Tillmann Rendel wrote:
Now consider a variant:
if' a b c = if a then b else c variant x = if' (p x) (f x) (g x)
I would say that if' has the same operational behavior as an if-then-else expressions, and therefore, (f x) and (g x) are still tail cails, even if they now appear in the context of another function call.
yes, and the if' is also a tail-call. I guess I would say that (f x) and (g x) are tail-calls out of if' and if' is a tail-call out of variant.
I think that a definition of tail calls in Haskell should take the strictness properties of functions into account.
sometimes the optimizer does this, and makes more functions behave like tail- call than you'd expect! But sometimes it doesn't.
And I think that this makes for a much more friendly migration path. Instead of telling people: "No, forget about tail calls, in Haskell, you need something else", you could tell them: "Yes, tail calls are the way to go, but in Haskell, they look different".
Sometimes you do need something different though... if you're *producing* a lazy data structure. foldr f z (x:xs) = x `f` (foldr f z xs) If "f" is ":" then this is an efficient non-tail-call (later someone will demand the later parts of the list) whereas foldl f z (x:xs) = foldl f (z `f` x) xs is tail-recursion, which means that it needs to traverse the whole input-list before producing *any* output -Isaac