Commit | Line | Data |
---|---|---|
7f918cf1 CE |
1 | <!DOCTYPE html>\r |
2 | <html lang="en">\r | |
3 | <head>\r | |
4 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">\r | |
5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
6 | <title>ObjectOrientedProgramming</title>\r | |
7 | <link rel="stylesheet" href="./asciidoc.css" type="text/css">\r | |
8 | <link rel="stylesheet" href="./pygments.css" type="text/css">\r | |
9 | \r | |
10 | \r | |
11 | <script type="text/javascript" src="./asciidoc.js"></script>\r | |
12 | <script type="text/javascript">\r | |
13 | /*<![CDATA[*/\r | |
14 | asciidoc.install();\r | |
15 | /*]]>*/\r | |
16 | </script>\r | |
17 | <link rel="stylesheet" href="./mlton.css" type="text/css">\r | |
18 | </head>\r | |
19 | <body class="article">\r | |
20 | <div id="banner">\r | |
21 | <div id="banner-home">\r | |
22 | <a href="./Home">MLton 20180207</a>\r | |
23 | </div>\r | |
24 | </div>\r | |
25 | <div id="header">\r | |
26 | <h1>ObjectOrientedProgramming</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 explicit support for\r | |
32 | object-oriented programming. Here are some papers that show how to\r | |
33 | express certain object-oriented concepts in SML.</p></div>\r | |
34 | <div class="ulist"><ul>\r | |
35 | <li>\r | |
36 | <p>\r | |
37 | <a href="References#Berthomieu00"> OO Programming styles in ML</a>\r | |
38 | </p>\r | |
39 | </li>\r | |
40 | <li>\r | |
41 | <p>\r | |
42 | <a href="References#ThorupTofte94"> Object-oriented programming and Standard ML</a>\r | |
43 | </p>\r | |
44 | </li>\r | |
45 | <li>\r | |
46 | <p>\r | |
47 | <a href="References#LarsenNiss04"> mGTK: An SML binding of Gtk+</a>\r | |
48 | </p>\r | |
49 | </li>\r | |
50 | <li>\r | |
51 | <p>\r | |
52 | <a href="References#FluetPucella06"> Phantom Types and Subtyping</a>\r | |
53 | </p>\r | |
54 | </li>\r | |
55 | </ul></div>\r | |
56 | <div class="paragraph"><p>The question of OO programming in SML comes up every now and then.\r | |
57 | The following discusses a simple object-oriented (OO) programming\r | |
58 | technique in Standard ML. The reader is assumed to be able to read\r | |
59 | Java and SML code.</p></div>\r | |
60 | </div>\r | |
61 | </div>\r | |
62 | <div class="sect1">\r | |
63 | <h2 id="_motivation">Motivation</h2>\r | |
64 | <div class="sectionbody">\r | |
65 | <div class="paragraph"><p>SML doesn’t provide subtyping, but it does provide parametric\r | |
66 | polymorphism, which can be used to encode some forms of subtyping.\r | |
67 | Most articles on OO programming in SML concentrate on such encoding\r | |
68 | techniques. While those techniques are interesting — and it is\r | |
69 | recommended to read such articles — and sometimes useful, it seems\r | |
70 | that basically all OO gurus agree that (deep) subtyping (or\r | |
71 | inheritance) hierarchies aren’t as practical as they were thought to\r | |
72 | be in the early OO days. "Good", flexible, "OO" designs tend to have\r | |
73 | a flat structure</p></div>\r | |
74 | <div class="listingblock">\r | |
75 | <div class="content monospaced">\r | |
76 | <pre> Interface\r | |
77 | ^\r | |
78 | |\r | |
79 | - - -+-------+-------+- - -\r | |
80 | | | |\r | |
81 | ImplA ImplB ImplC</pre>\r | |
82 | </div></div>\r | |
83 | <div class="paragraph"><p>and deep inheritance hierarchies</p></div>\r | |
84 | <div class="listingblock">\r | |
85 | <div class="content monospaced">\r | |
86 | <pre>ClassA\r | |
87 | ^\r | |
88 | |\r | |
89 | ClassB\r | |
90 | ^\r | |
91 | |\r | |
92 | ClassC\r | |
93 | ^\r | |
94 | |</pre>\r | |
95 | </div></div>\r | |
96 | <div class="paragraph"><p>tend to be signs of design mistakes. There are good underlying\r | |
97 | reasons for this, but a thorough discussion is not in the scope of\r | |
98 | this article. However, the point is that perhaps the encoding of\r | |
99 | subtyping is not as important as one might believe. In the following\r | |
100 | we ignore subtyping and rather concentrate on a very simple and basic\r | |
101 | dynamic dispatch technique.</p></div>\r | |
102 | </div>\r | |
103 | </div>\r | |
104 | <div class="sect1">\r | |
105 | <h2 id="_dynamic_dispatch_using_a_recursive_record_of_functions">Dynamic Dispatch Using a Recursive Record of Functions</h2>\r | |
106 | <div class="sectionbody">\r | |
107 | <div class="paragraph"><p>Quite simply, the basic idea is to implement a "virtual function\r | |
108 | table" using a record that is wrapped inside a (possibly recursive)\r | |
109 | datatype. Let’s first take a look at a simple concrete example.</p></div>\r | |
110 | <div class="paragraph"><p>Consider the following Java interface:</p></div>\r | |
111 | <div class="listingblock">\r | |
112 | <div class="content monospaced">\r | |
113 | <pre>public interface Counter {\r | |
114 | public void inc();\r | |
115 | public int get();\r | |
116 | }</pre>\r | |
117 | </div></div>\r | |
118 | <div class="paragraph"><p>We can translate the <span class="monospaced">Counter</span> interface to SML as follows:</p></div>\r | |
