[Haskell-cafe] Applying a value to a function generically

[Felipe Lessa]

On Wed, Jul 14, 2010 at 8:25 AM, Arnaud Bailly <arnaud.oqube at gmail.com> wrote:
> 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.html#v%3Acast

> 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.