On 7 Mar 2011, at 23:38, Alexander Solla wrote:
   _|_ /= (_|_,_|_)

> (undefined, undefined)
(*** Exception: Prelude.undefined

That is as close to Haskell-equality as you can get for a proto-value that does not have an Eq instance.  As a consequence of referential transparency, evaluation induces an equivalence relation.  This implies that (_|_, _|_) = _|_ = (_|_, _|_).

Surely the key thing is the '(' character which is produced immediately before the exception is encountered.

I'd say that demonstrates that in GHC  _|_ /= (_|_,_|_).

--Ben