[Haskell-cafe] Can this be improved?

[Chris Kuklewicz]

Happy Holidays,

I was wondering if it this small snippet of code could be altered to
require fewer OPTIONS -fallow-... switches.

It creates a show-what-i-mean function called "swim" that takes a
variable number of arguments, and treats strings as-is, calling show on
the other arguments.

> {-# OPTIONS -fglasgow-exts #-}
> {-# OPTIONS -fallow-undecidable-instances #-}
> {-# OPTIONS -fallow-incoherent-instances #-}
> {-# OPTIONS -fallow-overlapping-instances #-}
> 
> -- Want to show strings "as-is"
> 
> class (Show a) => MyShow a where
>   myShow :: a -> String
>   myShow = show
> 
> instance MyShow String where
>   myShow = id
> 
> instance (Show b) => MyShow b
> 
> -- Want to take variable number of arguments
> 
> class PVShow a r  where
>    pvShow :: String -> a -> r
> 
> instance (MyShow b) => PVShow b String where
>    pvShow oldString b = oldString ++ (myShow b)
> 
> instance (MyShow b,PVShow a r) => PVShow b (a->r) where
>    pvShow oldString b = pvShow (oldString ++ (myShow b))
> 
> -- swim is short for Show What I Mean
> 
> swim :: (PVShow a r) => a -> r
> swim = pvShow ""
> 
> -- Demonstates of polymorphic and first class nature of swim
> 
> test foo = foo " and " 7 " are "
> 
> -- Tests
> 
> main = do putStrLn $ swim "Hello" " " "World #" [17,18,19] "!"
>           putStrLn $ test (swim "I also think " [4,5,6]) "cool" "."

GHCI session:
Main> main
Hello World #[17,18,19]!
I also think [4,5,6] and 7 are cool.
*Main>

Could this be improved by TypeEq machinery?  Is there a Haskell 98 way
to make myShow work this way?

-- 
Chris Kuklewicz