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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>ReturnStatement</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>Programmers coming from languages that have a <span class="monospaced">return</span> statement, such\r | |
32 | as C, Java, and Python, often ask how one can translate functions that\r | |
33 | return early into SML. This page briefly describes a number of ways\r | |
34 | to translate uses of <span class="monospaced">return</span> to SML.</p></div>\r | |
35 | </div>\r | |
36 | </div>\r | |
37 | <div class="sect1">\r | |
38 | <h2 id="_conditional_iterator_function">Conditional iterator function</h2>\r | |
39 | <div class="sectionbody">\r | |
40 | <div class="paragraph"><p>A conditional iterator function, such as\r | |
41 | <a href="http://www.standardml.org/Basis/list.html#SIG:LIST.find:VAL"><span class="monospaced">List.find</span></a>,\r | |
42 | <a href="http://www.standardml.org/Basis/list.html#SIG:LIST.exists:VAL"><span class="monospaced">List.exists</span></a>,\r | |
43 | or\r | |
44 | <a href="http://www.standardml.org/Basis/list.html#SIG:LIST.all:VAL"><span class="monospaced">List.all</span></a>\r | |
45 | is probably what you want in most cases. Unfortunately, it might be\r | |
46 | the case that the particular conditional iteration pattern that you\r | |
47 | want isn’t provided for your data structure. Usually the best\r | |
48 | alternative in such a case is to implement the desired iteration\r | |
49 | pattern as a higher-order function. For example, to implement a\r | |
50 | <span class="monospaced">find</span> function for arrays (which already exists as\r | |
51 | <a href="http://www.standardml.org/Basis/array.html#SIG:ARRAY.findi:VAL"><span class="monospaced">Array.find</span></a>)\r | |
52 | one could write</p></div>\r | |
53 | <div class="listingblock">\r | |
54 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">find</span><span class="w"> </span><span class="n">predicate</span><span class="w"> </span><span class="n">array</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
55 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">loop</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
56 | <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Array</span><span class="p">.</span><span class="n">length</span><span class="w"> </span><span class="n">array</span><span class="w"> </span><span class="k">then</span><span class="w"></span>\r | |
57 | <span class="w"> </span><span class="n">NONE</span><span class="w"></span>\r | |
58 | <span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">predicate</span><span class="w"> </span><span class="p">(</span><span class="n">Array</span><span class="p">.</span><span class="n">sub</span><span class="w"> </span><span class="p">(</span><span class="n">array</span><span class="p">,</span><span class="w"> </span><span class="n">i</span><span class="p">))</span><span class="w"> </span><span class="k">then</span><span class="w"></span>\r | |
59 | <span class="w"> </span><span class="n">SOME</span><span class="w"> </span><span class="p">(</span><span class="n">Array</span><span class="p">.</span><span class="n">sub</span><span class="w"> </span><span class="p">(</span><span class="n">array</span><span class="p">,</span><span class="w"> </span><span class="n">i</span><span class="p">))</span><span class="w"></span>\r | |
60 | <span class="w"> </span><span class="k">else</span><span class="w"></span>\r | |
61 | <span class="w"> </span><span class="n">loop</span><span class="w"> </span><span class="p">(</span><span class="n">i+</span><span class="mi">1</span><span class="p">)</span><span class="w"></span>\r | |
62 | <span class="k">in</span><span class="w"></span>\r | |
63 | <span class="w"> </span><span class="n">loop</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>\r | |
64 | <span class="k">end</span><span class="w"></span>\r | |
65 | </pre></div></div></div>\r | |
66 | <div class="paragraph"><p>Of course, this technique, while probably the most common case in\r | |
67 | practice, applies only if you are essentially iterating over some data\r | |
68 | structure.</p></div>\r | |
69 | </div>\r | |
70 | </div>\r | |
71 | <div class="sect1">\r | |
72 | <h2 id="_escape_handler">Escape handler</h2>\r | |
73 | <div class="sectionbody">\r | |
74 | <div class="paragraph"><p>Probably the most direct way to translate code using <span class="monospaced">return</span>\r | |
75 | statements is to basically implement <span class="monospaced">return</span> using exception\r | |
76 | handling. The mechanism can be packaged into a reusable module with\r | |
77 | the signature\r | |
78 | (<a href="https://github.com/MLton/mltonlib/blob/master/com/ssh/extended-basis/unstable/public/control/exit.sig"><span class="monospaced">exit.sig</span></a>):</p></div>\r | |
79 | <div class="listingblock">\r | |
80 | <div class="content"><div class="highlight"><pre><span class="cm">(**</span>\r | |
81 | <span class="cm"> * Signature for exit (or escape) handlers.</span>\r | |
82 | <span class="cm"> *</span>\r | |
83 | <span class="cm"> * Note that the implementation necessarily uses exception handling. This</span>\r | |
84 | <span class="cm"> * is to make proper resource handling possible. Exceptions raised by the</span>\r | |
85 | <span class="cm"> * implementation can be caught by wildcard exception handlers. Wildcard</span>\r | |
86 | <span class="cm"> * exception handlers should generally reraise exceptions after performing</span>\r | |
87 | <span class="cm"> * their effects.</span>\r | |
88 | <span class="cm"> *)</span><span class="w"></span>\r | |
89 | <span class="k">signature</span><span class="w"> </span><span class="n">EXIT</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
90 | <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 | |
91 | <span class="w"> </span><span class="cm">(** The type of exits. *)</span><span class="w"></span>\r | |
