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<head><title><- -- assignment with left side evaluated</title>
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<div><a href="index.html" title="">Macaulay2Doc</a> > <a href="___The_sp__Macaulay2_splanguage.html" title="">The Macaulay2 language</a> > <a href="_operators.html" title="">operators</a> > <a href="__lt-.html" title="assignment with left side evaluated"><-</a></div>
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<div><h1><- -- assignment with left side evaluated</h1>
<div class="single"><h2>Description</h2>
<div><div><h2>assignment to symbols</h2>
<ul><li><div class="list"><dl class="element"><dt class="heading">Usage: </dt><dd class="value"><div><tt>x <- e</tt></div>
</dd></dl>
</div>
</li>
<li>Inputs:<ul><li><span><tt>x</tt>, <span>a <a href="___Symbol.html">symbol</a></span>, (evaluated)</span></li>
<li><span><tt>e</tt>, <span>a <a href="___Thing.html">thing</a></span></span></li>
</ul>
</li>
<li>Consequences:<ul><li>assuming the value of <tt>x</tt> is a symbol, <tt>t</tt>, say, the value of <tt>e</tt> is assigned to <tt>t</tt> so that future references to the value of <tt>t</tt> yield <tt>e</tt>.</li>
</ul>
</li>
<li>Outputs:<ul><li><span>the value of the expression is the value of <tt>e</tt></span></li>
</ul>
</li>
</ul>
<table class="examples"><tr><td><pre>i1 : x = t
o1 = t
o1 : Symbol</pre>
</td></tr>
<tr><td><pre>i2 : x <- 4
o2 = 4</pre>
</td></tr>
<tr><td><pre>i3 : x
o3 = t
o3 : Symbol</pre>
</td></tr>
<tr><td><pre>i4 : t
o4 = 4</pre>
</td></tr>
</table>
<p>Initially, the value of a symbol <tt>y</tt> is <tt>y</tt> itself, and then <tt>y = e</tt> and <tt>y <- e</tt> do the same thing.</p>
<table class="examples"><tr><td><pre>i5 : y <- 44
o5 = 44</pre>
</td></tr>
<tr><td><pre>i6 : y
o6 = 44</pre>
</td></tr>
</table>
<p>The code to the left of the arrow can be any expression whose value is a symbol.</p>
<table class="examples"><tr><td><pre>i7 : f = () -> symbol z
o7 = f
o7 : FunctionClosure</pre>
</td></tr>
<tr><td><pre>i8 : (f()) <- 44
o8 = 44</pre>
</td></tr>
<tr><td><pre>i9 : z
o9 = 44</pre>
</td></tr>
</table>
</div>
<div><h2>installing new assignment methods</h2>
<ul><li><div class="list"><dl class="element"><dt class="heading">Usage: </dt><dd class="value"><div><tt>installMethod(symbol <-, X, (x,e) -> ...)</tt></div>
</dd></dl>
</div>
</li>
<li>Inputs:<ul><li><span><tt>X</tt>, <span>a <a href="___Type.html">type</a></span></span></li>
<li><span><tt>(x,e) -> ...</tt>, <span>a <a href="___Function.html">function</a></span> of two arguments</span></li>
</ul>
</li>
<li>Consequences:<ul><li>the function <tt>(x,e) -> ...</tt> is installed as the method for assignment to objects of type <tt>X</tt>. See the next subsection below for using it.</li>
</ul>
</li>
<li>Outputs:<ul><li><span>the value returned is the function provided</span></li>
</ul>
</li>
</ul>
<table class="examples"><tr><td><pre>i10 : installMethod(symbol <-, String, peek)
o10 = peek
o10 : FunctionClosure</pre>
</td></tr>
</table>
</div>
<div><h2>using installed assignment methods</h2>
<ul><li><div class="list"><dl class="element"><dt class="heading">Usage: </dt><dd class="value"><div><tt>x <- e</tt></div>
</dd></dl>
</div>
</li>
<li>Inputs:<ul><li><span><tt>x</tt>, an object of type X</span></li>
<li><span><tt>e</tt>, <span>a <a href="___Thing.html">thing</a></span></span></li>
</ul>
</li>
<li>Consequences:<ul><li>the previously installed method for assignment to objects of type <tt>X</tt> is called with arguments <tt>(x,e)</tt>, unless <tt>x</tt> is a symbol, in which case the internal assignment method applies, as described above. If there is no method for <tt>X</tt>, then the ancestors of <tt>X</tt> are consulted, starting with the parent of <tt>X</tt> and ending with <a href="___Thing.html" title="the class of all things">Thing</a> (see <a href="_inheritance.html" title="">inheritance</a>).</li>
</ul>
</li>
<li>Outputs:<ul><li><span>the value of the expression is the value returned by the previously installed method</span></li>
