Louis-Julien Guillemette <[EMAIL PROTECTED]> wrote: > Say we are using a GADT to represent a simple language of boolean > constants and conditionals, > > data Term a where > B :: Bool -> Term Bool > Cnd :: Term Bool -> Term t -> Term t -> Term t > > and we would like to perform a type-safe CPS conversion over this language. > We > encode the relationship between the types in the source language and those > in > the CPS-converted language using a class and a fundep: > > class CpsForm a b | a -> b > instance CpsForm Bool Bool > > A natural formulation of the type of the CPS transformation would be > > cps :: CpsForm t cps_t => Term t -> (Term cps_t -> Term ()) -> Term () > [...] > Couldn't match the rigid variable `cps_t' against `Bool' [...] > > This case seems to be an instance of a problem discussed earlier. > (http://www.haskell.org/pipermail/haskell-cafe/2005-August/011053.html) > > Any suggestion on a way to get around this problem in current Haskell? >
My way around this is to replace instance derivations by GADT elements, something like: data CpsForm :: * -> * -> * where CpsBool :: CpsForm Bool Bool cps :: CpsForm t cps_t -> Term t -> (Term cps_t -> Term ()) -> Term () YMMV! Wolfram _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