92 | \r | |
93 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">within</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">t</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">CPS</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
94 | <span class="w"> </span><span class="cm">(**</span>\r | |
95 | <span class="cm"> * Sets up an exit and passes it to the given function. The function</span>\r | |
96 | <span class="cm"> * may then return normally or by calling {to} with the exit and a</span>\r | |
97 | <span class="cm"> * return value. For example,</span>\r | |
98 | <span class="cm"> *</span>\r | |
99 | <span class="cm"> *> Exit.within</span>\r | |
100 | <span class="cm"> *> (fn l =></span>\r | |
101 | <span class="cm"> *> if condition then</span>\r | |
102 | <span class="cm"> *> Exit.to l 1</span>\r | |
103 | <span class="cm"> *> else</span>\r | |
104 | <span class="cm"> *> 2)</span>\r | |
105 | <span class="cm"> *</span>\r | |
106 | <span class="cm"> * evaluates either to {1} or to {2} depending on the {condition}.</span>\r | |
107 | <span class="cm"> *</span>\r | |
108 | <span class="cm"> * Note that the function receiving the exit is called from a non-tail</span>\r | |
109 | <span class="cm"> * position.</span>\r | |
110 | <span class="cm"> *)</span><span class="w"></span>\r | |
111 | \r | |
112 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">to</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="p">-></span><span class="w"> </span><span class="n">'b</span><span class="w"></span>\r | |
113 | <span class="w"> </span><span class="cm">(**</span>\r | |
114 | <span class="cm"> * {to l v} returns from the {within} invocation that introduced the</span>\r | |
115 | <span class="cm"> * exit {l} with the value {v}. Evaluating {to l v} outside of the</span>\r | |
116 | <span class="cm"> * {within} invocation that introduced {l} is a programming error and</span>\r | |
117 | <span class="cm"> * raises an exception.</span>\r | |
118 | <span class="cm"> *</span>\r | |
119 | <span class="cm"> * Note that the type variable {'b} only appears as the return type.</span>\r | |
120 | <span class="cm"> * This means that {to} doesn't return normally to the caller and can</span>\r | |
121 | <span class="cm"> * be called from a context of any type.</span>\r | |
122 | <span class="cm"> *)</span><span class="w"></span>\r | |
123 | \r | |
124 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">call</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="p">-></span><span class="w"> </span><span class="n">'b</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">CPS</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
125 | <span class="w"> </span><span class="cm">(**</span>\r | |
126 | <span class="cm"> * Simpler, but less flexibly typed, interface to {within} and {to}.</span>\r | |
127 | <span class="cm"> * Specifically, {call f} is equivalent to {within (f o to)}.</span>\r | |
128 | <span class="cm"> *)</span><span class="w"></span>\r | |
129 | <span class="k">end</span><span class="w"></span>\r | |
130 | </pre></div></div></div>\r | |
131 | <div class="paragraph"><p>(<a href="References#HarperEtAl93"> Typing First-Class Continuations in ML</a>\r | |
132 | discusses the typing of a related construct.) The implementation\r | |
133 | (<a href="https://github.com/MLton/mltonlib/blob/master/com/ssh/extended-basis/unstable/detail/control/exit.sml"><span class="monospaced">exit.sml</span></a>)\r | |
134 | is straightforward:</p></div>\r | |
135 | <div class="listingblock">\r | |
136 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Exit</span><span class="w"> </span><span class="p">:></span><span class="w"> </span><span class="n">EXIT</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
137 | <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="p">-></span><span class="w"> </span><span class="n">exn</span><span class="w"></span>\r | |
138 | \r | |
139 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">within</span><span class="w"> </span><span class="n">block</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
140 | <span class="w"> </span><span class="k">exception</span><span class="w"> </span><span class="n">EscapedExit</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
141 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
142 | <span class="w"> </span><span class="n">block</span><span class="w"> </span><span class="n">EscapedExit</span><span class="w"></span>\r | |
143 | <span class="w"> </span><span class="k">handle</span><span class="w"> </span><span class="n">EscapedExit</span><span class="w"> </span><span class="n">value</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">value</span><span class="w"></span>\r | |
144 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
145 | \r | |
146 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">to</span><span class="w"> </span><span class="n">exit</span><span class="w"> </span><span class="n">value</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">exit</span><span class="w"> </span><span class="n">value</span><span class="w"></span>\r | |
147 | \r | |
148 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">call</span><span class="w"> </span><span class="n">block</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">within</span><span class="w"> </span><span class="p">(</span><span class="n">block</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">to</span><span class="p">)</span><span class="w"></span>\r | |
149 | <span class="k">end</span><span class="w"></span>\r | |
150 | </pre></div></div></div>\r | |
151 | <div class="paragraph"><p>Here is an example of how one could implement a <span class="monospaced">find</span> function given\r | |
152 | an <span class="monospaced">app</span> function:</p></div>\r | |
153 | <div class="listingblock">\r | |
