Thanks Stephen, that worked out just fine! On 18-11-2012 18:23, Jose A. Lopes wrote:
Thanks Stephen. I will try this!
On 18-11-2012 17:56, Stephen Tetley wrote:
With existentials an "extesible" version might look like this:
{-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-}
... class Action and datatypes A and B the same as before ...
-- some new ones... data C = C Int deriving (Read, Show)
instance Action C where run (C n) = n
data D = D Int deriving (Read, Show) instance Action D where run (D n) = n
The important one:
data PolyA = forall a. Action a => PolyA a
parseAction :: String -> PolyA parseAction str | "(A " `isPrefixOf` str = PolyA $ (read :: String -> A) str | "(B " `isPrefixOf` str = PolyA $ (read :: String -> B) str
-- can add new cases | "(C " `isPrefixOf` str = PolyA $ (read :: String -> C) str | "(D " `isPrefixOf` str = PolyA $ (read :: String -> D) str
This is "extensible" to some degree as you can add new cases of "different" types, but all you can do with these different types is run them to make an Int, so it is equivalent to the second version I gave previously:
parseAction :: String -> Int parseAction str | "(A " `isPrefixOf` str = run $ (read str :: A) | "(B " `isPrefixOf` str = run $ (read str :: B) | "(C " `isPrefixOf` str = run $ (read str :: C) | "(D " `isPrefixOf` str = run $ (read str :: D) i.e instead of using an existential type to hold something polymorphic that can be run to produce an Int, just call run at the point of use making an Int.
-- José António Branquinho de Oliveira Lopes Instituto Superior Técnico Technical University of Lisbon