On Mon, Jan 19, 2009 at 9:10 AM, ChrisK
Consider that the order of pattern matching can matter as well, the simplest common case being zip:
zip xs [] = [] zip [] ys = [] zip (x:xs) (y:ys) = (x,y) : zip xs ys
If you are obsessive about least-strictness and performance isn't a giant concern, this seems like a perfect use for Conal's unamb[1] operator. zipR xs [] = [] zipR [] ys = [] zipR (x:xs) (y:ys) = (x,y) : zip xs ys zipL [] ys = [] zipL xs [] = [] zipL (x:xs) (y:ys) = (x,y) : zip xs ys zip xs ys = unamb (zipL xs ys) (zipR xs ys) This runs both zipL and zipR in parallel until one of them gives a result; if neither of them is _|_ they are guaranteed to be identical, so we can "unambiguously choose" whichever one gives a result first. -- ryan [1] http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice/