Brian Hulley wrote:
Brian Hurt wrote:
nth 0 (x:xs) = Some x nth i (x:xs) = if i < 0 then Empty else nth (i-1) xs nth i [] = Empty [blows stack on large i]
As other people have pointed out, this is due to laziness. I'd write it like:
nth 0 (x:_) = Some x nth i (_:xs) = of i < 0 then Empty else (nth $! i-1) xs ^^^ -- oops!
nth _ [] = Empty
I think the above code would be more efficient written as: nth n xs = if n < 0 then Empty else nth' n xs where nth' 0 (x:_) = Some x nth' i (_:xs) = (nth' $! i - 1) xs nth' _ _ = Empty since the first 2 cases deal with every situation where we've got a non-empty list and an index >= 0 (so no need to keep checking that the index is >= 0 in the second case - instead the test can be moved outside the "loop") and there's no need to pattern match against [] in the last case because we already know this must be an empty list. (Though in general it's probably a good idea to make each pattern as independent as possible when writing a function so that if some cases of a function are later edited there's more chance of the compiler being able to lead you to errors via the "incomplete patterns" warning - hopefully the compiler can optimize out any redundant checks) Brian. -- http://www.metamilk.com