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| 25 | <div class="appendix" lang="en"> |
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| 26 | <div class="titlepage"><div><div><h2 class="title"> |
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| 27 | <a name="id2713626"></a>Appendix A. Rationale for some of the design decisions</h2></div></div></div> |
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| 28 | <div class="toc"> |
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| 29 | <p><b>Table of Contents</b></p> |
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| 30 | <dl><dt><span class="section"><a href="apa.html#lambda.why_weak_arity"> |
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| 31 | Lambda functor arity |
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| 32 | </a></span></dt></dl> |
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| 33 | </div> |
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| 34 | <div class="section" lang="en"> |
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| 35 | <div class="titlepage"><div><div><h4 class="title"> |
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| 36 | <a name="lambda.why_weak_arity"></a> |
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| 37 | Lambda functor arity |
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| 38 | </h4></div></div></div> |
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| 39 | <p> |
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| 40 | The highest placeholder index in a lambda expression determines the arity of the resulting function object. |
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| 41 | 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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| 42 | Consider the two bind expressions and their invocations below: |
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| 43 | |
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| 44 | </p> |
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| 45 | <pre class="programlisting"> |
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| 46 | bind(g, _3, _3, _3)(x, y, z); |
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| 47 | bind(g, _1, _1, _1)(x, y, z); |
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| 48 | </pre> |
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| 49 | <p> |
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| 50 | |
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| 51 | This first line discards arguments <code class="literal">x</code> and |
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| 52 | <code class="literal">y</code>, and makes the call: |
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| 53 | </p> |
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| 54 | <pre class="programlisting"> |
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| 55 | g(z, z, z) |
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| 56 | </pre> |
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| 57 | <p> |
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| 58 | whereas the second line discards arguments <code class="literal">y</code> and |
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| 59 | <code class="literal">z</code>, and calls: |
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| 60 | </p> |
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| 61 | <pre class="programlisting"> |
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| 62 | g(x, x, x) |
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| 63 | </pre> |
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| 64 | <p> |
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| 65 | In earlier versions of the library, the latter line resulted in a compile |
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| 66 | time error. |
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| 67 | |
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| 68 | This is basically a tradeoff between safety and flexibility, and the issue |
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| 69 | was extensively discussed during the Boost review period of the library. |
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| 70 | The main points for the <span class="emphasis"><em>strict arity</em></span> checking |
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| 71 | was that it might |
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| 72 | catch a programming error at an earlier time and that a lambda expression that |
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| 73 | explicitly discards its arguments is easy to write: |
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| 74 | </p> |
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| 75 | <pre class="programlisting"> |
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| 76 | (_3, bind(g, _1, _1, _1))(x, y, z); |
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| 77 | </pre> |
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| 78 | <p> |
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| 79 | This lambda expression takes three arguments. |
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| 80 | The left-hand argument of the comma operator does nothing, and as comma |
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| 81 | returns the result of evaluating the right-hand argument we end up with |
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| 82 | the call |
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| 83 | <code class="literal">g(x, x, x)</code> |
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| 84 | even with the strict arity. |
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| 85 | </p> |
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| 86 | <p> |
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| 87 | The main points against the strict arity checking were that the need to |
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| 88 | discard arguments is commonplace, and should therefore be straightforward, |
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| 89 | and that strict arity checking does not really buy that much more safety, |
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| 90 | particularly as it is not symmetric. |
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| 91 | For example, if the programmer wanted to write the expression |
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| 92 | <code class="literal">_1 + _2</code> but mistakenly wrote <code class="literal">_1 + 2</code>, |
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| 93 | with strict arity checking, the complier would spot the error. |
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| 94 | However, if the erroneous expression was <code class="literal">1 + _2</code> instead, |
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| 95 | the error would go unnoticed. |
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| 96 | Furthermore, weak arity checking simplifies the implementation a bit. |
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| 97 | Following the recommendation of the Boost review, strict arity checking |
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| 98 | was dropped. |
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| 99 | </p> |
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| 100 | </div> |
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| 101 | </div> |
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| 102 | <table width="100%"><tr> |
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| 103 | <td align="left"></td> |
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| 104 | <td align="right"><small>Copyright © 1999-2004 Jaakko Järvi, Gary Powell</small></td> |
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