</ul>
</li>
</ul>
<p>The following example used the method for assignment to strings installed above.</p>
<table class="examples"><tr><td><pre>i11 : "foo" <- "bar"
o11 = ("foo", "bar")</pre>
</td></tr>
</table>
<p>As before, the left hand side is evaluated, and in this case, it can be any expression whose value is a string.</p>
<table class="examples"><tr><td><pre>i12 : "foo" | "foo" <- "bar"
o12 = ("foofoo", "bar")</pre>
</td></tr>
</table>
</div>
<div><h2>assignment to an indexed variable</h2>
<ul><li><div class="list"><dl class="element"><dt class="heading">Usage: </dt><dd class="value"><div><tt>x <- e</tt></div>
</dd></dl>
</div>
</li>
<li>Inputs:<ul><li><span><tt>x</tt>, <span>an <a href="___Indexed__Variable.html">indexed variable</a></span></span></li>
<li><span><tt>e</tt>, <span>a <a href="___Thing.html">thing</a></span></span></li>
</ul>
</li>
<li>Consequences:<ul><li>assuming the value of <tt>x</tt> is <span>an <a href="___Indexed__Variable.html">indexed variable</a></span>, the value of e is assigned to it, so that future references to <tt>value x</tt> or to <tt>s_i</tt>, if that's what the value of <tt>x</tt> is, yield <tt>e</tt>. Moreover, the value of <tt>s</tt> is set to <tt>s</tt>.</li>
</ul>
</li>
<li>Outputs:<ul><li><span>the value of the expression is the value returned by the previously installed method</span></li>
</ul>
</li>
</ul>
<p>This assignment method is pre-installed.</p>
<table class="examples"><tr><td><pre>i13 : u = s_4
o13 = s
4
o13 : IndexedVariable</pre>
</td></tr>
<tr><td><pre>i14 : s = 3
o14 = 3</pre>
</td></tr>
<tr><td><pre>i15 : u <- 555
--warning: clearing value of symbol s to allow access to subscripted variables based on it
o15 = 555</pre>
</td></tr>
<tr><td><pre>i16 : s
o16 = s
o16 : IndexedVariableTable</pre>
</td></tr>
<tr><td><pre>i17 : s_4
o17 = 555</pre>
</td></tr>
<tr><td><pre>i18 : u
o18 = s
4
o18 : IndexedVariable</pre>
</td></tr>
<tr><td><pre>i19 : value u
o19 = 555</pre>
</td></tr>
</table>
</div>
<div><h2>parallel assignment</h2>
<ul><li><div class="list"><dl class="element"><dt class="heading">Usage: </dt><dd class="value"><div><tt>x <- e</tt></div>
</dd></dl>
</div>
</li>
<li>Inputs:<ul><li><span><tt>x</tt>, <span>a <a href="___Sequence.html">sequence</a></span></span></li>
<li><span><tt>e</tt>, <span>a <a href="___Sequence.html">sequence</a></span></span></li>
</ul>
</li>
<li>Consequences:<ul><li>assuming the values of <tt>x</tt> and of <tt>e</tt> are <span>a <a href="___Sequence.html">sequence</a></span>, with the same length, each member of <tt>e</tt> is assigned to the corresponding member of <tt>x</tt></li>
</ul>
</li>
<li>Outputs:<ul><li><span>the value of the expression is the value of <tt>e</tt></span></li>
</ul>
</li>
</ul>
<p>This assignment method is pre-installed.</p>
<table class="examples"><tr><td><pre>i20 : (symbol a, symbol b) <- (3,4)
o20 = (3, 4)
o20 : Sequence</pre>
</td></tr>
<tr><td><pre>i21 : a
o21 = 3</pre>
</td></tr>
<tr><td><pre>i22 : (symbol r_1 .. symbol r_3) <- (5,6,7)
o22 = (5, 6, 7)
o22 : Sequence</pre>
</td></tr>
<tr><td><pre>i23 : r_2
o23 = 6</pre>
</td></tr>
</table>
</div>
</div>
</div>
<div class="single"><h2>See also</h2>
<ul><li><span><a href="__eq.html" title="assignment">=</a> -- assignment</span></li>
<li><span><a href="__co_eq.html" title="assignment of method or new local variable">:=</a> -- assignment of method or new local variable</span></li>
<li><span><a href="__lt-.html" title="assignment with left side evaluated">IndexedVariable <- Thing</a> -- assignment with left side evaluated</span></li>
</ul>
</div>
<div class="waystouse"><h2>Ways to use <tt><-</tt> :</h2>
<ul><li>IndexedVariable <- Thing</li>
<li>Sequence <- Thing</li>
</ul>
</div>
<div class="waystouse"><h2>For the programmer</h2>
<p>The object <a href="__lt-.html" title="assignment with left side evaluated"><-</a> is <span>a <a href="___Keyword.html">keyword</a></span>.</p>
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