On Sat, Dec 11, 2010 at 08:45:22AM -0800, Russ Abbott wrote:

I have a data type for which I want to generate my own show string in a few cases, e.g.,

*data *T = T1 ... | T2 ... | T3 ... ...

I'd like to do something like this,

*instance *Show T *where* show T5 ... = <something special> show t = showTheDerived t

Is there a way to do something like that simply?

Unfortunately, there isn't quite. There are two solutions that I can think of, both slightly sub-optimal: (1) Just derive the normal Show instance for T. Then make a newtype wrapper around T, newtype NiceT = NiceT T and give T2 a show instance like instance Show T2 where show (NiceT (T5 ...)) = <something special> show (NiceT t) = show t that way when you want a T shown specially you can wrap it in a NiceT constructor. (2) Make a new type class class Pretty p where ppr :: p -> String and write an instance of it for T in terms of T's derived Show instance. This solution satisfies those people who argue that Show should always print out valid Haskell expressions that could be, say, pasted into ghci (and I admit to leaning in that direction personally) and using a different class for human-readable, pretty-printed representations. But of course the nice thing about Show is that it gets used automatically for values in ghci, and with this solution you have to explicitly wrap things in calls to ppr. -Brent