type class problem

[Martin Sulzmann]

There's another possible fix which makes use of scoped variables.

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::t1,t2::t2) = prj t
          ^^^^^^^^^^^^^^
         scoped variables

Martin



 > 
 > Dean Herington wrote:
 > > Can someone explain why the following doesn't work?
 > 
 > > {-# OPTIONS -fglasgow-exts #-}
 > 
 > > class R r where
 > >   rId :: r -> String
 > 
 > > class (R r) => RT r t where
 > >   rtId :: r -> t -> String
 > 
 > > data 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
 > 
 > You need a functional dependency. For example:
 > 
 > class TPair t t1 t2 | t->t1 t2 where
 >   prj :: t -> (t1,t2)
 >   inj :: (t1,t2) -> t
 > 
 > with this definition, the typechecker is satisfied.
 > 
 > Without the dependency, the compiler assumes that there may be several
 > instances:
 > 	TPair t t1 t2
 > and
 > 	TPair t t1' t2'
 > 
 > You claimed that RT r1 t1 and RT r2 t2 holds. But you didn't promise
 > that RT r1 t1' and RT r2 t2' will also hold. In other words,
 > 	(RT r1 t1, RT r2 t2, TPair t t1 t2)
 > reads as
 > 	(exists t1 t2. RT r1 t1, RT r2 t2, TPair t t1 t2)
 > rather than
 > 	(forall t1 t2. RT r1 t1, RT r2 t2, TPair t t1 t2)
 > (which you need to guarantee that the definition of (t1,t2) = prj t
 > can be typechecked). Notice that forall is _inside_ of parentheses,
 > on the assumption side (the negative side). 
 > 
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