119 | <div class="listingblock">\r | |
120 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">counter</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Counter</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">inc</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"> </span><span class="n">get</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">int</span><span class="p">}</span><span class="w"></span>\r | |
121 | </pre></div></div></div>\r | |
122 | <div class="paragraph"><p>Each value of type <span class="monospaced">counter</span> can be thought of as an object that\r | |
123 | responds to two messages <span class="monospaced">inc</span> and <span class="monospaced">get</span>. To actually send messages\r | |
124 | to a counter, it is useful to define auxiliary functions</p></div>\r | |
125 | <div class="listingblock">\r | |
126 | <div class="content"><div class="highlight"><pre><span class="k">local</span><span class="w"></span>\r | |
127 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">mk</span><span class="w"> </span><span class="n">m</span><span class="w"> </span><span class="p">(</span><span class="n">Counter</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">m</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
128 | <span class="k">in</span><span class="w"></span>\r | |
129 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">cGet</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">mk</span><span class="p">#</span><span class="n">get</span><span class="w"></span>\r | |
130 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">cInc</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">mk</span><span class="p">#</span><span class="n">inc</span><span class="w"></span>\r | |
131 | <span class="k">end</span><span class="w"></span>\r | |
132 | </pre></div></div></div>\r | |
133 | <div class="paragraph"><p>that basically extract the "function table" <span class="monospaced">t</span> from a counter object\r | |
134 | and then select the specified method <span class="monospaced">m</span> from the table.</p></div>\r | |
135 | <div class="paragraph"><p>Let’s then implement a simple function that increments a counter until a\r | |
136 | given maximum is reached:</p></div>\r | |
137 | <div class="listingblock">\r | |
138 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">incUpto</span><span class="w"> </span><span class="n">counter</span><span class="w"> </span><span class="n">max</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="n">cGet</span><span class="w"> </span><span class="n">counter</span><span class="w"> </span><span class="n"><</span><span class="w"> </span><span class="n">max</span><span class="w"> </span><span class="k">do</span><span class="w"> </span><span class="n">cInc</span><span class="w"> </span><span class="n">counter</span><span class="w"></span>\r | |
139 | </pre></div></div></div>\r | |
140 | <div class="paragraph"><p>You can easily verify that the above code compiles even without any\r | |
141 | concrete implementation of a counter, thus it is clear that it doesn’t\r | |
142 | depend on a particular counter implementation.</p></div>\r | |
143 | <div class="paragraph"><p>Let’s then implement a couple of counters. First consider the\r | |
144 | following Java class implementing the <span class="monospaced">Counter</span> interface given earlier.</p></div>\r | |
145 | <div class="listingblock">\r | |
146 | <div class="content monospaced">\r | |
147 | <pre>public class BasicCounter implements Counter {\r | |
148 | private int cnt;\r | |
149 | public BasicCounter(int initialCnt) { this.cnt = initialCnt; }\r | |
150 | public void inc() { this.cnt += 1; }\r | |
151 | public int get() { return this.cnt; }\r | |
152 | }</pre>\r | |
153 | </div></div>\r | |
154 | <div class="paragraph"><p>We can translate the above to SML as follows:</p></div>\r | |
155 | <div class="listingblock">\r | |
156 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">newBasicCounter</span><span class="w"> </span><span class="n">initialCnt</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
157 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">cnt</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="n">initialCnt</span><span class="w"></span>\r | |
158 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
159 | <span class="w"> </span><span class="n">Counter</span><span class="w"> </span><span class="p">{</span><span class="n">inc</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="n">cnt</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">!cnt</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>\r | |
160 | <span class="w"> </span><span class="n">get</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="n">!cnt</span><span class="p">}</span><span class="w"></span>\r | |
161 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
162 | </pre></div></div></div>\r | |
