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<a name="Input-variables-and-Scheme"></a>
Next: <a rel="next" accesskey="n" href="Internal-music-representation.es.html#Internal-music-representation">Internal music representation</a>,
Up: <a rel="up" accesskey="u" href="Programmer-interfaces.es.html#Programmer-interfaces">Programmer interfaces</a>
<hr>
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<h4 class="subsection">6.2.1 Input variables and Scheme</h4>
<p>The input format supports the notion of variables: in the following
example, a music expression is assigned to a variable with the name
<code>traLaLa</code>.
<pre class="example">traLaLa = { c'4 d'4 }
</pre>
<p class="noindent">There is also a form of scoping: in the following example, the
<code>\layout</code> block also contains a <code>traLaLa</code> variable, which is
independent of the outer <code>\traLaLa</code>.
<pre class="example">traLaLa = { c'4 d'4 }
\layout { traLaLa = 1.0 }
</pre>
<p>In effect, each input file is a scope, and all <code>\header</code>,
<code>\midi</code>, and <code>\layout</code> blocks are scopes nested inside that
toplevel scope.
<p>Both variables and scoping are implemented in the GUILE module system.
An anonymous Scheme module is attached to each scope. An assignment of
the form
<pre class="example">traLaLa = { c'4 d'4 }
</pre>
<p class="noindent">is internally converted to a Scheme definition
<pre class="example">(define traLaLa <var>Scheme value of `</var><code>... </code><var>'</var>)
</pre>
<p>This means that input variables and Scheme variables may be freely
mixed. In the following example, a music fragment is stored in the
variable <code>traLaLa</code>, and duplicated using Scheme. The result is
imported in a <code>\score</code> block by means of a second variable
<code>twice</code>:
<pre class="verbatim">
traLaLa = { c'4 d'4 }
%% dummy action to deal with parser lookahead
#(display "this needs to be here, sorry!")
#(define newLa (map ly:music-deep-copy
(list traLaLa traLaLa)))
#(define twice
(make-sequential-music newLa))
{ \twice }
</pre>
<p>
<a href="../73/lily-16e287d9.ly">
<img align="middle"
border="0" src="../73/lily-16e287d9.png" alt="[image of music]">
</a>
</p>
<p>Due to parser lookahead
<p>In this example, the assignment happens after parser has verified that
nothing interesting happens after <code>traLaLa = { ... }</code>. Without
the dummy statement in the above example, the <code>newLa</code> definition
is executed before <code>traLaLa</code> is defined, leading to a syntax
error.
<p>The above example shows how to ‘export’ music expressions from the
input to the Scheme interpreter. The opposite is also possible. By
wrapping a Scheme value in the function <code>ly:export</code>, a Scheme
value is interpreted as if it were entered in LilyPond syntax.
Instead of defining <code>\twice</code>, the example above could also have
been written as
<pre class="example">...
{ #(ly:export (make-sequential-music (list newLa))) }
</pre>
<p>Scheme code is evaluated as soon as the parser encounters it. To
define some Scheme code in a macro (to be called later), use
<a href="Void-functions.es.html#Void-functions">Void functions</a>, or
<pre class="example">#(define (nopc)
(ly:set-option 'point-and-click #f))
...
#(nopc)
{ c'4 }
</pre>
<p class="noindent">
<h5 class="subsubheading">Known issues and warnings</h5>
<p>Mixing Scheme and LilyPond variables is not possible with the
<code>--safe</code> option.
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