doc/guide/localhost/TipsForWritingConciseSML

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  22 <a href="./Home">MLton 20180207</a>
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  25 <div id="header">
  26 <h1>TipsForWritingConciseSML</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>SML is a rich enough language that there are often several ways to
  32 express things.  This page contains miscellaneous tips (ideas not
  33 rules) for writing concise SML.  The metric that we are interested in
  34 here is the number of tokens or words (rather than the number of
  35 lines, for example).</p></div>
  36 </div>
  37 </div>
  38 <div class="sect1">
  39 <h2 id="_datatypes_in_signatures">Datatypes in Signatures</h2>
  40 <div class="sectionbody">
  41 <div class="paragraph"><p>A seemingly frequent source of repetition in SML is that of datatype
  42 definitions in signatures and structures.  Actually, it isn&#8217;t
  43 repetition at all.  A datatype specification in a signature, such as,</p></div>
  44 <div class="listingblock">
  45 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">EXP</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>
  46 <span class="w">   </span><span class="k">datatype</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Fn</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">App</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Var</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"></span>
  47 <span class="k">end</span><span class="w"></span>
  48 </pre></div></div></div>
  49 <div class="paragraph"><p>is just a specification of a datatype that may be matched by multiple
  50 (albeit identical) datatype declarations.  For example, in</p></div>
  51 <div class="listingblock">
  52 <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">AnExp</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">EXP</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>
  53 <span class="w">   </span><span class="k">datatype</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Fn</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">App</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Var</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"></span>
  54 <span class="k">end</span><span class="w"></span>
  55
  56 <span class="k">structure</span><span class="w"> </span><span class="n">AnotherExp</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">EXP</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>
  57 <span class="w">   </span><span class="k">datatype</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Fn</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">App</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Var</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"></span>
  58 <span class="k">end</span><span class="w"></span>
  59 </pre></div></div></div>
  60 <div class="paragraph"><p>the types <span class="monospaced">AnExp.exp</span> and <span class="monospaced">AnotherExp.exp</span> are two distinct types.  If
  61 such <a href="GenerativeDatatype">generativity</a> isn&#8217;t desired or needed, you
  62 can avoid the repetition:</p></div>
  63 <div class="listingblock">
  64 <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Exp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>
  65 <span class="w">   </span><span class="k">datatype</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Fn</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">App</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Var</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"></span>
  66 <span class="k">end</span><span class="w"></span>
  67
  68 <span class="k">signature</span><span class="w"> </span><span class="n">EXP</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>
  69 <span class="w">   </span><span class="k">datatype</span><span class="w"> </span><span class="n">exp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">datatype</span><span class="w"> </span><span class="n">Exp</span><span class="p">.</span><span class="n">exp</span><span class="w"></span>
  70 <span class="k">end</span><span class="w"></span>
  71
  72 <span class="k">structure</span><span class="w"> </span><span class="n">Exp</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">EXP</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>
  73 <span class="w">   </span><span class="k">open</span><span class="w"> </span><span class="n">Exp</span><span class="w"></span>
  74 <span class="k">end</span><span class="w"></span>
  75 </pre></div></div></div>
  76 <div class="paragraph"><p>Keep in mind that this isn&#8217;t semantically equivalent to the original.</p></div>
  77 </div>
  78 </div>
  79 <div class="sect1">
  80 <h2 id="_clausal_function_definitions">Clausal Function Definitions</h2>
  81 <div class="sectionbody">
  82 <div class="paragraph"><p>The syntax of clausal function definitions is rather repetitive.  For
  83 example,</p></div>
  84 <div class="listingblock">
  85 <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">isSome</span><span class="w"> </span><span class="n">NONE</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">false</span><span class="w"></span>
  86 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">isSome</span><span class="w"> </span><span class="p">(</span><span class="n">SOME</span><span class="w"> </span><span class="p">_)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">true</span><span class="w"></span>
  87 </pre></div></div></div>
  88 <div class="paragraph"><p>is more verbose than</p></div>
  89 <div class="listingblock">
  90 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">isSome</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  91 <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">NONE</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">false</span><span class="w"></span>
  92 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">SOME</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">true</span><span class="w"></span>
  93 </pre></div></div></div>
  94 <div class="paragraph"><p>For recursive functions the break-even point is one clause higher.  For example,</p></div>
  95 <div class="listingblock">
  96 <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>
  97 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">1</span><span class="w"></span>
  98 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="p">(</span><span class="n">n-</span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="p">(</span><span class="n">n-</span><span class="mi">2</span><span class="p">)</span><span class="w"></span>
