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
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