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22 | <a href="./Home">MLton 20180207</a>\r | |
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25 | <div id="header">\r | |
26 | <h1>OptionalArguments</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><a href="StandardML">Standard ML</a> does not have built-in support for optional\r | |
32 | arguments. Nevertheless, using <a href="Fold">Fold</a>, it is easy to define\r | |
33 | functions that take optional arguments.</p></div>\r | |
34 | <div class="paragraph"><p>For example, suppose that we have the following definition of a\r | |
35 | function <span class="monospaced">f</span>.</p></div>\r | |
36 | <div class="listingblock">\r | |
37 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">r</span><span class="p">,</span><span class="w"> </span><span class="n">s</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
38 | <span class="w"> </span><span class="n">concat</span><span class="w"> </span><span class="p">[</span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="s">", "</span><span class="p">,</span><span class="w"> </span><span class="n">Real</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">r</span><span class="p">,</span><span class="w"> </span><span class="s">", "</span><span class="p">,</span><span class="w"> </span><span class="n">s</span><span class="p">]</span><span class="w"></span>\r | |
39 | </pre></div></div></div>\r | |
40 | <div class="paragraph"><p>Using the <span class="monospaced">OptionalArg</span> structure described below, we can define a\r | |
41 | function <span class="monospaced">f'</span>, an optionalized version of <span class="monospaced">f</span>, that takes 0, 1, 2, or\r | |
42 | 3 arguments. Embedded within <span class="monospaced">f'</span> will be default values for <span class="monospaced">i</span>,\r | |
43 | <span class="monospaced">r</span>, and <span class="monospaced">s</span>. If <span class="monospaced">f'</span> gets no arguments, then all the defaults are\r | |
44 | used. If <span class="monospaced">f'</span> gets one argument, then that will be used for <span class="monospaced">i</span>. Two\r | |
45 | arguments will be used for <span class="monospaced">i</span> and <span class="monospaced">r</span> respectively. Three arguments\r | |
46 | will override all default values. Calls to <span class="monospaced">f'</span> will look like the\r | |
47 | following.</p></div>\r | |
48 | <div class="listingblock">\r | |
49 | <div class="content"><div class="highlight"><pre><span class="n">f'</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
50 | <span class="n">f'</span><span class="w"> </span><span class="n">`</span><span class="mi">2</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
51 | <span class="n">f'</span><span class="w"> </span><span class="n">`</span><span class="mi">2</span><span class="w"> </span><span class="n">`</span><span class="mf">3.0</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
52 | <span class="n">f'</span><span class="w"> </span><span class="n">`</span><span class="mi">2</span><span class="w"> </span><span class="n">`</span><span class="mf">3.0</span><span class="w"> </span><span class="n">`</span><span class="s">"four"</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
53 | </pre></div></div></div>\r | |
54 | <div class="paragraph"><p>The optional argument indicator, <span class="monospaced">`</span>, is not special syntax ---\r | |
55 | it is a normal SML value, defined in the <span class="monospaced">OptionalArg</span> structure\r | |
56 | below.</p></div>\r | |
57 | <div class="paragraph"><p>Here is the definition of <span class="monospaced">f'</span> using the <span class="monospaced">OptionalArg</span> structure, in\r | |
58 | particular, <span class="monospaced">OptionalArg.make</span> and <span class="monospaced">OptionalArg.D</span>.</p></div>\r | |
59 | <div class="listingblock">\r | |
60 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">f'</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
61 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
62 | <span class="w"> </span><span class="k">let</span><span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">OptionalArg</span><span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
63 | <span class="w"> </span><span class="n">make</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="mf">2.0</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="s">"three"</span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
64 | <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">r</span><span class="p">,</span><span class="w"> </span><span class="n">s</span><span class="p">))</span><span class="w"></span>\r | |
65 | <span class="w"> </span><span class="n">z</span><span class="w"></span>\r | |
66 | </pre></div></div></div>\r | |
67 | <div class="paragraph"><p>The definition of <span class="monospaced">f'</span> is eta expanded as with all uses of fold. A\r | |
68 | call to <span class="monospaced">OptionalArg.make</span> is supplied with a variable number of\r | |
69 | defaults (in this case, three), the end-of-arguments terminator, <span class="monospaced">$</span>,\r | |
70 | and the function to run, taking its arguments as an n-ary\r | |
71 | <a href="ProductType">product</a>. In this case, the function simply converts\r | |
72 | the product to an ordinary tuple and calls <span class="monospaced">f</span>. Often, the function\r | |
73 | body will simply be written directly.</p></div>\r | |
74 | <div class="paragraph"><p>In general, the definition of an optional-argument function looks like\r | |
75 | the following.</p></div>\r | |
76 | <div class="listingblock">\r | |
77 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
78 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
79 | <span class="w"> </span><span class="k">let</span><span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">OptionalArg</span><span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
80 | <span class="w"> </span><span class="n">make</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="n"><default1></span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="n"><default2></span><span class="p">)</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="n"><defaultn></span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
81 | <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">x2</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">xn</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
82 | <span class="w"> </span><span class="n"><function</span><span class="w"> </span><span class="n">code</span><span class="w"> </span><span class="n">goes</span><span class="w"> </span><span class="n">here></span><span class="p">)</span><span class="w"></span>\r | |