  99 </pre></div></div></div>
 100 <div class="paragraph"><p>isn&#8217;t less verbose than</p></div>
 101 <div class="listingblock">
 102 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="k">rec</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 103 <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>
 104 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="mi">1</span><span class="w"></span>
 105 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="p">(</span><span class="n">n-</span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">fib</span><span class="w"> </span><span class="p">(</span><span class="n">n-</span><span class="mi">2</span><span class="p">)</span><span class="w"></span>
 106 </pre></div></div></div>
 107 <div class="paragraph"><p>It is quite often the case that a curried function primarily examines
 108 just one of its arguments.  Such functions can be written particularly
 109 concisely by making the examined argument last.  For example, instead
 110 of</p></div>
 111 <div class="listingblock">
 112 <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">eval</span><span class="w"> </span><span class="p">(</span><span class="n">Fn</span><span class="w"> </span><span class="p">(</span><span class="n">v</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">))</span><span class="w"> </span><span class="n">env</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 113 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">eval</span><span class="w"> </span><span class="p">(</span><span class="n">App</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="n">env</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 114 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">eval</span><span class="w"> </span><span class="p">(</span><span class="n">Var</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </span><span class="n">env</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 115 </pre></div></div></div>
 116 <div class="paragraph"><p>consider writing</p></div>
 117 <div class="listingblock">
 118 <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">eval</span><span class="w"> </span><span class="n">env</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 119 <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">Fn</span><span class="w"> </span><span class="p">(</span><span class="n">v</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 120 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">App</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 121 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">Var</span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 122 </pre></div></div></div>
 123 </div>
 124 </div>
 125 <div class="sect1">
 126 <h2 id="_parentheses">Parentheses</h2>
 127 <div class="sectionbody">
 128 <div class="paragraph"><p>It is a good idea to avoid using lots of irritating superfluous
 129 parentheses.  An important rule to know is that prefix function
 130 application in SML has higher precedence than any infix operator.  For
 131 example, the outer parentheses in</p></div>
 132 <div class="listingblock">
 133 <div class="content"><div class="highlight"><pre><span class="p">(</span><span class="n">square</span><span class="w"> </span><span class="p">(</span><span class="mi">5</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="mi">1</span><span class="p">))</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="p">(</span><span class="n">square</span><span class="w"> </span><span class="p">(</span><span class="mi">5</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="mi">2</span><span class="p">))</span><span class="w"></span>
 134 </pre></div></div></div>
 135 <div class="paragraph"><p>are superfluous.</p></div>
 136 <div class="paragraph"><p>People trained in other languages often use superfluous parentheses in
 137 a number of places.  In particular, the parentheses in the following
 138 examples are practically always superfluous and are best avoided:</p></div>
 139 <div class="listingblock">
 140 <div class="content"><div class="highlight"><pre><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">condition</span><span class="p">)</span><span class="w"> </span><span class="k">then</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 141 <span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">condition</span><span class="p">)</span><span class="w"> </span><span class="k">do</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 142 </pre></div></div></div>
 143 <div class="paragraph"><p>The same basically applies to case expressions:</p></div>
 144 <div class="listingblock">
 145 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="p">(</span><span class="n">expression</span><span class="p">)</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 146 </pre></div></div></div>
 147 <div class="paragraph"><p>It is not uncommon to match a tuple of two or more values:</p></div>
 148 <div class="listingblock">
 149 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">)</span><span class="w"> </span><span class="k">of</span><span class="w"></span>
 150 <span class="w">   </span><span class="p">(</span><span class="n">A1</span><span class="p">,</span><span class="w"> </span><span class="n">B1</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 151 <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">(</span><span class="n">A2</span><span class="p">,</span><span class="w"> </span><span class="n">B2</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 152 </pre></div></div></div>
 153 <div class="paragraph"><p>Such case expressions can be written more concisely with an
 154 <a href="ProductType">infix product constructor</a>:</p></div>
 155 <div class="listingblock">
 156 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="n">&amp;</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="k">of</span><span class="w"></span>
 157 <span class="w">   </span><span class="n">A1</span><span class="w"> </span><span class="n">&amp;</span><span class="w"> </span><span class="n">B1</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 158 <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">A2</span><span class="w"> </span><span class="n">&amp;</span><span class="w"> </span><span class="n">B2</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 159 </pre></div></div></div>
 160 </div>
 161 </div>
 162 <div class="sect1">
 163 <h2 id="_conditionals">Conditionals</h2>
 164 <div class="sectionbody">
