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22 | <a href="./Home">MLton 20180207</a>\r | |
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25 | <div id="header">\r | |
26 | <h1>StandardMLGotchas</h1>\r | |
27 | <div id="toc"> | |
28 | <div id="toctitle">Table of Contents</div> | |
29 | <noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript> | |
30 | </div>\r | |
31 | </div>\r | |
32 | <div id="content">\r | |
33 | <div id="preamble">\r | |
34 | <div class="sectionbody">\r | |
35 | <div class="paragraph"><p>This page contains brief explanations of some recurring sources of\r | |
36 | confusion and problems that SML newbies encounter.</p></div>\r | |
37 | <div class="paragraph"><p>Many confusions about the syntax of SML seem to arise from the use of\r | |
38 | an interactive REPL (Read-Eval Print Loop) while trying to learn the\r | |
39 | basics of the language. While writing your first SML programs, you\r | |
40 | should keep the source code of your programs in a form that is\r | |
41 | accepted by an SML compiler as a whole.</p></div>\r | |
42 | </div>\r | |
43 | </div>\r | |
44 | <div class="sect1">\r | |
45 | <h2 id="_the_span_class_monospaced_and_span_keyword">The <span class="monospaced">and</span> keyword</h2>\r | |
46 | <div class="sectionbody">\r | |
47 | <div class="paragraph"><p>It is a common mistake to misuse the <span class="monospaced">and</span> keyword or to not know how\r | |
48 | to introduce mutually recursive definitions. The purpose of the <span class="monospaced">and</span>\r | |
49 | keyword is to introduce mutually recursive definitions of functions\r | |
50 | and datatypes. For example,</p></div>\r | |
51 | <div class="listingblock">\r | |
52 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">isEven</span><span class="w"> </span><span class="mi">0w0</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">true</span><span class="w"></span>\r | |
53 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">isEven</span><span class="w"> </span><span class="mi">0w1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">false</span><span class="w"></span>\r | |
54 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">isEven</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">isOdd</span><span class="w"> </span><span class="p">(</span><span class="n">n-</span><span class="mi">0w1</span><span class="p">)</span><span class="w"></span>\r | |
55 | <span class="k">and</span><span class="w"> </span><span class="n">isOdd</span><span class="w"> </span><span class="mi">0w0</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">false</span><span class="w"></span>\r | |
56 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">isOdd</span><span class="w"> </span><span class="mi">0w1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">true</span><span class="w"></span>\r | |
57 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">isOdd</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">isEven</span><span class="w"> </span><span class="p">(</span><span class="n">n-</span><span class="mi">0w1</span><span class="p">)</span><span class="w"></span>\r | |
58 | </pre></div></div></div>\r | |
59 | <div class="paragraph"><p>and</p></div>\r | |
60 | <div class="listingblock">\r | |
61 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">decl</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">VAL</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">pat</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">expr</span><span class="w"></span>\r | |
62 | <span class="w"> </span><span class="cm">(* | ... *)</span><span class="w"></span>\r | |
63 | <span class="w"> </span><span class="k">and</span><span class="w"> </span><span class="n">expr</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">LET</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">decl</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">expr</span><span class="w"></span>\r | |
64 | <span class="w"> </span><span class="cm">(* | ... *)</span><span class="w"></span>\r | |
65 | </pre></div></div></div>\r | |
66 | <div class="paragraph"><p>You can also use <span class="monospaced">and</span> as a shorthand in a couple of other places, but\r | |
67 | it is not necessary.</p></div>\r | |
68 | </div>\r | |
69 | </div>\r | |
70 | <div class="sect1">\r | |
71 | <h2 id="_constructed_patterns">Constructed patterns</h2>\r | |
72 | <div class="sectionbody">\r | |
73 | <div class="paragraph"><p>It is a common mistake to forget to parenthesize constructed patterns\r | |
74 | in <span class="monospaced">fun</span> bindings. Consider the following invalid definition:</p></div>\r | |
