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

[ChrisK]

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

(This needs -fglasgow-exts or some flags)

> module Main where
> 
> class Interface x where
>   withName :: x -> String
> 
> data A = A String
> 
> instance Interface A where
>   withName (A string) = "< Interface A with " ++ string ++ " >"
> 
> data B = B Int
> 
> instance Interface B where
>   withName (B int) = "< Interface B with " ++ show int ++ " >"
> 
> data WrapInterface where
>        WrapInterface :: forall z. Interface z => z -> WrapInterface
> 
> a :: A
> a = A "seven"
> 
> b :: B
> b = B 7
> 
> listOfWrapInterface :: [WrapInterface]
> listOfWrapInterface = [ WrapInterface a
>                       , WrapInterface b
>                       , WrapInterface (A "()")
>                       , WrapInterface (B (-2007))
>                       ]
> 
> nameOfWrapped :: WrapInterface -> String
> nameOfWrapped (WrapInterface q) = withName q
> 
> instance Interface WrapInterface where
>   withName = nameOfWrapped
> 
> main = do
>   putStrLn (show (map nameOfWrapped listOfWrapInterface))
>   putStrLn (show (map withName listOfWrapInterface))
> 

In ghci this prints:

*Main> main
["< Interface A with seven >","< Interface B with 7 >","< Interface A with ()
>","< Interface B with -2007 >"]
["< Interface A with seven >","< Interface B with 7 >","< Interface A with ()
>","< Interface B with -2007 >"]