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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id1999367"></a>Rationale for some of the design decisions</h2></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="s10.html#lambda.why_weak_arity">
Lambda functor arity
</a></span></dt></dl></div>
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<div class="titlepage"><div><div><h3 class="title">
<a name="lambda.why_weak_arity"></a>
Lambda functor arity
</h3></div></div></div>
<p>
The highest placeholder index in a lambda expression determines the arity of the resulting function object.
However, this is just the minimal arity, as the function object can take arbitrarily many arguments; those not needed are discarded.
Consider the two bind expressions and their invocations below:

</p>
<pre class="programlisting">
bind(g, _3, _3, _3)(x, y, z); 
bind(g, _1, _1, _1)(x, y, z); 
</pre>
<p>

This first line discards arguments <code class="literal">x</code> and
<code class="literal">y</code>, and makes the call:
</p>
<pre class="programlisting">
g(z, z, z) 
</pre>
<p>
whereas the second line discards arguments <code class="literal">y</code> and
<code class="literal">z</code>, and calls:
</p>
<pre class="programlisting">
g(x, x, x)
</pre>
<p>
In earlier versions of the library, the latter line resulted in a compile 
time error.

This is basically a tradeoff between safety and flexibility, and the issue
was extensively discussed during the Boost review period of the library.
The main points for the <span class="emphasis"><em>strict arity</em></span> checking
was that it might
catch a programming error at an earlier time and that a lambda expression that
explicitly discards its arguments is easy to write:
</p>
<pre class="programlisting">
(_3, bind(g, _1, _1, _1))(x, y, z);
</pre>
<p>
This lambda expression takes three arguments.
The left-hand argument of the comma operator does nothing, and as comma 
returns the result of evaluating the right-hand argument we end up with 
the call
<code class="literal">g(x, x, x)</code>
even with the strict arity.
</p>
<p>
The main points against the strict arity checking were that the need to 
discard arguments is commonplace, and should therefore be straightforward, 
and that strict arity checking does not really buy that much more safety, 
particularly as it is not symmetric.
For example, if the programmer wanted to write the expression 
<code class="literal">_1 + _2</code> but mistakenly wrote <code class="literal">_1 + 2</code>, 
with strict arity checking, the complier would spot the error.
However, if the erroneous expression was <code class="literal">1 + _2</code> instead,
the error would go unnoticed.
Furthermore, weak arity checking simplifies the implementation a bit.
Following the recommendation of the Boost review, strict arity checking 
was dropped.
</p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 1999-2004 Jaakko J&#228;rvi, Gary Powell<p>Use, modification and distribution is subject to the Boost
    Software License, Version 1.0. (See accompanying file
    <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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