On 8 May 2011 06:14, Nicholas Tung
Dear all, I'd like to write a function "maybeShow :: a -> Maybe String", which runs "show" if its argument is of class Show.
I'm pretty sure this is not readily possible - there might be some hack through Typeable but that would oblige but Show and Typeable constraints on the type of "a".
The context and motivation for this are as follows. I have a GADT type which encapsulates abstract-value computation (or constants or error codes), a snippet of which is below. data AV t where AVLeft :: AV a -> AV (Either a b) This is used to implement an arrow transformer, and due to Arrows mapping all Haskell functions, I cannot put some kind of qualification on the constructor, like "AVLeft :: Show a => ...".
Yes you can, from the GHC docs: http://haskell.org/ghc/docs/7.0-latest/html/users_guide/data-type-extensions... data Showable where MkShowable :: Show a => a -> Showable