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121 lines
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HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Why Use Keywords? (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Keywords.html" rel="up" title="Keywords">
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<div class="subsubsection-level-extent" id="Why-Use-Keywords_003f">
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<p>
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Next: <a href="Coding-With-Keywords.html" accesskey="n" rel="next">Coding With Keywords</a>, Up: <a href="Keywords.html" accesskey="u" rel="up">Keywords</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<h4 class="subsubsection" id="Why-Use-Keywords_003f-1"><span>6.6.7.1 Why Use Keywords?<a class="copiable-link" href="#Why-Use-Keywords_003f-1"> ¶</a></span></h4>
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<p>Keywords are useful in contexts where a program or procedure wants to be
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able to accept a large number of optional arguments without making its
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interface unmanageable.
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</p>
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<p>To illustrate this, consider a hypothetical <code class="code">make-window</code>
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procedure, which creates a new window on the screen for drawing into
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using some graphical toolkit. There are many parameters that the caller
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might like to specify, but which could also be sensibly defaulted, for
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example:
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</p>
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<ul class="itemize mark-bullet">
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<li>color depth – Default: the color depth for the screen
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</li><li>background color – Default: white
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</li><li>width – Default: 600
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</li><li>height – Default: 400
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</li></ul>
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<p>If <code class="code">make-window</code> did not use keywords, the caller would have to
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pass in a value for each possible argument, remembering the correct
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argument order and using a special value to indicate the default value
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for that argument:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(make-window 'default ;; Color depth
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'default ;; Background color
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800 ;; Width
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100 ;; Height
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...) ;; More make-window arguments
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</pre></div>
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<p>With keywords, on the other hand, defaulted arguments are omitted, and
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non-default arguments are clearly tagged by the appropriate keyword. As
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a result, the invocation becomes much clearer:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(make-window #:width 800 #:height 100)
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</pre></div>
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<p>On the other hand, for a simpler procedure with few arguments, the use
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of keywords would be a hindrance rather than a help. The primitive
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procedure <code class="code">cons</code>, for example, would not be improved if it had to
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be invoked as
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(cons #:car x #:cdr y)
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</pre></div>
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<p>So the decision whether to use keywords or not is purely pragmatic: use
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them if they will clarify the procedure invocation at point of call.
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</p>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Coding-With-Keywords.html">Coding With Keywords</a>, Up: <a href="Keywords.html">Keywords</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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