Simon Peyton-Jones wrote:
I agree that if (n+k) patterns go, then so should k patterns. Both are overloaded, and that's the root of their complexity.
Personally I think ~ patterns are great, and we are now talking about ! patterns, a kind of dual to ~ patterns. So at least I think we should un-couple the two discussions.
I agree that it is sensible to decouple the two discussions, so just add k patterns to n+k patterns. However, ~ patterns are really currently the most complicated patterns and ! patterns match them in their complexity. Personally I believe that programmers should strive for more laziness, rarely for more strictness. I do not like that you can add ! in lots of places where it doesn't make any difference, e.g. f [] !x = rhs1 f (y:ys) !x = rhs2 is the same as f [] !x = rhs1 f (y:ys) x = rhs2 Your motivating example f2 !x !y | g x = rhs1 | otherwise = rhs2 I would express as f2 x y = x `seq` y `seq` if g x then rhs1 else rhs2 Now you will probably counter with a definition where you can fall through the guard to the next equation. In my opinion that just shows how horrible guards are (and I would propose their removal if I saw any chance of success). Ciao, Olaf