Rebinding (:) -- built-in syntax, or just another constructor?

[dons]

This program is valid in Hugs 2005, but not in GHC:

import Prelude (print,(<),Bool(..))

data Cond a = a : a

infixl 0 ?
infixl 1 :

(?) :: Bool -> Cond a -> a
True  ? (x : _) = x
False ? (_ : y) = y

main = print (1 < 2 ? "yeah" : "no!")

Hugs responds with:

Main> main
"yeah"

GHCi says:

$ ghci A.hs
A.hs:4:16: Illegal binding of built-in syntax: :

Which one is right?

[simonmar]

GHC is right here, I think. It's disallowed by the Haskell 98 syntax: a data declaration can define a consym, which excludes all reservedop, and : is one of the reservedops.

Of course it's convenient for the compiler to allow you to define :, because then the Prelude can define the built-in list type, which is probably why Hugs allows this. GHC also does this, but adds extra checks to disallow defining : outisde of the standard libraries, as per Haskell 98.