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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id-1.3.21.12"></a>Rationale for some of the design decisions</h2></div></div></div>
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<div class="toc"><dl class="toc"><dt><span class="section"><a href="s10.html#lambda.why_weak_arity">
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Lambda functor arity
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</a></span></dt></dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="lambda.why_weak_arity"></a>
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Lambda functor arity
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</h3></div></div></div>
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<p>
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The highest placeholder index in a lambda expression determines the arity of the resulting function object.
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However, this is just the minimal arity, as the function object can take arbitrarily many arguments; those not needed are discarded.
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Consider the two bind expressions and their invocations below:
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<pre class="programlisting">
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bind(g, _3, _3, _3)(x, y, z);
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bind(g, _1, _1, _1)(x, y, z);
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</pre>
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<p>
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This first line discards arguments <code class="literal">x</code> and
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<code class="literal">y</code>, and makes the call:
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</p>
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<pre class="programlisting">
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g(z, z, z)
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</pre>
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<p>
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whereas the second line discards arguments <code class="literal">y</code> and
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<code class="literal">z</code>, and calls:
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</p>
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<pre class="programlisting">
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g(x, x, x)
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</pre>
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<p>
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In earlier versions of the library, the latter line resulted in a compile
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time error.
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This is basically a tradeoff between safety and flexibility, and the issue
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was extensively discussed during the Boost review period of the library.
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The main points for the <span class="emphasis"><em>strict arity</em></span> checking
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was that it might
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catch a programming error at an earlier time and that a lambda expression that
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explicitly discards its arguments is easy to write:
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</p>
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<pre class="programlisting">
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(_3, bind(g, _1, _1, _1))(x, y, z);
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</pre>
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<p>
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This lambda expression takes three arguments.
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The left-hand argument of the comma operator does nothing, and as comma
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returns the result of evaluating the right-hand argument we end up with
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the call
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<code class="literal">g(x, x, x)</code>
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even with the strict arity.
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</p>
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<p>
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The main points against the strict arity checking were that the need to
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discard arguments is commonplace, and should therefore be straightforward,
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and that strict arity checking does not really buy that much more safety,
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particularly as it is not symmetric.
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For example, if the programmer wanted to write the expression
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<code class="literal">_1 + _2</code> but mistakenly wrote <code class="literal">_1 + 2</code>,
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with strict arity checking, the complier would spot the error.
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However, if the erroneous expression was <code class="literal">1 + _2</code> instead,
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the error would go unnoticed.
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Furthermore, weak arity checking simplifies the implementation a bit.
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Following the recommendation of the Boost review, strict arity checking
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was dropped.
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</p>
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<td align="right"><div class="copyright-footer">Copyright © 1999-2004 Jaakko Järvi, Gary Powell<p>Use, modification and distribution is subject to the Boost
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Software License, Version 1.0. (See accompanying file
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<code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
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