[Haskell-cafe] Re: how would this be done? type classes?
Benjamin.Rudiak-Gould at cl.cam.ac.uk
Fri Mar 17 10:46:56 EST 2006
Matthias Fischmann wrote:
> now i want to create a list of a type similar to
> [r1, r2, r3] :: (Resource a) => [a]
> but with r1 being pizza, r2 being crude oil, and so on.
The type you actually want here is [exists a. (Resource a) && a], but no
Haskell implementation supports that.
> data Rs = forall a . (Resource a) => Rs a
> unRs (Rs a) = a
> rsName :: Rs -> String
> rsName = resourceName . unRs
> but what is the type of unRs?
Its type is (Rs -> (exists a. (Resource a) && a)), but again no Haskell
implementation supports that.
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