Tail Call - Description

Description

When a function is called, the computer must "remember" the place it was called from, the return address, so that it can return to that location with the result once the call is complete. Typically, this information is saved on the call stack, a simple list of return locations in order of the times that the call locations they describe were reached. For tail calls, there is no need to remember the place we are calling from — instead, we can perform tail call elimination by leaving the stack alone (except possibly for function arguments and local variables), and the newly called function will return its result directly to the original caller. Note that the tail call doesn't have to appear lexically after all other statements in the source code; it is only important that the calling function return immediately after the tail call, returning the tail call's result if any, since the calling function will never get a chance to do anything after the call if the optimization is performed.

For non-recursive function calls, this is usually an optimization that saves little time and space, since there are not that many different functions available to call. When dealing with recursive or mutually recursive functions where recursion happens through tail calls, however, the stack space and the number of returns saved can grow to be very significant, since a function can call itself, directly or indirectly, many times. In fact, it often asymptotically reduces stack space requirements from linear, or O(n), to constant, or O(1). Tail call elimination is thus required by the standard definitions of some programming languages, such as Scheme, and languages in the ML family among others. In the case of Scheme, the language definition formalizes the intuitive notion of tail position exactly, by specifying which syntactic forms allow having results in tail context. Implementations allowing an unlimited number of tail calls to be active at the same moment, thanks to tail call elimination, can also be called 'properly tail-recursive'.

Besides space and execution efficiency, tail call elimination is important in the functional programming idiom known as continuation passing style (CPS), which would otherwise quickly run out of stack space.

Read more about this topic:  Tail Call

Famous quotes containing the word description:

    Do not require a description of the countries towards which you sail. The description does not describe them to you, and to- morrow you arrive there, and know them by inhabiting them.
    Ralph Waldo Emerson (1803–1882)

    Why does philosophy use concepts and why does faith use symbols if both try to express the same ultimate? The answer, of course, is that the relation to the ultimate is not the same in each case. The philosophical relation is in principle a detached description of the basic structure in which the ultimate manifests itself. The relation of faith is in principle an involved expression of concern about the meaning of the ultimate for the faithful.
    Paul Tillich (1886–1965)

    The great object in life is Sensation—to feel that we exist, even though in pain; it is this “craving void” which drives us to gaming, to battle, to travel, to intemperate but keenly felt pursuits of every description whose principal attraction is the agitation inseparable from their accomplishment.
    George Gordon Noel Byron (1788–1824)