154 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">appToFind</span><span class="w"> </span><span class="p">(</span><span class="n">app</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="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">)</span><span class="w"> </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="n">unit</span><span class="p">)</span><span class="w"></span>\r | |
155 | <span class="w"> </span><span class="p">(</span><span class="n">predicate</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">bool</span><span class="p">)</span><span class="w"></span>\r | |
156 | <span class="w"> </span><span class="p">(</span><span class="n">data</span><span class="w"> </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>\r | |
157 | <span class="w"> </span><span class="n">Exit</span><span class="p">.</span><span class="n">call</span><span class="w"></span>\r | |
158 | <span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">return</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
159 | <span class="w"> </span><span class="p">(</span><span class="n">app</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>\r | |
160 | <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">predicate</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">then</span><span class="w"></span>\r | |
161 | <span class="w"> </span><span class="n">return</span><span class="w"> </span><span class="p">(</span><span class="n">SOME</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"></span>\r | |
162 | <span class="w"> </span><span class="k">else</span><span class="w"></span>\r | |
163 | <span class="w"> </span><span class="p">())</span><span class="w"></span>\r | |
164 | <span class="w"> </span><span class="n">data</span><span class="w"></span>\r | |
165 | <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 | |
166 | </pre></div></div></div>\r | |
167 | <div class="paragraph"><p>In the above, as soon as the expression <span class="monospaced">predicate x</span> evaluates to\r | |
168 | <span class="monospaced">true</span> the <span class="monospaced">app</span> invocation is terminated.</p></div>\r | |
169 | </div>\r | |
170 | </div>\r | |
171 | <div class="sect1">\r | |
172 | <h2 id="_continuation_passing_style_cps">Continuation-passing Style (CPS)</h2>\r | |
173 | <div class="sectionbody">\r | |
174 | <div class="paragraph"><p>A general way to implement complex control patterns is to use\r | |
175 | <a href="http://en.wikipedia.org/wiki/Continuation-passing_style">CPS</a>. In CPS,\r | |
176 | instead of returning normally, functions invoke a function passed as\r | |
177 | an argument. In general, multiple continuation functions may be\r | |
178 | passed as arguments and the ordinary return continuation may also be\r | |
179 | used. As an example, here is a function that finds the leftmost\r | |
180 | element of a binary tree satisfying a given predicate:</p></div>\r | |
181 | <div class="listingblock">\r | |
182 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">tree</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">LEAF</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">BRANCH</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">tree</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">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">tree</span><span class="w"></span>\r | |
183 | \r | |
184 | <span class="k">fun</span><span class="w"> </span><span class="n">find</span><span class="w"> </span><span class="n">predicate</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
185 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">recurse</span><span class="w"> </span><span class="n">continue</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
186 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">LEAF</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
187 | <span class="w"> </span><span class="n">continue</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
188 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">BRANCH</span><span class="w"> </span><span class="p">(</span><span class="n">lhs</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">,</span><span class="w"> </span><span class="n">rhs</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
189 | <span class="w"> </span><span class="n">recurse</span><span class="w"></span>\r | |
190 | <span class="w"> </span><span class="p">(</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 | |
191 | <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">predicate</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="k">then</span><span class="w"></span>\r | |
192 | <span class="w"> </span><span class="n">SOME</span><span class="w"> </span><span class="n">elem</span><span class="w"></span>\r | |
193 | <span class="w"> </span><span class="k">else</span><span class="w"></span>\r | |
194 | <span class="w"> </span><span class="n">recurse</span><span class="w"> </span><span class="n">continue</span><span class="w"> </span><span class="n">rhs</span><span class="p">)</span><span class="w"></span>\r | |
195 | <span class="w"> </span><span class="n">lhs</span><span class="w"></span>\r | |
196 | <span class="k">in</span><span class="w"></span>\r | |
197 | <span class="w"> </span><span class="n">recurse</span><span class="w"> </span><span class="p">(</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 | |
198 | <span class="k">end</span><span class="w"></span>\r | |
199 | </pre></div></div></div>\r | |
200 | <div class="paragraph"><p>Note that the above function returns as soon as the leftmost element\r | |
201 | satisfying the predicate is found.</p></div>\r | |
202 | </div>\r | |
203 | </div>\r | |
204 | </div>\r | |
205 | <div id="footnotes"><hr></div>\r | |
206 | <div id="footer">\r | |
207 | <div id="footer-text">\r | |
208 | </div>\r | |
209 | <div id="footer-badges">\r | |
210 | </div>\r | |
211 | </div>\r | |
212 | </body>\r | |
213 | </html>\r |