed Apr 22 03:18:54 EDT 2009

Daryoush Mehrtash-2 wrote:
> I am not sure I follow how the endofunctor gave me the 2nd functor.
> As I read the transformation there are two catagories C and D and two
> functors F and G between the same two catagories.  My problem is that I
> only
> have one functor between the Hask and List catagories.  So where does the
> 2nd functor come into picture that also maps between the same C and D
> catagories?

singleton :: a -> [a]
singleton x = [x]

Here F is the identity functor, and G is the list functor. And yes, C=D=
category of (a subset of) Haskell types.

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