[Haskell-cafe] Re: Curious Functor Class

Ashley Yakeley ashley at semantic.org
Wed Sep 27 23:10:15 EDT 2006

I wrote:
> Perhaps the key is that there exist types P and Q s.t. there's an 
> isomorphism
>   F a <=> (P -> a,Q)
> This seems to be intuitively Napierian:
>   ln (P -> a,Q) = (P,ln a) | ln Q
> I can believe that Hoistables are in fact Idioms, though I know there 
> are Idioms that are not Hoistables (Maybe and Either, for instance).

Ah, that's not correct, not all Hoistables are Idioms. For instance, 
this type can be made an instance of Hoistable but not of Idiom:

   data Extra a = MkExtra Int a

Ashley Yakeley
Seattle WA

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