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<h4 class="subsection">6.3.4 Adding articulation to notes (example)</h4>
<p>The easy way to add articulation to notes is to merge two music
expressions into one context, as explained in
<a href="Creating-contexts.html#Creating-contexts">Creating contexts</a>. However, suppose that we want to write
a music function which does this.
<p>A <code>$variable</code> inside the <code>#{...#}</code> notation is like
using a regular <code>\variable</code> in classical LilyPond
notation. We know that
<pre class="example">{ \music -. -> }
</pre>
<p class="noindent">will not work in LilyPond. We could avoid this problem by attaching
the articulation to a fake note,
<pre class="example">{ << \music s1*0-.-> }
</pre>
<p class="noindent">but for the sake of this example, we will learn how to do this in
Scheme. We begin by examining our input and desired output,
<pre class="example">% input
\displayMusic c4
===>
(make-music
'EventChord
'elements
(list (make-music
'NoteEvent
'duration
(ly:make-duration 2 0 1 1)
'pitch
(ly:make-pitch -1 0 0))))
=====
% desired output
\displayMusic c4->
===>
(make-music
'EventChord
'elements
(list (make-music
'NoteEvent
'duration
(ly:make-duration 2 0 1 1)
'pitch
(ly:make-pitch -1 0 0))
(make-music
'ArticulationEvent
'articulation-type
"marcato")))
</pre>
<p>We see that a note (<code>c4</code>) is represented as an <code>EventChord</code>
expression, with a <code>NoteEvent</code> expression in its elements list. To
add a marcato articulation, an <code>ArticulationEvent</code> expression must
be added to the elements property of the <code>EventChord</code>
expression.
<p>To build this function, we begin with
<pre class="example">(define (add-marcato event-chord)
"Add a marcato ArticulationEvent to the elements of `event-chord',
which is supposed to be an EventChord expression."
(let ((result-event-chord (ly:music-deep-copy event-chord)))
(set! (ly:music-property result-event-chord 'elements)
(cons (make-music 'ArticulationEvent
'articulation-type "marcato")
(ly:music-property result-event-chord 'elements)))
result-event-chord))
</pre>
<p>The first line is the way to define a function in Scheme: the function
name is <code>add-marcato</code>, and has one variable called
<code>event-chord</code>. In Scheme, the type of variable is often clear
from its name. (this is good practice in other programming languages,
too!)
<pre class="example">"Add a marcato..."
</pre>
<p class="noindent">is a description of what the function does. This is not strictly
necessary, but just like clear variable names, it is good practice.
<pre class="example">(let ((result-event-chord (ly:music-deep-copy event-chord)))
</pre>
<p><code>let</code> is used to declare local variables. Here we use one local
variable, named <code>result-event-chord</code>, to which we give the value
<code>(ly:music-deep-copy event-chord)</code>. <code>ly:music-deep-copy</code> is
a function specific to LilyPond, like all functions prefixed by
<code>ly:</code>. It is use to make a copy of a music
expression. Here we copy <code>event-chord</code> (the parameter of the
function). Recall that our purpose is to add a marcato to an
<code>EventChord</code> expression. It is better to not modify the
<code>EventChord</code> which was given as an argument, because it may be
used elsewhere.
<p>Now we have a <code>result-event-chord</code>, which is a
<code>NoteEventChord</code> expression and is a copy of <code>event-chord</code>. We
add the marcato to its elements list property.
<pre class="example">(set! place new-value)
</pre>
<p>Here, what we want to set (the ‘place’) is the ‘elements’ property of
<code>result-event-chord</code> expression.
<pre class="example">(ly:music-property result-event-chord 'elements)
</pre>
<p><code>ly:music-property</code> is the function used to access music properties
(the <code>'elements</code>, <code>'duration</code>, <code>'pitch</code>, etc, that we
see in the <code>\displayMusic</code> output above). The new value is the
former elements property, with an extra item: the
<code>ArticulationEvent</code> expression, which we copy from the
<code>\displayMusic</code> output,
<pre class="example">(cons (make-music 'ArticulationEvent
'articulation-type "marcato")
(ly:music-property result-event-chord 'elements))
</pre>
<p><code>cons</code> is used to add an element to a list without modifying the
original list. This is what we
want: the same list as before, plus the new <code>ArticulationEvent</code>
expression. The order inside the elements property is not important here.
<p>Finally, once we have added the marcato articulation to its <code>elements</code>
property, we can return <code>result-event-chord</code>, hence the last line of
the function.
<p>Now we transform the <code>add-marcato</code> function into a music
function,
<pre class="example">addMarcato = #(define-music-function (parser location event-chord)
(ly:music?)
"Add a marcato ArticulationEvent to the elements of `event-chord',
which is supposed to be an EventChord expression."
(let ((result-event-chord (ly:music-deep-copy event-chord)))
(set! (ly:music-property result-event-chord 'elements)
(cons (make-music 'ArticulationEvent
'articulation-type "marcato")
(ly:music-property result-event-chord 'elements)))
result-event-chord))
</pre>
<p>We may verify that this music function works correctly,
<pre class="example">\displayMusic \addMarcato c4
</pre>
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