Re: [Haskell-cafe] type/class question: toString

On Tue, 2007-11-06 at 09:18 -0500, Graham Fawcett wrote:

Hi folks,

Is there a way to declare a 'toString' function, such that

toString x | <x is a String> = x toString x | = show x

Perhaps, in the type system, there's a way to declare a ToString class, and somehow "inherit" all instances of Show as ToString instances?

I think the simpler solution (for your particular problem) is to tag strings that should be printed as-is: newtype Literal = Literal String instance Show Literal where show (Literal x) = x lit :: String -> Literal lit = Literal I generally try to keep the law read . show == id Thus, for anything that needs to be printed in a nicer way I use something like this: class PPrint a where pretty :: a -> Doc