[Haskell-cafe] Re: Dynamically find out instances of classes (pluginsystem for haskell)

Jason Dusek wrote:

ChrisK wrote:

...

the standard way to do that is use an existential wrapper:

Does this relate to the "basket of fruit" problem in object oriented languages?

You created the existential wrapper to allow a multimorphic list type?

When you access the wrapped data then *_ONLY_* thing you can do with it is via the type class(es) that the GADT was wrapping. The example I gave had a single type class:

data WrapInterface where WrapInterface :: forall z. Interface z => z -> WrapInterface

One could have multiple interfaces:

data WrapInterface where WrapInterface :: (Interface z,Show z,Num z) => z -> WrapInterface

One could have more than one piece of data, note that WrapInterface takes three parameters:

data WrapInterface where WrapInterface :: Interface z => z -> z -> z -> WrapInterface

One could do both:

data WrapInterface where WrapInterface :: (Interface z1, Show z2,Num z3) => z1 -> z2 -> z3 -> WrapInterface

And so on. You can even write something like:> data WrapInterface' where

WrapInterface' :: a -> (a->String) -> WrapInterface'

listExample = [ WrapInterface' "Hello" (show . (++ "World")) , WrapInterface' 17 (show . succ) , WrapInterface' True (show . not) ]

apply :: WrapInterface' -> String apply (WrapInterface' item function) = function item

main = do putStrLn (show (map apply listExample))

Now a WrapInterface' holds item "a" and a function "a->String". When you unwrap this in a case statement you can then apply the function to the item to get the String. The output in ghci is: *Main> main ["\"HelloWorld\"","18","False"] -- Chris