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Static Variables for Functions

Problem

Solution

(define function-with-static-var
  (let ((static-a 0) (static-b '()))
     (lambda (a-number)
        (set! static-b 
              (cons (list a-number 
                         (set! static-a (+ a-number static-a))
                    static-b)))))

(function-with-static-var 1) => ((1 1))
(function-with-static-var 2) => ((2 3)(1 1))
(function-with-static-var 3) => ((3 6)(2 3)(1 1))
(function-with-static-var 4) => ((4 10)(3 6)(2 3)(1 1))

Discussion

This is also a good way to remove so-called "magic numbers" from code. By defining constants within a let expression they are hidden from the global namespace. This use of the let-before-the-lambda trick remains a pure-functional method of programming.

This method can also avoids unnecessary allocation in any code that uses constant values. For example the code below allocates a new string to hold the magic phrase every time it is called.

(define (magic-phrase name)
  (let ((phrase "abracadabra"))
    (string-append name " the magic phrase is " phrase)))

By using the static variable idiom we can ensure only a single allocation:

(define magic-phrase
  (let ((phrase "abracadabra"))
    (lambda (name)
      (string-append name " the magic phrase is " phrase))))

The general case of this optimisation is known as lambda-lifting.

The term static variables comes from C. C does not provide closures but are limited form of closure can be imitated by declaring variables as static.

Comments about this recipe

This is my first contribution... Hopefully it doesn't suck.

I like it. -- NoelWelsh - 14 Sep 2004

Contributors

-- JonathanArkell - 07 Sep 2004