83 | <span class="w"> </span><span class="n">z</span><span class="w"></span>\r | |
84 | </pre></div></div></div>\r | |
85 | <div class="paragraph"><p>Here is the definition of <span class="monospaced">OptionalArg</span>.</p></div>\r | |
86 | <div class="listingblock">\r | |
87 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">OptionalArg</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
88 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
89 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">make</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
90 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
91 | <span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">fold</span><span class="w"></span>\r | |
92 | <span class="w"> </span><span class="p">((</span><span class="n">id</span><span class="p">,</span><span class="w"> </span><span class="k">fn</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">x</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">x</span><span class="p">),</span><span class="w"></span>\r | |
93 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">d</span><span class="p">,</span><span class="w"> </span><span class="n">r</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">func</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
94 | <span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">fold</span><span class="w"> </span><span class="p">((</span><span class="n">id</span><span class="p">,</span><span class="w"> </span><span class="n">d</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="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">d</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
95 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
96 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="p">(</span><span class="n">id</span><span class="p">,</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">d</span><span class="p">)</span><span class="w"></span>\r | |
97 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
98 | <span class="w"> </span><span class="n">func</span><span class="w"> </span><span class="n">d</span><span class="w"></span>\r | |
99 | <span class="w"> </span><span class="k">end</span><span class="p">))</span><span class="w"></span>\r | |
100 | <span class="w"> </span><span class="n">z</span><span class="w"></span>\r | |
101 | \r | |
102 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">D</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">step0</span><span class="w"> </span><span class="p">(</span><span class="k">fn</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">r</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
103 | <span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">ds</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">d</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">ds</span><span class="p">),</span><span class="w"></span>\r | |
104 | <span class="w"> </span><span class="k">fn</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="w"> </span><span class="n">&</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="n">r</span><span class="w"> </span><span class="p">(</span><span class="k">fn</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">f</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">x</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">)))</span><span class="w"></span>\r | |
105 | \r | |
106 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">`</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
107 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
108 | <span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">step1</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">d</span><span class="p">))</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">d</span><span class="p">),</span><span class="w"> </span><span class="n">d</span><span class="p">))</span><span class="w"></span>\r | |
109 | <span class="w"> </span><span class="n">z</span><span class="w"></span>\r | |
110 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
111 | </pre></div></div></div>\r | |
112 | <div class="paragraph"><p><span class="monospaced">OptionalArg.make</span> uses a nested fold. The first <span class="monospaced">fold</span> accumulates\r | |
113 | the default values in a product, associated to the right, and a\r | |
114 | reversal function that converts a product (of the same arity as the\r | |
115 | number of defaults) from right associativity to left associativity.\r | |
116 | The accumulated defaults are used by the second fold, which recurs\r | |
117 | over the product, replacing the appropriate component as it encounters\r | |
118 | optional arguments. The second fold also constructs a "fill"\r | |
119 | function, <span class="monospaced">f</span>, that is used to reconstruct the product once the\r | |
120 | end-of-arguments is reached. Finally, the finisher reconstructs the\r | |
121 | product and uses the reversal function to convert the product from\r | |
122 | right associative to left associative, at which point it is passed to\r | |
123 | the user-supplied function.</p></div>\r | |
124 | <div class="paragraph"><p>Much of the complexity comes from the fact that while recurring over a\r | |
125 | product from left to right, one wants it to be right-associative,\r | |
126 | e.g., look like</p></div>\r | |
127 | <div class="listingblock">\r | |
128 | <div class="content"><div class="highlight"><pre><span class="n">a</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="p">(</span><span class="n">b</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="p">(</span><span class="n">c</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">d</span><span class="p">))</span><span class="w"></span>\r | |
129 | </pre></div></div></div>\r | |
130 | <div class="paragraph"><p>but the user function in the end wants the product to be left\r | |
131 | associative, so that the product argument pattern can be written\r | |
132 | without parentheses (since <span class="monospaced">&</span> is left associative).</p></div>\r | |
133 | </div>\r | |
134 | </div>\r | |
135 | <div class="sect1">\r | |
136 | <h2 id="_labelled_optional_arguments">Labelled optional arguments</h2>\r | |
137 | <div class="sectionbody">\r | |
138 | <div class="paragraph"><p>In addition to the positional optional arguments described above, it\r | |
139 | is sometimes useful to have labelled optional arguments. These allow\r | |
140 | one to define a function, <span class="monospaced">f</span>, with defaults, say <span class="monospaced">a</span> and <span class="monospaced">b</span>. Then,\r | |