163 | <div class="paragraph"><p>The SML function <span class="monospaced">newBasicCounter</span> can be described as a constructor\r | |
164 | function for counter objects of the <span class="monospaced">BasicCounter</span> "class". We can\r | |
165 | also have other counter implementations. Here is the constructor for\r | |
166 | a counter decorator that logs messages:</p></div>\r | |
167 | <div class="listingblock">\r | |
168 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">newLoggedCounter</span><span class="w"> </span><span class="n">counter</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
169 | <span class="w"> </span><span class="n">Counter</span><span class="w"> </span><span class="p">{</span><span class="n">inc</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="n">print</span><span class="w"> </span><span class="s">"inc</span><span class="se">\n</span><span class="s">"</span><span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="n">cInc</span><span class="w"> </span><span class="n">counter</span><span class="p">),</span><span class="w"></span>\r | |
170 | <span class="w"> </span><span class="n">get</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="n">print</span><span class="w"> </span><span class="s">"get</span><span class="se">\n</span><span class="s">"</span><span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="n">cGet</span><span class="w"> </span><span class="n">counter</span><span class="p">)}</span><span class="w"></span>\r | |
171 | </pre></div></div></div>\r | |
172 | <div class="paragraph"><p>The <span class="monospaced">incUpto</span> function works just as well with objects of either\r | |
173 | class:</p></div>\r | |
174 | <div class="listingblock">\r | |
175 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">aCounter</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">newBasicCounter</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>\r | |
176 | <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">incUpto</span><span class="w"> </span><span class="n">aCounter</span><span class="w"> </span><span class="mi">5</span><span class="w"></span>\r | |
177 | <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">print</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="p">(</span><span class="n">cGet</span><span class="w"> </span><span class="n">aCounter</span><span class="p">)</span><span class="w"> </span><span class="n">^</span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span><span class="w"></span>\r | |
178 | \r | |
179 | <span class="k">val</span><span class="w"> </span><span class="n">aCounter</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">newLoggedCounter</span><span class="w"> </span><span class="p">(</span><span class="n">newBasicCounter</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"></span>\r | |
180 | <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">incUpto</span><span class="w"> </span><span class="n">aCounter</span><span class="w"> </span><span class="mi">5</span><span class="w"></span>\r | |
181 | <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">print</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="p">(</span><span class="n">cGet</span><span class="w"> </span><span class="n">aCounter</span><span class="p">)</span><span class="w"> </span><span class="n">^</span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span><span class="w"></span>\r | |
182 | </pre></div></div></div>\r | |
183 | <div class="paragraph"><p>In general, a dynamic dispatch interface is represented as a record\r | |
184 | type wrapped inside a datatype. Each field of the record corresponds\r | |
185 | to a public method or field of the object:</p></div>\r | |
186 | <div class="listingblock">\r | |
187 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">interface</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
188 | <span class="w"> </span><span class="n">Interface</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">method</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">t1</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">t2</span><span class="p">,</span><span class="w"></span>\r | |
189 | <span class="w"> </span><span class="n">immutableField</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="p">,</span><span class="w"></span>\r | |
190 | <span class="w"> </span><span class="n">mutableField</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">ref</span><span class="p">}</span><span class="w"></span>\r | |
191 | </pre></div></div></div>\r | |
192 | <div class="paragraph"><p>The reason for wrapping the record inside a datatype is that records,\r | |
193 | in SML, can not be recursive. However, SML datatypes can be\r | |
194 | recursive. A record wrapped in a datatype can contain fields that\r | |
195 | contain the datatype. For example, an interface such as <span class="monospaced">Cloneable</span></p></div>\r | |
196 | <div class="listingblock">\r | |
