[Haskell-cafe] Parsing different types, same typeclass

["José A. Lopes"]

Hey Stephen,

Thank you for the reply!

Can you elaborate on how I can use existentials?
I always up for learning new (Haskell) stuff :)

Cheers,
José

On 18-11-2012 00:19, Stephen Tetley wrote:
> Being concrete, all you can do is:
>
> parseAction :: String -> Either A B
> parseAction str
>      | "(A " `isPrefixOf` str = Left $ read str
>      | "(B " `isPrefixOf` str = Right $ read str
>
>
> parseAction :: String -> Int
> parseAction str
>      | "(A " `isPrefixOf` str = run $ (read str :: A)
>      | "(B " `isPrefixOf` str = run $ (read str :: B)
>
> As you can't return a polymorphic /a/ when you have typed the cases to A or B.
>
> Being less concrete, no doubt you can use existentials, but at this
> point I'd recommend you re-evaluate what you are trying to do.

-- 
José António Branquinho de Oliveira Lopes
Instituto Superior Técnico
Technical University of Lisbon