75 | <div class="listingblock">\r | |
76 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="n">nil</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>\r | |
77 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="n">h</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
78 | </pre></div></div></div>\r | |
79 | <div class="dlist"><dl>\r | |
80 | <dt class="hdlist1">\r | |
81 | The pattern `h \r | |
82 | </dt>\r | |
83 | <dd>\r | |
84 | <p>\r | |
85 | t` needs to be parenthesized:\r | |
86 | </p>\r | |
87 | </dd>\r | |
88 | </dl></div>\r | |
89 | <div class="listingblock">\r | |
90 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="n">nil</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>\r | |
91 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="p">(</span><span class="n">h</span><span class="w"> </span><span class="n">::</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="mi">1</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
92 | </pre></div></div></div>\r | |
93 | <div class="paragraph"><p>The parentheses are needed, because a <span class="monospaced">fun</span> definition may have\r | |
94 | multiple consecutive constructed patterns through currying.</p></div>\r | |
95 | <div class="paragraph"><p>The same applies to nonfix constructors. For example, the parentheses\r | |
96 | in</p></div>\r | |
97 | <div class="listingblock">\r | |
98 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">valOf</span><span class="w"> </span><span class="n">NONE</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">Option</span><span class="w"></span>\r | |
99 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">valOf</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><span class="p">=</span><span class="w"> </span><span class="n">x</span><span class="w"></span>\r | |
100 | </pre></div></div></div>\r | |
101 | <div class="paragraph"><p>are required. However, the outermost constructed pattern in a <span class="monospaced">fn</span> or\r | |
102 | <span class="monospaced">case</span> expression need not be parenthesized, because in those cases\r | |
103 | there is always just one constructed pattern. So, both</p></div>\r | |
104 | <div class="listingblock">\r | |
105 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">valOf</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">NONE</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">Option</span><span class="w"></span>\r | |
106 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">SOME</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">x</span><span class="w"></span>\r | |
107 | </pre></div></div></div>\r | |
108 | <div class="paragraph"><p>and</p></div>\r | |
109 | <div class="listingblock">\r | |
110 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">valOf</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
111 | <span class="w"> </span><span class="n">NONE</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">Option</span><span class="w"></span>\r | |
112 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">SOME</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">x</span><span class="w"></span>\r | |
113 | </pre></div></div></div>\r | |
114 | <div class="paragraph"><p>are fine.</p></div>\r | |
115 | </div>\r | |
116 | </div>\r | |
117 | <div class="sect1">\r | |
118 | <h2 id="_declarations_and_expressions">Declarations and expressions</h2>\r | |
119 | <div class="sectionbody">\r | |
120 | <div class="paragraph"><p>It is a common mistake to confuse expressions and declarations.\r | |
121 | Normally an SML source file should only contain declarations. The\r | |
122 | following are declarations:</p></div>\r | |
123 | <div class="listingblock">\r | |
124 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">dt</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
125 | <span class="k">fun</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
126 | <span class="k">functor</span><span class="w"> </span><span class="n">Fn</span><span class="w"> </span><span class="p">(...)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
127 | <span class="k">infix</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
128 | <span class="k">infixr</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
129 | <span class="k">local</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
130 | <span class="k">nonfix</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
131 | <span class="k">open</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
132 | <span class="k">signature</span><span class="w"> </span><span class="n">SIG</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
