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21.7.2 Coding With Keywords

If a procedure wants to support keywords, it should take a rest argument and then use whatever means is convenient to extract keywords and their corresponding arguments from the contents of that rest argument.

The following example illustrates the principle: the code for make-window uses a helper procedure called get-keyword-value to extract individual keyword arguments from the rest argument.

(define (get-keyword-value args keyword default)
  (let ((kv (memq keyword args)))
    (if (and kv (>= (length kv) 2))
        (cadr kv)
        default)))

(define (make-window . args)
  (let ((depth  (get-keyword-value args #:depth  screen-depth))
        (bg     (get-keyword-value args #:bg     "white"))
        (width  (get-keyword-value args #:width  800))
        (height (get-keyword-value args #:height 100))
        ...)
    ...))

But you don't need to write get-keyword-value. The (ice-9 optargs) module provides a set of powerful macros that you can use to implement keyword-supporting procedures like this:

(use-modules (ice-9 optargs))

(define (make-window . args)
  (let-keywords args #f ((depth  screen-depth)
                         (bg     "white")
                         (width  800)
                         (height 100))
    ...))

Or, even more economically, like this:

(use-modules (ice-9 optargs))

(define* (make-window #:key (depth  screen-depth)
                            (bg     "white")
                            (width  800)
                            (height 100))
  ...)

For further details on let-keywords, define* and other facilities provided by the (ice-9 optargs) module, see Optional Arguments.