type class problem

29 Sep 2003

      Can someone explain why the following doesn't work?  Is there some other 
way to achieve the same effect (declaring a set of instances for pair-like 
types in one go)?

Thanks.
Dean

swan(108)% cat Test1.hs
{-# OPTIONS -fglasgow-exts #-}

class R r where
  rId :: r -> String

class (R r) => RT r t where
  rtId :: r -> t -> String

data (R r1, R r2) => RPair r1 r2 = RPair r1 r2

instance (R r1, R r2) => R (RPair r1 r2) where
  rId (RPair r1 r2) = "RPair " ++ rId r1 ++ " " ++ rId r2

class TPair t t1 t2 where
  prj :: t -> (t1,t2)
  inj :: (t1,t2) -> t

instance (RT r1 t1, RT r2 t2, TPair t t1 t2) => RT (RPair r1 r2) t where
  rtId (RPair r1 r2) t = "RT (RPair " ++ rtId r1 t1 ++ " " ++ rtId r2 t2 ++ ")"
   where (t1,t2) = prj t

main = return ()
swan(109)% ghci Test1.hs
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 5.04.3, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... linking ... done.
Loading package haskell98 ... linking ... done.
Compiling Main             ( Test1.hs, interpreted )

Test1.hs:19:
    Could not deduce (RT r2 t21, RT r1 t11)
        from the context (RT (RPair r1 r2) t,
                          RT r1 t1,
                          RT r2 t2,
                          TPair t t1 t2,
                          R (RPair r1 r2))
    Probable fix:
        Add (RT r2 t21, RT r1 t11) to the class or instance method `rtId'
    arising from use of `rtId' at Test1.hs:19
    In the first argument of `(++)', namely `rtId r2 t2'
    In the second argument of `(++)', namely `(rtId r2 t2) ++ ")"'

Test1.hs:20:
    Could not deduce (TPair t t11 t21)
        from the context (RT (RPair r1 r2) t,
                          RT r1 t1,
                          RT r2 t2,
                          TPair t t1 t2,
                          R (RPair r1 r2))
    Probable fix:
        Add (TPair t t11 t21) to the class or instance method `rtId'
    arising from use of `prj' at Test1.hs:20
    In a pattern binding: prj t
Failed, modules loaded: none.
Prelude>

type class problem

Dean Herington