Simon Peyton-Jones wrote:
Concerning your application
| I am having a box like | > data Box = forall a. Cxt a => Box a | and want to write a Read instances for it.
I don’t see how it helps to defer the Read instance.
I would defer the instance declaration till the point where I know all types that will ever go into the box (for instance in my Main module). The Show instance of Box would write the representation of a together with it's type (this means Cxt should contain at least Show and Typeable). The Read instance will then contain something like ======================================================== String aType <- lexP result <- case aType of "[Int]" -> (readPrec :: ReadPrec [Int]) >>= (return . Box) "SomeOtherType" -> (readPrec :: ReadPrec SomeOtherType) >>= (return . Box) -- <more of such cases> _ -> error (aType ++ " cannot be read inside of Box") ======================================================== Not very elegant of course, but it seems to be the only way to Read existentials.
But I think the case is stronger for top-level constraints, and I will bear it in mind. If you would like to open a Trac feature request, please do so.
Ok, I will. Cheers, Misha