197 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">cloneable</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Cloneable</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">clone</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">cloneable</span><span class="p">}</span><span class="w"></span>\r | |
198 | </pre></div></div></div>\r | |
199 | <div class="paragraph"><p>can be represented using recursive datatypes.</p></div>\r | |
200 | <div class="paragraph"><p>Like in OO languages, interfaces are abstract and can not be\r | |
201 | instantiated to produce objects. To be able to instantiate objects,\r | |
202 | the constructors of a concrete class are needed. In SML, we can\r | |
203 | implement constructors as simple functions from arbitrary arguments to\r | |
204 | values of the interface type. Such a constructor function can\r | |
205 | encapsulate arbitrary private state and functions using lexical\r | |
206 | closure. It is also easy to share implementations of methods between\r | |
207 | two or more constructors.</p></div>\r | |
208 | <div class="paragraph"><p>While the <span class="monospaced">Counter</span> example is rather trivial, it should not be\r | |
209 | difficult to see that this technique quite simply doesn’t require a huge\r | |
210 | amount of extra verbiage and is more than usable in practice.</p></div>\r | |
211 | </div>\r | |
212 | </div>\r | |
213 | <div class="sect1">\r | |
214 | <h2 id="_sml_modules_and_dynamic_dispatch">SML Modules and Dynamic Dispatch</h2>\r | |
215 | <div class="sectionbody">\r | |
216 | <div class="paragraph"><p>One might wonder about how SML modules and the dynamic dispatch\r | |
217 | technique work together. Let’s investigate! Let’s use a simple\r | |
218 | dispenser framework as a concrete example. (Note that this isn’t\r | |
219 | intended to be an introduction to the SML module system.)</p></div>\r | |
220 | <div class="sect2">\r | |
221 | <h3 id="_programming_with_sml_modules">Programming with SML Modules</h3>\r | |
222 | <div class="paragraph"><p>Using SML signatures we can specify abstract data types (ADTs) such as\r | |
223 | dispensers. Here is a signature for an "abstract" functional (as\r | |
224 | opposed to imperative) dispenser:</p></div>\r | |
225 | <div class="listingblock">\r | |
226 | <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">ABSTRACT_DISPENSER</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
227 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
228 | <span class="w"> </span><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 | |
229 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">push</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">*</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">t</span><span class="w"></span>\r | |
230 | <span class="w"> </span><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="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">option</span><span class="w"></span>\r | |
231 | <span class="k">end</span><span class="w"></span>\r | |
232 | </pre></div></div></div>\r | |
233 | <div class="paragraph"><p>The term "abstract" in the name of the signature refers to the fact that\r | |
234 | the signature gives no way to instantiate a dispenser. It has nothing to\r | |
235 | do with the concept of abstract data types.</p></div>\r | |
236 | <div class="paragraph"><p>Using SML functors we can write "generic" algorithms that manipulate\r | |
237 | dispensers of an unknown type. Here are a couple of very simple\r | |
238 | algorithms:</p></div>\r | |
239 | <div class="listingblock">\r | |
240 | <div class="content"><div class="highlight"><pre><span class="k">functor</span><span class="w"> </span><span class="n">DispenserAlgs</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">ABSTRACT_DISPENSER</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
241 | <span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">D</span><span class="w"></span>\r | |
242 | \r | |
243 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">pushAll</span><span class="w"> </span><span class="p">(</span><span class="n">xs</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><span class="n">foldl</span><span class="w"> </span><span class="n">push</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="n">xs</span><span class="w"></span>\r | |
244 | \r | |
245 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">popAll</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">let</span><span class="w"></span>\r | |
246 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">lp</span><span class="w"> </span><span class="p">(</span><span class="n">xs</span><span class="p">,</span><span class="w"> </span><span class="n">NONE</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">rev</span><span class="w"> </span><span class="n">xs</span><span class="w"></span>\r | |
