contents/glyph/book/extending/interpreting.html
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title: "Glyph - Interpreting Glyph Code"
content-type: page
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<p>What if you need to evaluate some Glyph code <em>within</em> a macro? Say for example you want to transform a
parameter in a link, and you want to make sure that link gets validated exactly like the others, in this case, you
can use the <code>interpret</code> method, as follows:</p>
<div class="CodeRay">
<div class="code">
<pre><span class="line-numbers"><a href="#n1" name="n1">1</a></span>macro <span class="symbol">:fmi</span> <span class="keyword">do</span>
<span class="line-numbers"><a href="#n2" name="n2">2</a></span> topic, href = <span class="instance-variable">@params</span>
<span class="line-numbers"><a href="#n3" name="n3">3</a></span> link = placeholder <span class="keyword">do</span> |document|
<span class="line-numbers"><a href="#n4" name="n4">4</a></span> interpret <span class="string"><span class="delimiter">"</span><span class="content">link[</span><span class="inline"><span class="inline-delimiter">#{</span>href<span class="inline-delimiter">}</span></span><span class="content">]</span><span class="delimiter">"</span></span>
<span class="line-numbers"><a href="#n5" name="n5">5</a></span> <span class="keyword">end</span>
<span class="line-numbers"><a href="#n6" name="n6">6</a></span> <span class="string"><span class="delimiter">%{</span><span class="content"><span class="fmi">for more information on </span><span class="inline"><span class="inline-delimiter">#{</span>topic<span class="inline-delimiter">}</span></span><span class="content">, see </span><span class="inline"><span class="inline-delimiter">#{</span>link<span class="inline-delimiter">}</span></span><span class="content"></span></span><span class="delimiter">}</span></span>
<span class="line-numbers"><a href="#n7" name="n7">7</a></span><span class="keyword">end</span></pre>
</div>
</div>
<p>When the <code>interpret</code> method is called, the following happens:</p>
<ol>
<li>A new Glyph document is created from the <code>String</code> passed to the method.</li>
<li>Document-specific objects (bookmarks, headers, snippet, fragments, placeholders, etc.) are passed from the main
document to the new one. Because they are stored in arrays and hashes, they are passed by reference, so for
example any new bookmark stored in the new document will also become available in the main document.</li>
<li>Any macro included in the <code>String</code> is evaluated, and the resulting text is returned by the method.
Note that this new document does not get finalized: in other words, placeholders will be left as they are, and
they’ll eventually be replaced when <em>the main document</em> is finalized.</li>
</ol>
<section class="section">
<header>
<h3 id="h_93" class="toc">Dispatching</h1>
</header>
<p><a href="/glyph/book/text_editing/macro_composition.html#composition">Macro Composition</a> can be useful to
remove nesting, but you can also use it to create your own macro <em>dispatchers</em>. What is a macro
dispatcher? The easies way to understand this is by looking at the source code of one of them, the <a
href="/glyph/book/macros/macros_core.html#m_s"><code>s</code></a> macro:</p>
<div class="CodeRay">
<div class="code">
<pre><span class="line-numbers"> <a href="#n1" name="n1">1</a></span>macro <span class="symbol">:s</span> <span class="keyword">do</span>
<span class="line-numbers"> <a href="#n2" name="n2">2</a></span> dispatch <span class="keyword">do</span> |node|
<span class="line-numbers"> <a href="#n3" name="n3">3</a></span> forbidden = [<span class="symbol">:each</span>, <span class="symbol">:each_line</span>, <span class="symbol">:each_byte</span>, <span class="symbol">:upto</span>, <span class="symbol">:intern</span>, <span class="symbol">:to_sym</span>, <span class="symbol">:to_f</span>]
<span class="line-numbers"> <a href="#n4" name="n4">4</a></span> meth = node[<span class="symbol">:name</span>]
<span class="line-numbers"> <a href="#n5" name="n5">5</a></span> infer_type = lambda <span class="keyword">do</span> |str|
<span class="line-numbers"> <a href="#n6" name="n6">6</a></span> <span class="comment"># Code omitted...</span>
<span class="line-numbers"> <a href="#n7" name="n7">7</a></span> <span class="keyword">end</span>
<span class="line-numbers"> <a href="#n8" name="n8">8</a></span> macro_error <span class="string"><span class="delimiter">"</span><span class="content">Macro 's/</span><span class="inline"><span class="inline-delimiter">#{</span>meth<span class="inline-delimiter">}</span></span><span class="content">' takes at least one parameter</span><span class="delimiter">"</span></span> <span class="keyword">unless</span> node.params.length > <span class="integer">0</span>
<span class="line-numbers"> <a href="#n9" name="n9">9</a></span> macro_error <span class="string"><span class="delimiter">"</span><span class="content">String method '</span><span class="inline"><span class="inline-delimiter">#{</span>meth<span class="inline-delimiter">}</span></span><span class="content">' is not supported</span><span class="delimiter">"</span></span> <span class="keyword">if</span> meth.in?(forbidden) || meth.to_s.match(<span class="regexp"><span class="delimiter">/</span><span class="content">!$</span><span class="delimiter">/</span></span>)
<span class="line-numbers"><strong><a href="#n10" name="n10">10</a></strong></span> str = node.param(<span class="integer">0</span>).evaluate(node, <span class="symbol">:params</span> => <span class="predefined-constant">true</span>)
<span class="line-numbers"><a href="#n11" name="n11">11</a></span> <span class="keyword">begin</span>
