[Haskell-cafe] A problem with Show

[Donald Bruce Stewart]

zdenes:
> Hello,
> 
> I made a simple datatype called Pair and I'd like to make it an instance of Show class. But when I try to do that I run into troubles:
> 
> 
> data Pair a b = Pair a b
> instance Show a b => Show (Pair a b) where show (Pair a b) = show a ++ "_" ++ show b
> 
> 
> In Hugs I get this error: "Haskell does not support multiple parameter classes" When I run it in ghci I get a kind error "Show is applied to too many type arguments in the instance declaration for Show (Pair a b)"
> 
> Am I jsut using wrong syntax or is it not possible to do?
> 

Missing some parens and a comma:

> data Pair a b = Pair a b
>
> instance (Show a, Show b) => Show (Pair a b) where
>     show (Pair a b) = show a ++ "_" ++ show b

$ hugs A.hs
Main> show (Pair 'x' ())
"'x'_()"

Alternately:

> data Pair a b = Pair a b
>       deriving Show

-- Don