247 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">lp</span><span class="w"> </span><span class="p">(</span><span class="n">xs</span><span class="p">,</span><span class="w"> </span><span class="n">SOME</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">d</span><span class="p">))</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">lp</span><span class="w"> </span><span class="p">(</span><span class="n">x::xs</span><span class="p">,</span><span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="n">d</span><span class="p">)</span><span class="w"></span>\r | |
248 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
249 | <span class="w"> </span><span class="n">lp</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">d</span><span class="p">)</span><span class="w"></span>\r | |
250 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
251 | \r | |
252 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">cp</span><span class="w"> </span><span class="p">(</span><span class="n">from</span><span class="p">,</span><span class="w"> </span><span class="n">to</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">pushAll</span><span class="w"> </span><span class="p">(</span><span class="n">popAll</span><span class="w"> </span><span class="n">from</span><span class="p">,</span><span class="w"> </span><span class="n">to</span><span class="p">)</span><span class="w"></span>\r | |
253 | <span class="k">end</span><span class="w"></span>\r | |
254 | </pre></div></div></div>\r | |
255 | <div class="paragraph"><p>As one can easily verify, the above compiles even without any concrete\r | |
256 | dispenser structure. Functors essentially provide a form a static\r | |
257 | dispatch that one can use to break compile-time dependencies.</p></div>\r | |
258 | <div class="paragraph"><p>We can also give a signature for a concrete dispenser</p></div>\r | |
259 | <div class="listingblock">\r | |
260 | <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">DISPENSER</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
261 | <span class="w"> </span><span class="k">include</span><span class="w"> </span><span class="n">ABSTRACT_DISPENSER</span><span class="w"></span>\r | |
262 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">empty</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>\r | |
263 | <span class="k">end</span><span class="w"></span>\r | |
264 | </pre></div></div></div>\r | |
265 | <div class="paragraph"><p>and write any number of concrete structures implementing the signature.\r | |
266 | For example, we could implement stacks</p></div>\r | |
267 | <div class="listingblock">\r | |
268 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Stack</span><span class="w"> </span><span class="p">:></span><span class="w"> </span><span class="n">DISPENSER</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
269 | <span class="w"> </span><span class="k">type</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">list</span><span class="w"></span>\r | |
270 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">empty</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">[]</span><span class="w"></span>\r | |
271 | <span class="w"> </span><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">null</span><span class="w"></span>\r | |
272 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">push</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="n">::</span><span class="w"></span>\r | |
273 | <span class="w"> </span><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">List</span><span class="p">.</span><span class="n">getItem</span><span class="w"></span>\r | |
274 | <span class="k">end</span><span class="w"></span>\r | |
275 | </pre></div></div></div>\r | |
276 | <div class="paragraph"><p>and queues</p></div>\r | |
277 | <div class="listingblock">\r | |
278 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Queue</span><span class="w"> </span><span class="p">:></span><span class="w"> </span><span class="n">DISPENSER</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
279 | <span class="w"> </span><span class="k">datatype</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">T</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">list</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">list</span><span class="w"></span>\r | |
280 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">empty</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">([],</span><span class="w"> </span><span class="p">[])</span><span class="w"></span>\r | |
281 | <span class="w"> </span><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="k">fn</span><span class="w"> </span><span class="n">T</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><span class="n">true</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><span class="n">false</span><span class="w"></span>\r | |
282 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">normalize</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="n">ys</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="n">rev</span><span class="w"> </span><span class="n">ys</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><span class="n">q</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">q</span><span class="w"></span>\r | |