141 | a caller of <span class="monospaced">f</span> can supply values for <span class="monospaced">a</span> and <span class="monospaced">b</span> by name. If no\r | |
142 | value is supplied then the default is used.</p></div>\r | |
143 | <div class="paragraph"><p>Labelled optional arguments are a simple extension of\r | |
144 | <a href="FunctionalRecordUpdate">FunctionalRecordUpdate</a> using post composition. Suppose, for\r | |
145 | example, that one wants a function <span class="monospaced">f</span> with labelled optional\r | |
146 | arguments <span class="monospaced">a</span> and <span class="monospaced">b</span> with default values <span class="monospaced">0</span> and <span class="monospaced">0.0</span> respectively.\r | |
147 | If one has a functional-record-update function <span class="monospaced">updateAB</span> for records\r | |
148 | with <span class="monospaced">a</span> and <span class="monospaced">b</span> fields, then one can define <span class="monospaced">f</span> in the following way.</p></div>\r | |
149 | <div class="listingblock">\r | |
150 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
151 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
152 | <span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">post</span><span class="w"></span>\r | |
153 | <span class="w"> </span><span class="p">(</span><span class="n">updateAB</span><span class="w"> </span><span class="p">{</span><span class="n">a</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mf">0.0</span><span class="p">},</span><span class="w"></span>\r | |
154 | <span class="w"> </span><span class="k">fn</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="p">=></span><span class="w"> </span><span class="n">print</span><span class="w"> </span><span class="p">(</span><span class="n">concat</span><span class="w"> </span><span class="p">[</span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="s">" "</span><span class="p">,</span><span class="w"></span>\r | |
155 | <span class="w"> </span><span class="n">Real</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">]))</span><span class="w"></span>\r | |
156 | <span class="w"> </span><span class="n">z</span><span class="w"></span>\r | |
157 | </pre></div></div></div>\r | |
158 | <div class="paragraph"><p>The idea is that <span class="monospaced">f</span> is the post composition (using <span class="monospaced">Fold.post</span>) of\r | |
159 | the actual code for the function with a functional-record updater that\r | |
160 | starts with the defaults.</p></div>\r | |
161 | <div class="paragraph"><p>Here are some example calls to <span class="monospaced">f</span>.</p></div>\r | |
162 | <div class="listingblock">\r | |
163 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
164 | <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">a</span><span class="w"> </span><span class="mi">13</span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
165 | <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">a</span><span class="w"> </span><span class="mi">13</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">b</span><span class="w"> </span><span class="mf">17.5</span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
166 | <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">b</span><span class="w"> </span><span class="mf">17.5</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">a</span><span class="w"> </span><span class="mi">13</span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
167 | </pre></div></div></div>\r | |
168 | <div class="paragraph"><p>Notice that a caller can supply neither of the arguments, either of\r | |
169 | the arguments, or both of the arguments, and in either order. All\r | |
170 | that matter is that the arguments be labelled correctly (and of the\r | |
171 | right type, of course).</p></div>\r | |
172 | <div class="paragraph"><p>Here is another example.</p></div>\r | |
173 | <div class="listingblock">\r | |
174 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
175 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
176 | <span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">post</span><span class="w"></span>\r | |
177 | <span class="w"> </span><span class="p">(</span><span class="n">updateBCD</span><span class="w"> </span><span class="p">{</span><span class="n">b</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mf">0.0</span><span class="p">,</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="s">"<>"</span><span class="p">},</span><span class="w"></span>\r | |
178 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">{</span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">c</span><span class="p">,</span><span class="w"> </span><span class="n">d</span><span class="p">}</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
179 | <span class="w"> </span><span class="n">print</span><span class="w"> </span><span class="p">(</span><span class="n">concat</span><span class="w"> </span><span class="p">[</span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="s">" "</span><span class="p">,</span><span class="w"></span>\r | |
180 | <span class="w"> </span><span class="n">Real</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">c</span><span class="p">,</span><span class="w"> </span><span class="s">" "</span><span class="p">,</span><span class="w"></span>\r | |
181 | <span class="w"> </span><span class="n">d</span><span class="p">,</span><span class="w"> </span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">]))</span><span class="w"></span>\r | |
182 | <span class="w"> </span><span class="n">z</span><span class="w"></span>\r | |
183 | </pre></div></div></div>\r | |
184 | <div class="paragraph"><p>Here are some example calls.</p></div>\r | |
185 | <div class="listingblock">\r | |
186 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
187 | <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">d</span><span class="w"> </span><span class="s">"goodbye"</span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
188 | <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">d</span><span class="w"> </span><span class="s">"hello"</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">b</span><span class="w"> </span><span class="mi">17</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="n">U</span><span class="p">#</span><span class="n">c</span><span class="w"> </span><span class="mf">19.3</span><span class="p">)</span><span class="w"> </span><span class="n">$</span><span class="w"></span>\r | |
189 | </pre></div></div></div>\r | |
190 | </div>\r | |
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