doc/parts/extensions/program-flow.sxml

   1 <?xml version="1.0" encoding="UTF-8"?>
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  18 <!--                                                                        -->
  19 <!-- Copyright 2009 Henrik Tidefelt                                         -->
  20
  21
  22 <book>
  23         <title>program-flow</title>
  24         <description>
  25                 <p>Abstractions for program flow control.</p>
  26         </description>
  27         <meta-selflink><extension-href name="program-flow" /></meta-selflink>
  28         <base href=<!--#expand-next-string-->"$(BASE)" />
  29         <examples-home href=<!--#expand-next-string-->"$(EXAMPLES)" />
  30         <up-link><a part="extensions">All extensions</a></up-link>
  31         <prelude />
  32         <needs></needs>
  33         <top>
  34                 <alphabetical-index/>
  35                 <p>The primary tools that the <str-Shapes /> core language provides for program flow control and conditional evaluation, are the escape continuations and the <binding name="if" /> function.  This extension provides abstractions for some other common conditional constructs found in other programming languages.</p>
  36         </top>
  37
  38         <section id="extensions/program-flow/cond">
  39                 <title>Multi-clause conditionals</title>
  40                 <body>
  41                         <p>At the moment, there is just one function here.</p>
  42
  43                         <system-binding identifier="cond">
  44                                 <function>
  45                                         <case>
  46                                                 <arguments>
  47                                                         <sink>
  48                                                                 <type><named-type name="ConsPair" /></type>
  49                                                         </sink>
  50                                                 </arguments>
  51                                                 <result>
  52                                                         <type><named-type name="Value" /></type>
  53                                                 </result>
  54                                                 <dynamic-references></dynamic-references>
  55                                                 <description>
  56                                                         <p>Takes any number of <named-type name="ConsPair" /> pairs as arguments, and searches from left for a pair with true <field name="car" /> (predicate) part, returning the corresponding <field name="cdr" /> (consequence) part.</p>
  57                                                         <p>To provide a default result one must simply pass a <named-type name="ConsPair" /> with a predicate part which is just <inline>true</inline>.</p>
  58 <example-with-output title="Using the cond function" internal-id="program-flow/cond-example">
  59 <source file="features/error.shape">
  60 <![CDATA[<!--#include depth="0" virtual="$(BUILDDIR)$(EXAMPLES)features/error.shape" -->]]>
  61 </source>
  62 <stdout>
  63 <![CDATA[<!--#include depth="0" virtual="$(BUILDDIR)$(EXAMPLES)features/error.stdout" -->]]>
  64 </stdout>
  65 <caption>
  66         <p>One application of <binding name="cond" />, and one case where <binding name="cond" /> could have been used, but wasn't.</p>
  67 </caption>
  68 </example-with-output>
  69                                                 </description>
  70                                         </case>
  71                                 </function>
  72                         </system-binding>
  73
  74                 </body>
  75         </section>
  76
  77 </book>