 165 <div class="paragraph"><p>Repeated sequences of conditionals such as</p></div>
 166 <div class="listingblock">
 167 <div class="content"><div class="highlight"><pre><span class="k">if</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="n">&lt;</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="k">then</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 168 <span class="k">else</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="k">then</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 169 <span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 170 </pre></div></div></div>
 171 <div class="paragraph"><p>can often be written more concisely as case expressions such as</p></div>
 172 <div class="listingblock">
 173 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="n">Int</span><span class="p">.</span><span class="n">compare</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"> </span><span class="k">of</span><span class="w"></span>
 174 <span class="w">   </span><span class="n">LESS</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 175 <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">EQUAL</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 176 <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">GREATER</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 177 </pre></div></div></div>
 178 <div class="paragraph"><p>For a custom comparison, you would then define an appropriate datatype
 179 and a reification function.  An alternative to using datatypes is to
 180 use dispatch functions</p></div>
 181 <div class="listingblock">
 182 <div class="content"><div class="highlight"><pre><span class="n">comparing</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"></span>
 183 <span class="p">{</span><span class="n">lt</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...,</span><span class="w"></span>
 184 <span class="w"> </span><span class="n">eq</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...,</span><span class="w"></span>
 185 <span class="w"> </span><span class="n">gt</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...}</span><span class="w"></span>
 186 </pre></div></div></div>
 187 <div class="paragraph"><p>where</p></div>
 188 <div class="listingblock">
 189 <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">comparing</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="n">lt</span><span class="p">,</span><span class="w"> </span><span class="n">eq</span><span class="p">,</span><span class="w"> </span><span class="n">gt</span><span class="p">}</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 190 <span class="w">    </span><span class="p">(</span><span class="k">case</span><span class="w"> </span><span class="n">Int</span><span class="p">.</span><span class="n">compare</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"> </span><span class="k">of</span><span class="w"></span>
 191 <span class="w">        </span><span class="n">LESS</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">lt</span><span class="w"></span>
 192 <span class="w">      </span><span class="p">|</span><span class="w"> </span><span class="n">EQUAL</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">eq</span><span class="w"></span>
 193 <span class="w">      </span><span class="p">|</span><span class="w"> </span><span class="n">GREATER</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">gt</span><span class="p">)</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
 194 </pre></div></div></div>
 195 <div class="paragraph"><p>An advantage is that no datatype definition is needed.  A disadvantage
 196 is that you can&#8217;t combine multiple dispatch results easily.</p></div>
 197 </div>
 198 </div>
 199 <div class="sect1">
 200 <h2 id="_command_query_fusion">Command-Query Fusion</h2>
 201 <div class="sectionbody">
 202 <div class="paragraph"><p>Many are familiar with the
 203 <a href="http://en.wikipedia.org/wiki/Command-Query_Separation">Command-Query
 204 Separation Principle</a>.  Adhering to the principle, a signature for an
 205 imperative stack might contain specifications</p></div>
 206 <div class="listingblock">
 207 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">isEmpty</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">bool</span><span class="w"></span>
 208 <span class="k">val</span><span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"></span>
 209 </pre></div></div></div>
 210 <div class="paragraph"><p>and use of a stack would look like</p></div>
 211 <div class="listingblock">
 212 <div class="content"><div class="highlight"><pre><span class="k">if</span><span class="w"> </span><span class="n">isEmpty</span><span class="w"> </span><span class="n">stack</span><span class="w"></span>
 213 <span class="k">then</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="n">stack</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 214 <span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 215 </pre></div></div></div>
 216 <div class="paragraph"><p>or, when the element needs to be named,</p></div>
 217 <div class="listingblock">
 218 <div class="content"><div class="highlight"><pre><span class="k">if</span><span class="w"> </span><span class="n">isEmpty</span><span class="w"> </span><span class="n">stack</span><span class="w"></span>
 219 <span class="k">then</span><span class="w"> </span><span class="k">let</span><span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="n">stack</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 220 <span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 221 </pre></div></div></div>
 222 <div class="paragraph"><p>For efficiency, correctness, and conciseness, it is often better to
 223 combine the query and command and return the result as an option:</p></div>
 224 <div class="listingblock">
 225 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="n">option</span><span class="w"></span>
 226 </pre></div></div></div>
 227 <div class="paragraph"><p>A use of a stack would then look like this:</p></div>
 228 <div class="listingblock">
 229 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="n">stack</span><span class="w"> </span><span class="k">of</span><span class="w"></span>
 230 <span class="w">   </span><span class="n">NONE</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 231 <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">SOME</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 232 </pre></div></div></div>
 233 </div>
 234 </div>
 235 </div>
 236 <div id="footnotes"><hr></div>
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