On 10 Dec 2004, at 15:06, GoldPython wrote:
I'm missing something, a functional idiom, whatever. I know there's "length" to count the elements in a list and it works fine, but the function below stack overflows on a large list.
countLines [] = 0 countLines (_:ls) = 1 + countLines ls
I would have thought that this was tail recursive and would be flattened into iteration by the compiler. Can anyone explain why? Is it because the call is embedded in an expression?
If you write it in prefix form: countLines (_:ls) = (+)(1,countLines ls) ...you can see that it is not tail recursive. Tail recursive means that the result of the function *is* the result of the recursive call. The technique you have to use to make functions like this tail recursive is to pass along an accumulating parameter. See: http://www.haskell.org/hawiki/TailRecursive The textbook answer to your problem is something like this: countLines l = countLines' 0 l where countLines' n [] = n countLines' n (_:ls) = countLines' (n+1) ls Jules