283 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">push</span><span class="w"> </span><span class="p">(</span><span class="n">y</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">(</span><span class="n">xs</span><span class="p">,</span><span class="w"> </span><span class="n">ys</span><span class="p">))</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">(</span><span class="n">normalize</span><span class="w"> </span><span class="p">(</span><span class="n">xs</span><span class="p">,</span><span class="w"> </span><span class="n">y::ys</span><span class="p">))</span><span class="w"></span>\r | |
284 | <span class="w"> </span><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="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">T</span><span class="w"> </span><span class="p">(</span><span class="n">x::xs</span><span class="p">,</span><span class="w"> </span><span class="n">ys</span><span class="p">))</span><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="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">(</span><span class="n">normalize</span><span class="w"> </span><span class="p">(</span><span class="n">xs</span><span class="p">,</span><span class="w"> </span><span class="n">ys</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><span class="p">=></span><span class="w"> </span><span class="n">NONE</span><span class="w"></span>\r | |
285 | <span class="k">end</span><span class="w"></span>\r | |
286 | </pre></div></div></div>\r | |
287 | <div class="paragraph"><p>One can now write code that uses either the <span class="monospaced">Stack</span> or the <span class="monospaced">Queue</span>\r | |
288 | dispenser. One can also instantiate the previously defined functor to\r | |
289 | create functions for manipulating dispensers of a type:</p></div>\r | |
290 | <div class="listingblock">\r | |
291 | <div class="content"><div class="highlight"><pre><span class="k">structure</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">DispenserAlgs</span><span class="w"> </span><span class="p">(</span><span class="n">Stack</span><span class="p">)</span><span class="w"></span>\r | |
292 | <span class="k">val</span><span class="w"> </span><span class="p">[</span><span class="mi">4</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">1</span><span class="p">]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">popAll</span><span class="w"> </span><span class="p">(</span><span class="n">S</span><span class="p">.</span><span class="n">pushAll</span><span class="w"> </span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">],</span><span class="w"> </span><span class="n">Stack</span><span class="p">.</span><span class="n">empty</span><span class="p">))</span><span class="w"></span>\r | |
293 | \r | |
294 | <span class="k">structure</span><span class="w"> </span><span class="n">Q</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">DispenserAlgs</span><span class="w"> </span><span class="p">(</span><span class="n">Queue</span><span class="p">)</span><span class="w"></span>\r | |
295 | <span class="k">val</span><span class="w"> </span><span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Q</span><span class="p">.</span><span class="n">popAll</span><span class="w"> </span><span class="p">(</span><span class="n">Q</span><span class="p">.</span><span class="n">pushAll</span><span class="w"> </span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">],</span><span class="w"> </span><span class="n">Queue</span><span class="p">.</span><span class="n">empty</span><span class="p">))</span><span class="w"></span>\r | |
296 | </pre></div></div></div>\r | |
297 | <div class="paragraph"><p>There is no dynamic dispatch involved at the module level in SML. An\r | |
298 | attempt to do dynamic dispatch</p></div>\r | |
299 | <div class="listingblock">\r | |
300 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">q</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Q</span><span class="p">.</span><span class="n">push</span><span class="w"> </span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">Stack</span><span class="p">.</span><span class="n">empty</span><span class="p">)</span><span class="w"></span>\r | |
301 | </pre></div></div></div>\r | |
302 | <div class="paragraph"><p>will give a type error.</p></div>\r | |
303 | </div>\r | |
304 | <div class="sect2">\r | |
305 | <h3 id="_combining_sml_modules_and_dynamic_dispatch">Combining SML Modules and Dynamic Dispatch</h3>\r | |
306 | <div class="paragraph"><p>Let’s then combine SML modules and the dynamic dispatch technique\r | |
307 | introduced in this article. First we define an interface for\r | |
308 | dispensers:</p></div>\r | |
309 | <div class="listingblock">\r | |
