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22 | <a href="./Home">MLton 20180207</a>\r | |
23 | </div>\r | |
24 | </div>\r | |
25 | <div id="header">\r | |
26 | <h1>TipsForWritingConciseSML</h1>\r | |
27 | </div>\r | |
28 | <div id="content">\r | |
29 | <div id="preamble">\r | |
30 | <div class="sectionbody">\r | |
31 | <div class="paragraph"><p>SML is a rich enough language that there are often several ways to\r | |
32 | express things. This page contains miscellaneous tips (ideas not\r | |
33 | rules) for writing concise SML. The metric that we are interested in\r | |
34 | here is the number of tokens or words (rather than the number of\r | |
35 | lines, for example).</p></div>\r | |
36 | </div>\r | |
37 | </div>\r | |
38 | <div class="sect1">\r | |
39 | <h2 id="_datatypes_in_signatures">Datatypes in Signatures</h2>\r | |
40 | <div class="sectionbody">\r | |
41 | <div class="paragraph"><p>A seemingly frequent source of repetition in SML is that of datatype\r | |
42 | definitions in signatures and structures. Actually, it isn’t\r | |
43 | repetition at all. A datatype specification in a signature, such as,</p></div>\r | |
44 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
47 | <span class="k">end</span><span class="w"></span>\r | |
48 | </pre></div></div></div>\r | |
49 | <div class="paragraph"><p>is just a specification of a datatype that may be matched by multiple\r | |
50 | (albeit identical) datatype declarations. For example, in</p></div>\r | |
51 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
54 | <span class="k">end</span><span class="w"></span>\r | |
55 | \r | |
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>\r | |
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>\r | |
58 | <span class="k">end</span><span class="w"></span>\r | |
59 | </pre></div></div></div>\r | |
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\r | |
61 | such <a href="GenerativeDatatype">generativity</a> isn’t desired or needed, you\r | |
62 | can avoid the repetition:</p></div>\r | |
63 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
66 | <span class="k">end</span><span class="w"></span>\r | |
67 | \r | |
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>\r | |
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>\r | |
70 | <span class="k">end</span><span class="w"></span>\r | |
71 | \r | |
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>\r | |
73 | <span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">Exp</span><span class="w"></span>\r | |
74 | <span class="k">end</span><span class="w"></span>\r | |
75 | </pre></div></div></div>\r | |
76 | <div class="paragraph"><p>Keep in mind that this isn’t semantically equivalent to the original.</p></div>\r | |
77 | </div>\r | |
78 | </div>\r | |
79 | <div class="sect1">\r | |
80 | <h2 id="_clausal_function_definitions">Clausal Function Definitions</h2>\r | |
81 | <div class="sectionbody">\r | |
82 | <div class="paragraph"><p>The syntax of clausal function definitions is rather repetitive. For\r | |
83 | example,</p></div>\r | |
84 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
87 | </pre></div></div></div>\r | |
88 | <div class="paragraph"><p>is more verbose than</p></div>\r | |
89 | <div class="listingblock">\r | |
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>\r | |
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">=></span><span class="w"> </span><span class="n">false</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="n">true</span><span class="w"></span>\r | |
93 | </pre></div></div></div>\r | |
94 | <div class="paragraph"><p>For recursive functions the break-even point is one clause higher. For example,</p></div>\r | |
95 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
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>\r | |
99 | </pre></div></div></div>\r | |
100 | <div class="paragraph"><p>isn’t less verbose than</p></div>\r | |
101 | <div class="listingblock">\r | |
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>\r | |
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">=></span><span class="w"> </span><span class="mi">0</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="mi">1</span><span class="w"></span>\r | |
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">=></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>\r | |
106 | </pre></div></div></div>\r | |
107 | <div class="paragraph"><p>It is quite often the case that a curried function primarily examines\r | |
108 | just one of its arguments. Such functions can be written particularly\r | |
109 | concisely by making the examined argument last. For example, instead\r | |
110 | of</p></div>\r | |
111 | <div class="listingblock">\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
115 | </pre></div></div></div>\r | |
116 | <div class="paragraph"><p>consider writing</p></div>\r | |
117 | <div class="listingblock">\r | |
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>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
122 | </pre></div></div></div>\r | |
123 | </div>\r | |
124 | </div>\r | |
125 | <div class="sect1">\r | |
126 | <h2 id="_parentheses">Parentheses</h2>\r | |
127 | <div class="sectionbody">\r | |
128 | <div class="paragraph"><p>It is a good idea to avoid using lots of irritating superfluous\r | |
129 | parentheses. An important rule to know is that prefix function\r | |
130 | application in SML has higher precedence than any infix operator. For\r | |
131 | example, the outer parentheses in</p></div>\r | |
132 | <div class="listingblock">\r | |
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>\r | |
134 | </pre></div></div></div>\r | |
135 | <div class="paragraph"><p>are superfluous.</p></div>\r | |
136 | <div class="paragraph"><p>People trained in other languages often use superfluous parentheses in\r | |
137 | a number of places. In particular, the parentheses in the following\r | |
138 | examples are practically always superfluous and are best avoided:</p></div>\r | |
