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