5.5.8.1 Why Use Keywords?

Keywords are useful in contexts where a program or procedure wants to be able to accept a large number of optional arguments without making its interface unmanageable.

To illustrate this, consider a hypothetical make-window procedure, which creates a new window on the screen for drawing into using some graphical toolkit. There are many parameters that the caller might like to specify, but which could also be sensibly defaulted, for example:

• color depth – Default: the color depth for the screen
• background color – Default: white
• width – Default: 600
• height – Default: 400

If make-window did not use keywords, the caller would have to pass in a value for each possible argument, remembering the correct argument order and using a special value to indicate the default value for that argument:

     (make-window 'default              ;; Color depth
                  'default              ;; Background color
                  800                   ;; Width
                  100                   ;; Height
                  ...)                  ;; More make-window arguments

With keywords, on the other hand, defaulted arguments are omitted, and non-default arguments are clearly tagged by the appropriate keyword. As a result, the invocation becomes much clearer:

     (make-window #:width 800 #:height 100)

On the other hand, for a simpler procedure with few arguments, the use of keywords would be a hindrance rather than a help. The primitive procedure cons, for example, would not be improved if it had to be invoked as

     (cons #:car x #:cdr y)

So the decision whether to use keywords or not is purely pragmatic: use them if they will clarify the procedure invocation at point of call.