139 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
142 | </pre></div></div></div>\r | |
143 | <div class="paragraph"><p>The same basically applies to case expressions:</p></div>\r | |
144 | <div class="listingblock">\r | |
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>\r | |
146 | </pre></div></div></div>\r | |
147 | <div class="paragraph"><p>It is not uncommon to match a tuple of two or more values:</p></div>\r | |
148 | <div class="listingblock">\r | |
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>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
152 | </pre></div></div></div>\r | |
153 | <div class="paragraph"><p>Such case expressions can be written more concisely with an\r | |
154 | <a href="ProductType">infix product constructor</a>:</p></div>\r | |
155 | <div class="listingblock">\r | |
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">&</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
157 | <span class="w"> </span><span class="n">A1</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">B1</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">&</span><span class="w"> </span><span class="n">B2</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
159 | </pre></div></div></div>\r | |
160 | </div>\r | |
161 | </div>\r | |
162 | <div class="sect1">\r | |
163 | <h2 id="_conditionals">Conditionals</h2>\r | |
164 | <div class="sectionbody">\r | |
165 | <div class="paragraph"><p>Repeated sequences of conditionals such as</p></div>\r | |
166 | <div class="listingblock">\r | |
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"><</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>\r | |
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>\r | |
169 | <span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
170 | </pre></div></div></div>\r | |
171 | <div class="paragraph"><p>can often be written more concisely as case expressions such as</p></div>\r | |
172 | <div class="listingblock">\r | |
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>\r | |
174 | <span class="w"> </span><span class="n">LESS</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
177 | </pre></div></div></div>\r | |
178 | <div class="paragraph"><p>For a custom comparison, you would then define an appropriate datatype\r | |
179 | and a reification function. An alternative to using datatypes is to\r | |
180 | use dispatch functions</p></div>\r | |
181 | <div class="listingblock">\r | |
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>\r | |
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">=></span><span class="w"> </span><span class="p">...,</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...,</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...}</span><span class="w"></span>\r | |
186 | </pre></div></div></div>\r | |
187 | <div class="paragraph"><p>where</p></div>\r | |
188 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
191 | <span class="w"> </span><span class="n">LESS</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">lt</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="n">eq</span><span class="w"></span>\r | |
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">=></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>\r | |
194 | </pre></div></div></div>\r | |
195 | <div class="paragraph"><p>An advantage is that no datatype definition is needed. A disadvantage\r | |
196 | is that you can’t combine multiple dispatch results easily.</p></div>\r | |
197 | </div>\r | |
198 | </div>\r | |
199 | <div class="sect1">\r | |
200 | <h2 id="_command_query_fusion">Command-Query Fusion</h2>\r | |
201 | <div class="sectionbody">\r | |
202 | <div class="paragraph"><p>Many are familiar with the\r | |
203 | <a href="http://en.wikipedia.org/wiki/Command-Query_Separation">Command-Query\r | |
204 | Separation Principle</a>. Adhering to the principle, a signature for an\r | |
205 | imperative stack might contain specifications</p></div>\r | |
206 | <div class="listingblock">\r | |
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">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">bool</span><span class="w"></span>\r | |
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">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
209 | </pre></div></div></div>\r | |
210 | <div class="paragraph"><p>and use of a stack would look like</p></div>\r | |
211 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
214 | <span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
215 | </pre></div></div></div>\r | |
216 | <div class="paragraph"><p>or, when the element needs to be named,</p></div>\r | |
217 | <div class="listingblock">\r | |
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>\r | |
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>\r | |
220 | <span class="k">else</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
221 | </pre></div></div></div>\r | |
222 | <div class="paragraph"><p>For efficiency, correctness, and conciseness, it is often better to\r | |
223 | combine the query and command and return the result as an option:</p></div>\r | |
224 | <div class="listingblock">\r | |
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">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">option</span><span class="w"></span>\r | |
226 | </pre></div></div></div>\r | |
227 | <div class="paragraph"><p>A use of a stack would then look like this:</p></div>\r | |
228 | <div class="listingblock">\r | |
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>\r | |
230 | <span class="w"> </span><span class="n">NONE</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
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">=></span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
232 | </pre></div></div></div>\r | |
233 | </div>\r | |
234 | </div>\r | |
235 | </div>\r | |
236 | <div id="footnotes"><hr></div>\r | |
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