List types

[Fergus Henderson]

On 01-Dec-2000, Eric Allen Wohlstadter <wohlstad@cs.ucdavis.edu> wrote:
> Is there a way to make a list that contains multiple types? If not, why,
> and isn't this a serious restriction? I would like to be able to say:
> 
> map show ['a',5]
> 
> Since both Char and Num derive Show it seems like a reasonable thing to
> do.

You can't do it in standard Haskell, but with ghc extensions you can
achieve the same effect.  For example, the following program

	data Showable = forall a . Show a => Showable a
	instance Show Showable where
		show (Showable x) = show x
	
	example = map show [Showable 'a', Showable 5]

	main = print example

produces the output

	["'a'","5"]

See http://www.haskell.org/ghc/docs/latest/set/existential-quantification.html
for more details.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.