On Wed, Jul 14, 2010 at 8:25 AM, Arnaud Bailly
Hello, I would like to construct a collection of function-like objects on which I could apply some value, in a typesafe and clean way.
You could use Data.Typeable.cast [1] [1] http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Data-Typeable...
Let's say I have something like this:
data Fun = forall a b . F (a -> b) type Callables = Map String Fun
Sorry, but using GADT syntax: data Fun where F :: (Typeable a, Typeable b) => (a -> b) -> Fun
I would like to be able to write:
invoke :: Callables -> a -> String -> b invoke m d k = case lookup k m of Just (F f) -> f d Nothing -> error $ "unable to find function for key " ++ k
Untested: invoke :: (Typeable a, Typeable b) => Callables -> a -> String -> Maybe b invoke m d k = do F f <- lookup k m arg <- cast d cast (f arg) HTH! -- Felipe.