Lennart Augustsson wrote:
Sorting by constructor tag is perfectly safe when done right. You can read about how to do it in my 1985 FPCA paper or in Simon's book.
When pattern matching against against things that that are not constructors (like literals etc) it's much trickier to reorder them since you have to prove harder pattern commutation properties.
I don't think there is any controversy at all about Haskell pattern matching semantics. As you say, it's pretty clearly spelled out. (It wouldn't hurt to have it written down as a denotational semantics, though.)
And ghc happens to have a bug.
Just thought I'd mention this other bug in the same area: http://hackage.haskell.org/trac/ghc/ticket/246 (Wrong pat-match order for records) Cheers, Simon