310 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Dispenser</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
311 | <span class="w"> </span><span class="k">datatype</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>\r | |
312 | <span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">isEmpty</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">bool</span><span class="p">,</span><span class="w"></span>\r | |
313 | <span class="w"> </span><span class="n">push</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</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="p">,</span><span class="w"></span>\r | |
314 | <span class="w"> </span><span class="n">pop</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">option</span><span class="p">}</span><span class="w"></span>\r | |
315 | \r | |
316 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">O</span><span class="w"> </span><span class="n">m</span><span class="w"> </span><span class="p">(</span><span class="n">I</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">m</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
317 | \r | |
318 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">isEmpty</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">O</span><span class="p">#</span><span class="n">isEmpty</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
319 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">push</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">t</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">O</span><span class="p">#</span><span class="n">push</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">v</span><span class="w"></span>\r | |
320 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">pop</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">O</span><span class="p">#</span><span class="n">pop</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
321 | <span class="k">end</span><span class="w"></span>\r | |
322 | </pre></div></div></div>\r | |
323 | <div class="paragraph"><p>The <span class="monospaced">Dispenser</span> module, which we can think of as an interface for\r | |
324 | dispensers, implements the <span class="monospaced">ABSTRACT_DISPENSER</span> signature using\r | |
325 | the dynamic dispatch technique, but we leave the signature ascription\r | |
326 | until later.</p></div>\r | |
327 | <div class="paragraph"><p>Then we define a <span class="monospaced">DispenserClass</span> functor that makes a "class" out of\r | |
328 | a given dispenser module:</p></div>\r | |
329 | <div class="listingblock">\r | |
330 | <div class="content"><div class="highlight"><pre><span class="k">functor</span><span class="w"> </span><span class="n">DispenserClass</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">DISPENSER</span><span class="p">)</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">DISPENSER</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
331 | <span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">Dispenser</span><span class="w"></span>\r | |
332 | \r | |
333 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">make</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
334 | <span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="p">{</span><span class="n">isEmpty</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="n">D</span><span class="p">.</span><span class="n">isEmpty</span><span class="w"> </span><span class="n">d</span><span class="p">,</span><span class="w"></span>\r | |
335 | <span class="w"> </span><span class="n">push</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">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">make</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="p">.</span><span class="n">push</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">d</span><span class="p">)),</span><span class="w"></span>\r | |
336 | <span class="w"> </span><span class="n">pop</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>\r | |
337 | <span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">D</span><span class="p">.</span><span class="n">pop</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
338 | <span class="w"> </span><span class="n">NONE</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">NONE</span><span class="w"></span>\r | |
339 | <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="n">x</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><span class="n">SOME</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">make</span><span class="w"> </span><span class="n">d</span><span class="p">)}</span><span class="w"></span>\r | |
340 | \r | |
341 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">empty</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
342 | <span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="p">{</span><span class="n">isEmpty</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="n">true</span><span class="p">,</span><span class="w"></span>\r | |
