[Haskell-cafe] Anonymous, Unique Types, maybe

[Felipe Almeida Lessa]

On Sun, Dec 4, 2011 at 3:53 AM, Scott Lawrence <bytbox at gmail.com> wrote:
> type AList = [Event]
> type BList = [Event]
> type CList = [Event]
>
> myMapish :: AList -> AList
> mySelect :: AList -> (Event -> Bool) -> BList
> myOtherSelect :: BList -> CList

A suggestion:

  data Exists f = forall a. Exists f a

  data List a = List [Event] -- your list type

  myMapish :: List a -> List a
  myDoSomething :: List a -> (List a, List a)
  myPair :: (Event -> Event -> Event) -> List a -> List a -> List a
  mySelect :: List a -> Exists List

So the "anonymous, unique type" would be enclosed on the existential.
Its real type doesn't really matter, it may be always (), but code
that uses mySelect can't use this fact.  It's not able to do even
something simple like

  let Exists b = mySelect a
      Exists c = mySelect a
  in myPair f b c

even though we know that in this case this is valid =).  But you can always make

  unsafeCastList :: List a -> List b
  unsafeCastList (List a) = List a

Cheers,

-- 
Felipe.