[Haskell] Parsing bug in GHC 6.4.1 ?

[Cale Gibbard]

On 08/01/06, Bruno Oliveira <bruno.oliveira at comlab.ox.ac.uk> 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