343 | <span class="w"> </span><span class="n">push</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">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">make</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="p">.</span><span class="n">push</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">D</span><span class="p">.</span><span class="n">empty</span><span class="p">)),</span><span class="w"></span>\r | |
344 | <span class="w"> </span><span class="n">pop</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="n">NONE</span><span class="p">}</span><span class="w"></span>\r | |
345 | <span class="k">end</span><span class="w"></span>\r | |
346 | </pre></div></div></div>\r | |
347 | <div class="paragraph"><p>Finally we seal the <span class="monospaced">Dispenser</span> module:</p></div>\r | |
348 | <div class="listingblock">\r | |
349 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Dispenser</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">ABSTRACT_DISPENSER</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Dispenser</span><span class="w"></span>\r | |
350 | </pre></div></div></div>\r | |
351 | <div class="paragraph"><p>This isn’t necessary for type safety, because the unsealed <span class="monospaced">Dispenser</span>\r | |
352 | module does not allow one to break encapsulation, but makes sure that\r | |
353 | only the <span class="monospaced">DispenserClass</span> functor can create dispenser classes\r | |
354 | (because the constructor <span class="monospaced">Dispenser.I</span> is no longer accessible).</p></div>\r | |
355 | <div class="paragraph"><p>Using the <span class="monospaced">DispenserClass</span> functor we can turn any concrete dispenser\r | |
356 | module into a dispenser class:</p></div>\r | |
357 | <div class="listingblock">\r | |
358 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">StackClass</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">DispenserClass</span><span class="w"> </span><span class="p">(</span><span class="n">Stack</span><span class="p">)</span><span class="w"></span>\r | |
359 | <span class="k">structure</span><span class="w"> </span><span class="n">QueueClass</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">DispenserClass</span><span class="w"> </span><span class="p">(</span><span class="n">Queue</span><span class="p">)</span><span class="w"></span>\r | |
360 | </pre></div></div></div>\r | |
361 | <div class="paragraph"><p>Each dispenser class implements the same dynamic dispatch interface\r | |
362 | and the <span class="monospaced">ABSTRACT_DISPENSER</span> -signature.</p></div>\r | |
363 | <div class="paragraph"><p>Because the dynamic dispatch <span class="monospaced">Dispenser</span> module implements the\r | |
364 | <span class="monospaced">ABSTRACT_DISPENSER</span>-signature, we can use it to instantiate the\r | |
365 | <span class="monospaced">DispenserAlgs</span>-functor:</p></div>\r | |
366 | <div class="listingblock">\r | |
367 | <div class="content"><div class="highlight"><pre><span class="k">structure</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">DispenserAlgs</span><span class="w"> </span><span class="p">(</span><span class="n">Dispenser</span><span class="p">)</span><span class="w"></span>\r | |
368 | </pre></div></div></div>\r | |
369 | <div class="paragraph"><p>The resulting <span class="monospaced">D</span> module, like the <span class="monospaced">Dispenser</span> module, works with\r | |
370 | any dispenser class and uses dynamic dispatch:</p></div>\r | |
371 | <div class="listingblock">\r | |
372 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="p">[</span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">D</span><span class="p">.</span><span class="n">popAll</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="p">.</span><span class="n">pushAll</span><span class="w"> </span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">],</span><span class="w"> </span><span class="n">StackClass</span><span class="p">.</span><span class="n">empty</span><span class="p">))</span><span class="w"></span>\r | |
373 | <span class="k">val</span><span class="w"> </span><span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">D</span><span class="p">.</span><span class="n">popAll</span><span class="w"> </span><span class="p">(</span><span class="n">D</span><span class="p">.</span><span class="n">pushAll</span><span class="w"> </span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">],</span><span class="w"> </span><span class="n">QueueClass</span><span class="p">.</span><span class="n">empty</span><span class="p">))</span><span class="w"></span>\r | |
374 | </pre></div></div></div>\r | |
375 | </div>\r | |
376 | </div>\r | |
377 | </div>\r | |
378 | </div>\r | |
379 | <div id="footnotes"><hr></div>\r | |
380 | <div id="footer">\r | |
381 | <div id="footer-text">\r | |
382 | </div>\r | |
383 | <div id="footer-badges">\r | |
384 | </div>\r | |
385 | </div>\r | |
386 | </body>\r | |
387 | </html>\r |