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Optional arguments

Problem

Some functions take arguments that only in exceptional cases are different from a default value. Optional arguments makes it possible only to give the extra arguments in the exceptional case.

Solution

Two different solutions are presented below.

The most direct us using opt-lambda. The opt-lambda form behaves like a normal lambda form, except that a default value can be attached to a parameter by replacing parameter with [parameter default-value-expression]. Note that all variables after an optional argument must also have default value expression.

The second solution uses case-lambda to catch the cases where the optional arguments are missing. They are then provided in a recursive call.

The example features the function count-down. In most cases a count down to 0 is wanted, but the end point can be given.

;; (count-down n m) = (list n n-1 n-2 ... m)
;; (count-down n)   = (list n n-1 ... 0)

; (interval s t) = (s s+1 ... s+(t-s))
(define (interval s t)
  (if (> s t)
    '()
    (cons s (interval (+ s 1) t))))


(define count-down
  (opt-lambda (n [m 0])
    (reverse (interval (+ (- n m) 1) m))))

(count-down 10)    ; => (10 9 8 7 6 5 4 3 2 1 0)
(count-down 10 4)  ; => (10 9 8 7 6 5 4)

(define count-down
  (case-lambda
    [(n)   (count-down n 0)]
    [(n m) (reverse (interval (+ (- n m) 1) m))]))

(count-down 10)    ; => (10 9 8 7 6 5 4 3 2 1 0)
(count-down 10 4)  ; => (10 9 8 7 6 5 4)

Discussion

The case-lambda construct is a primitive in PLT Scheme. In other Schemes remember to load SRFI 16 (Syntax for procedures of variable arity. )

The opt-lambda macro is implemented in terms of case-lambda.

Contributors

-- JensAxelSoegaard -- NoelWelsh

Comments

I think it would be best not to require SRFI-1 to use "iota" in an example. The use looked complicated, and is not standard Scheme, when the intent is to illustrate optional arguments. Below is an R5RS count-down, but perhaps simpler example like incremented would be best (if that's not too simple):

(require (lib "etc.ss"))

(define count-down
  (opt-lambda (start (end 0))
    (let loop ((i start))
      (if (< i end)
          '()
          (cons i (loop (- i 1)))))))

(require (lib "etc.ss"))

(define incremented
  (opt-lambda (number (increment 1))
    (+ number increment)))

(incremented 42)    ; => 43
(incremented 42 27) ; => 69

Also we should note that opt-lambda requires (lib "etc.ss").

-- NeilVanDyke - 17 May 2004

I renamed iota to interval and defined without using SRFI9.

-- JensAxelSoegaard - 21 May 2004

I just did some minor corrections: - Added ")" at the end of "interval", which was missing. - Changed the description above "interval", which now uses only s and t parameters to explain the function.

-- RudiBonfiglioli - 01 August 2010