[Haskell-cafe] Re: how would this be done? type classes? existential types?

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. -- Ben