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

Matthias Fischmann fis at wiwi.hu-berlin.de
Thu Mar 16 06:57:54 EST 2006


this is one of those situations that always make scheme and perl
hackers laugh at me: i have written a piece of code that is
intuitively clear, and now i am trying to turn it into something that
compiles.  and here it goes.

i have a type class that looks something like this:

  class Resource a where
    resourceName      :: a -> String
    resourceAdvance   :: a -> a
    resourceStarved   :: a -> Bool
    resourceSpend     :: a -> Int -> a
    resourceEarn      :: a -> Int -> a

resource types are rice, crude oil, pizza, software code, and so on.
they all have a different internal structure and the same abstract
interface, that's why i have defined this type class.

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.  my first idea
was this:

  data Rs = forall a . (Resource a) => Rs a
  unRs (Rs a) = a
  rsName :: Rs -> String
  rsName = resourceName . unRs

and then export Rs as an abstract data type.  this would allow for
lists of type [Rs], which is exactly what i want.

but what is the type of unRs?  or better: can i make it type at all?
and isn't this solution a little redundant and verbose?  should i do
it like in the example for existentially quantified types in the ghc


but wouldnt't the code become really messy?  or should i do the type
class and instances, and then do Rs the existentially quantified way,
with all class methods arguments to the Rs constructor?  or is there a
completely different way to do this (besides using scheme or perl :-)?

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