Bas van Dijk writes (to the haskell-prime list):
The Clean Report[1] is not really clear on that but to my knowledge nested guards do not have fall-through semantics.
The report does mention this:
"To ensure that at least one of the alternatives of a nested guard will be successful, a nested guarded alternative must always have a 'default case' as last alternative."
I don't think this explanation is accurate. The reason that a nested guard must have a default case is syntactical, otherwise there could be the dangling-else ambiguity. If I remember correctly, the compiler can handle nested- guards with fall-throughs just fine. There have been proposals to use the offside rule to resolve the ambiguity, but I can't recall if this was implemented. See <http://mailman.science.ru.nl/pipermail/clean-list/ 1997/000175.html> for some examples. Cheers, Ronny Wichers Schreur