On Sat, May 19, 2007 at 04:10:29PM +0100, Jon Fairbairn wrote: [...]
foldl f z l = case l of (x:xs) -> foldl f (f z x) xs [] -> z
which reveals that foldl in the RHS isn't really the leftmost outermost function, but case is -- the tail call is to case. It happens that case is a kind of function that makes a choice -- it returns one of its arguments but doesn't do anything more to it. In a strict language only some predefined operators like case and if have this property, but lazy languages allow the definition of new ones, which is why we need more care when thinking about tail calls.
By definition, tail recursive function is recursive function in which the value returned from a recursive call is immediately returned without modification as value of the top-level invocation, which is true for foldl defined as above.
-- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk