Draft Scheme Standard R6RS Released

Watson Ladd writes, "The new version of the official Scheme standard has been released as a draft (PDF)." From the draft: "[This] report gives a defining description of the programming language Scheme. Scheme is a statically scoped and properly tail-recursive dialect of the Lisp programming language invented by Guy Lewis Steele Jr. and Gerald Jay Sussman. It was designed to have an exceptionally clear and simple semantics and few different ways to form expressions. A wide variety of programming paradigms, including imperative, functional, and message passing styles, find convenient expression in Scheme."

Comments

Tail Recursion

[RAMMS EIN]

For those of you who don't know what "properly tail recursive" means, a quick explanation. Consider the following code:

(define (f x) (x x))
(f f)

This defines a function, f, which takes one argument, x, which should be a function (yay, first-class functions!), and calls x upon itself. Then, it calls f on f.

Of course, this will cause f to call f upon itself. Again. And again. Infinite recursion!

Now, proper tail recursion means that if a function call returns in tail position (meaning it is the last thing the surrounding function does before it returns), the activation frame for the surrounding function is replaced by that of the function it calls. Contrast this with normal recursion, where a _new_ activation frame would be created for the called function.

Tail recursion makes the example code above run in bounded memory...looping forever. :-)