(guile.info.gz) Coding With Keywords

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(guile.info.gz) Keyword Read Syntax

 
 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.
 

Info Catalog

(guile.info.gz) Why Use Keywords?

(guile.info.gz) Keywords

(guile.info.gz) Keyword Read Syntax