<span class="line-numbers"><a href="#n12" name="n12">12</a></span> <span class="keyword">if</span> node.param(<span class="integer">1</span>) <span class="keyword">then</span>
<span class="line-numbers"><a href="#n13" name="n13">13</a></span> meth_params = node.params[<span class="integer">1</span>..node.params.length-<span class="integer">1</span>].map <span class="keyword">do</span> |p|
<span class="line-numbers"><a href="#n14" name="n14">14</a></span> infer_type.call(p.evaluate(node, <span class="symbol">:params</span> => <span class="predefined-constant">true</span>))
<span class="line-numbers"><a href="#n15" name="n15">15</a></span> <span class="keyword">end</span>
<span class="line-numbers"><a href="#n16" name="n16">16</a></span> str.send(meth, *meth_params).to_s
<span class="line-numbers"><a href="#n17" name="n17">17</a></span> <span class="keyword">else</span>
<span class="line-numbers"><a href="#n18" name="n18">18</a></span> str.send(meth).to_s
<span class="line-numbers"><a href="#n19" name="n19">19</a></span> <span class="keyword">end</span>
<span class="line-numbers"><strong><a href="#n20" name="n20">20</a></strong></span> <span class="keyword">rescue</span> <span class="constant">Exception</span> => e
<span class="line-numbers"><a href="#n21" name="n21">21</a></span> <span class="comment"># Code omittted</span>
<span class="line-numbers"><a href="#n22" name="n22">22</a></span> <span class="keyword">end</span>
<span class="line-numbers"><a href="#n23" name="n23">23</a></span> <span class="keyword">end</span>
<span class="line-numbers"><a href="#n24" name="n24">24</a></span><span class="keyword">end</span></pre>
</div>
</div>
<p>See the <code>dispatch</code> method at the very beginning? This method takes a block with a <code>node</code>
parameter, corresponding to the MacroNode of the macro which is being composed with <code>s</code>. So, for
example, if you write <code>s/sub[my string|/my/|your]</code> the node of a macro called <code>sub</code> will
be passed to the block. Of course there’s no <code>sub</code> macro defined in Glyph, but it doesn’t
matter: its name will be interpreted as the name of a method of the Ruby String class in this case, so no
worries.</p>
<p>Got it? Tricky, but damn useful to create your own “dynamic” macros.</p>
</section>
<section class="section">
<header>
<h3 id="rewriting" class="toc">Defining macros using Glyph</h1>
</header>
<p>While the <code>interpret</code> method is useful to evaluate Glyph code in a macro while performing other
actions (storing a bookmark, checking for the presence of an anchor, etc.), in some cases it may not be
necessary. If you simply want your macro to be converted into existing Glyph macro without performing any action
excepting parameter substitution, you can just use the <a
href="/glyph/book/macros/macros_core.html#m_define_"><code>define:</code></a> macro within your Glyph
document</p>
<p>Consider the following macro definition:</p>
<div class="CodeRay">
<div class="code">
<pre><span class="line-numbers"><a href="#n1" name="n1">1</a></span>macro <span class="symbol">:issue</span> <span class="keyword">do</span>
<span class="line-numbers"><a href="#n2" name="n2">2</a></span> interpret <span class="string"><span class="delimiter">%{</span><span class="content"></span></span>
<span class="line-numbers"><a href="#n3" name="n3">3</a></span><span class="string"><span class="content"> tr[</span></span>
<span class="line-numbers"><a href="#n4" name="n4">4</a></span><span class="string"><span class="content"> td[/=>[http://github.com/h3rald/glyph/issues/closed#issue/</span><span class="inline"><span class="inline-delimiter">#{</span>param[<span class="integer">0</span>]<span class="inline-delimiter">}</span></span><span class="content">|#</span><span class="inline"><span class="inline-delimiter">#{</span>param(<span class="integer">0</span>)<span class="inline-delimiter">}</span></span><span class="content">]]</span></span>
<span class="line-numbers"><a href="#n5" name="n5">5</a></span><span class="string"><span class="content"> td[txt[</span><span class="inline"><span class="inline-delimiter">#{</span>param(<span class="integer">1</span>)<span class="inline-delimiter">}</span></span><span class="content">]]</span></span>
<span class="line-numbers"><a href="#n6" name="n6">6</a></span><span class="string"><span class="content"> ]</span></span>
<span class="line-numbers"><a href="#n7" name="n7">7</a></span><span class="string"><span class="content"> </span><span class="delimiter">}</span></span>
<span class="line-numbers"><a href="#n8" name="n8">8</a></span><span class="keyword">end</span></pre>
</div>
</div>
<p>The <code>issue</code> macro is only rewriting existing Glyph code around the two parameters provided. In this
case, it is possible to do exactly the same thing using the <a
href="/glyph/book/macros/macros_core.html#m_define_"><code>define:</code></a> macro (aliased by @def:@):</p>
<div class="CodeRay">
<div class="code">
<pre><span class="line-numbers"><a href="#n1" name="n1">1</a></span>define:[issue|
<span class="line-numbers"><a href="#n2" name="n2">2</a></span> tr[
<span class="line-numbers"><a href="#n3" name="n3">3</a></span> td[/=>[http://github.com/h3rald/glyph/issues/closed#issue/{{0}}|#{{0}}]]
<span class="line-numbers"><a href="#n4" name="n4">4</a></span> td[txt[{{1}}]]
<span class="line-numbers"><a href="#n5" name="n5">5</a></span> ]
<span class="line-numbers"><a href="#n6" name="n6">6</a></span>]</pre>
</div>
</div>
<p>Within the <a href="/glyph/book/macros/macros_core.html#m_define_"><code>define:</code></a> macro, it is possible
to use a special syntax to call the <code>raw_attr</code> or <code>raw_param</code> methods: <br />
<code>{{</code><em>parameter_number</em> or <em>attribute_name</em><code>}}</code>
</p>
</section>
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