[Haskell-cafe] Can this be improved?
u.stenzel at web.de
Fri Dec 30 08:21:53 EST 2005
Chris Kuklewicz wrote:
> > *MyShow> main
> > Hello" ""World #"[17,18,19]"!"
> > I also think [4,5,6]" and "7" are ""cool""."
> The extra double quotes being what I am trying to avoid with MyShow
This is your only special case, a single list of a sigle type that is
different from other lists? To avoid undecidable, incoherent and other
nasty instances, just do what the Prelude does:
class MyShow t where
swim :: t -> String
swimList :: [t] -> String
swimList  = ""
swimList xs = '[' : foldr1 (\x -> (swim x ++) . (',' :)) "]" xs
instance MyShow Char where
swim = (:)
swimList = id
Untested, but you get the idea. It's pure Haskell 98. Heck, it's
almost Prelude.Show. It won't scale to more type constructors easily,
but you didn't ask for that ability, either ;-)
"As far as Clinton supposedly cheating on his wife, what do people think
he's going to do? Be president of another country while he's president
-- Tom R., age 12
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