John Meacham wrote:

I was wondering if we could make Text.Printf extensible to user defined types.

as far as I can tell, all that is needed is to export PrintfArg so that users can define new instances. it would also be nice if UPrintf could be switched from the current union type to a higher order function, since the only thing UPrintf is used for is formating strings. I am thinking something like

newtype UPrintf = UPrintf (UFmt -> ShowS)

data Adjustment = LeftAdjust | ZeroPad

data UFmt = UFmt { fmtAdjust :: Maybe Adjustment, fmtFieldWidth :: Maybe Int, fmtPrecision :: Maybe Int, fmtCharacter :: Char }

Or do away with UPrintf completely: class PrintfArg a where formatArg :: a -> UFmt -> Maybe ShowS (in other words, type UPrintf = UFmt -> Maybe ShowS). It must return a Maybe because the formatting char might be wrong. We would probably have to supply some combinators for building PrintfArg instances, they share a lot of code. Cheers, Simon