On 08/01/06, Bruno Oliveira <[EMAIL PROTECTED]> wrote: > Hello, > > The following class definition: > > > class Foo o where > > (:+) :: o -> o -> o > > and even the following function definition: > > > bar f (x,y) = x :+ y > > are accepted by GHC. However, when I try to create > one instance of Foo: > > > instance Foo Int where > > x :+ y = x + y > > I get the following error message: > > Parsing.lhs:7:5: > Pattern bindings (except simple variables) not allowed in instance > declarations > x :+ y = x + y > > The same error still occurs if I change the infix operator to be (:+:). > However, if I define: > > > class Foo3 o where > > (<+>) :: o -> o -> o > > > instance Foo3 Int where > > x <+> y = x + y > > Everything works as expected. > > The only explanation that I have is that this is a (parsing) bug in GHC... > > This is probably related to the fact that > > > (:+) :: Int -> Int -> Int > > f :+ g = f + g > > is an invalid definition (it complains that ":+" is not a data constructor). > > I have not tried this code in other Haskell compiler (like Hugs) or even > previous versions of GHC. I would be interested to know how do those > behave. > > Cheers, > > Bruno >
Infix operators which start with a colon are reserved for use as data constructors. Names starting with an uppercase letter are reserved in the same way. You can define a type: data Complex a = a :+ a and write values of type Complex Double like (1.0 :+ pi), but you can't use :+ as the name of an ordinary function. I'm not sure if it's ideal that the class declaration is allowing that type signature to occur, but afaict, the syntax does permit it. - Cale _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
