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26<div class="titlepage"><div><div><h2 class="title">
27<a name="id2713626"></a>Appendix A. Rationale for some of the design decisions</h2></div></div></div>
28<div class="toc">
29<p><b>Table of Contents</b></p>
30<dl><dt><span class="section"><a href="apa.html#lambda.why_weak_arity">
31Lambda functor arity
32</a></span></dt></dl>
33</div>
34<div class="section" lang="en">
35<div class="titlepage"><div><div><h4 class="title">
36<a name="lambda.why_weak_arity"></a>
37Lambda functor arity
38</h4></div></div></div>
39<p>
40The highest placeholder index in a lambda expression determines the arity of the resulting function object.
41However, this is just the minimal arity, as the function object can take arbitrarily many arguments; those not needed are discarded.
42Consider the two bind expressions and their invocations below:
43
44</p>
45<pre class="programlisting">
46bind(g, _3, _3, _3)(x, y, z);
47bind(g, _1, _1, _1)(x, y, z);
48</pre>
49<p>
50
51This first line discards arguments <code class="literal">x</code> and
52<code class="literal">y</code>, and makes the call:
53</p>
54<pre class="programlisting">
55g(z, z, z)
56</pre>
57<p>
58whereas the second line discards arguments <code class="literal">y</code> and
59<code class="literal">z</code>, and calls:
60</p>
61<pre class="programlisting">
62g(x, x, x)
63</pre>
64<p>
65In earlier versions of the library, the latter line resulted in a compile
66time error.
67
68This is basically a tradeoff between safety and flexibility, and the issue
69was extensively discussed during the Boost review period of the library.
70The main points for the <span class="emphasis"><em>strict arity</em></span> checking
71was that it might
72catch a programming error at an earlier time and that a lambda expression that
73explicitly discards its arguments is easy to write:
74</p>
75<pre class="programlisting">
76(_3, bind(g, _1, _1, _1))(x, y, z);
77</pre>
78<p>
79This lambda expression takes three arguments.
80The left-hand argument of the comma operator does nothing, and as comma
81returns the result of evaluating the right-hand argument we end up with
82the call
83<code class="literal">g(x, x, x)</code>
84even with the strict arity.
85</p>
86<p>
87The main points against the strict arity checking were that the need to
88discard arguments is commonplace, and should therefore be straightforward,
89and that strict arity checking does not really buy that much more safety,
90particularly as it is not symmetric.
91For example, if the programmer wanted to write the expression
92<code class="literal">_1 + _2</code> but mistakenly wrote <code class="literal">_1 + 2</code>,
93with strict arity checking, the complier would spot the error.
94However, if the erroneous expression was <code class="literal">1 + _2</code> instead,
95the error would go unnoticed.
96Furthermore, weak arity checking simplifies the implementation a bit.
97Following the recommendation of the Boost review, strict arity checking
98was dropped.
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