5.5.8.2 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.