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.
To handle keyword arguments from procedures implemented in C,
use scm_c_bind_keyword_arguments (see Keyword Procedures).