133 | <span class="k">structure</span><span class="w"> </span><span class="n">Struct</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
134 | <span class="k">type</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 | |
135 | <span class="k">val</span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">...</span><span class="w"></span>\r | |
136 | </pre></div></div></div>\r | |
137 | <div class="paragraph"><p>Note that</p></div>\r | |
138 | <div class="listingblock">\r | |
139 | <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
140 | </pre></div></div></div>\r | |
141 | <div class="paragraph"><p>isn’t a declaration.</p></div>\r | |
142 | <div class="paragraph"><p>To specify a side-effecting computation in a source file, you can write:</p></div>\r | |
143 | <div class="listingblock">\r | |
144 | <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="p">...</span><span class="w"></span>\r | |
145 | </pre></div></div></div>\r | |
146 | </div>\r | |
147 | </div>\r | |
148 | <div class="sect1">\r | |
149 | <h2 id="_equality_types">Equality types</h2>\r | |
150 | <div class="sectionbody">\r | |
151 | <div class="paragraph"><p>SML has a fairly intricate built-in notion of equality. See\r | |
152 | <a href="EqualityType">EqualityType</a> and <a href="EqualityTypeVariable">EqualityTypeVariable</a> for a thorough\r | |
153 | discussion.</p></div>\r | |
154 | </div>\r | |
155 | </div>\r | |
156 | <div class="sect1">\r | |
157 | <h2 id="_nested_cases">Nested cases</h2>\r | |
158 | <div class="sectionbody">\r | |
159 | <div class="paragraph"><p>It is a common mistake to write nested case expressions without the\r | |
160 | necessary parentheses. See <a href="UnresolvedBugs">UnresolvedBugs</a> for a discussion.</p></div>\r | |
161 | </div>\r | |
162 | </div>\r | |
163 | <div class="sect1">\r | |
164 | <h2 id="_op">(op *)</h2>\r | |
165 | <div class="sectionbody">\r | |
166 | <div class="paragraph"><p>It used to be a common mistake to parenthesize <span class="monospaced">op *</span> as <span class="monospaced">(op *)</span>.\r | |
167 | Before SML’97, <span class="monospaced">*)</span> was considered a comment terminator in SML and\r | |
168 | caused a syntax error. At the time of writing, <a href="SMLNJ">SML/NJ</a> still\r | |
169 | rejects the code. An extra space may be used for portability:\r | |
170 | <span class="monospaced">(op * )</span>. However, parenthesizing <span class="monospaced">op</span> is redundant, even though it\r | |
171 | is a widely used convention.</p></div>\r | |
172 | </div>\r | |
173 | </div>\r | |
174 | <div class="sect1">\r | |
175 | <h2 id="_overloading">Overloading</h2>\r | |
176 | <div class="sectionbody">\r | |
177 | <div class="paragraph"><p>A number of standard operators (<span class="monospaced">+</span>, <span class="monospaced">-</span>, <span class="monospaced">~</span>, <span class="monospaced">*</span>, <span class="monospaced"><</span>, <span class="monospaced">></span>, …) and\r | |
178 | numeric constants are overloaded for some of the numeric types (<span class="monospaced">int</span>,\r | |
179 | <span class="monospaced">real</span>, <span class="monospaced">word</span>). It is a common surprise that definitions using\r | |
180 | overloaded operators such as</p></div>\r | |
181 | <div class="listingblock">\r | |
182 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">min</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="n"><</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">then</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="n">x</span><span class="w"></span>\r | |
183 | </pre></div></div></div>\r | |
184 | <div class="paragraph"><p>are not overloaded themselves. SML doesn’t really support\r | |
185 | (user-defined) overloading or other forms of ad hoc polymorphism. In\r | |
186 | cases such as the above where the context doesn’t resolve the\r | |
187 | overloading, expressions using overloaded operators or constants get\r | |
188 | assigned a default type. The above definition gets the type</p></div>\r | |
189 | <div class="listingblock">\r | |
190 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">min</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">int</span><span class="w"></span>\r | |
191 | </pre></div></div></div>\r | |
192 | <div class="paragraph"><p>See <a href="Overloading">Overloading</a> and <a href="TypeIndexedValues">TypeIndexedValues</a> for further discussion.</p></div>\r | |
193 | </div>\r | |
194 | </div>\r | |
195 | <div class="sect1">\r | |
196 | <h2 id="_semicolons">Semicolons</h2>\r | |
197 | <div class="sectionbody">\r | |
198 | <div class="paragraph"><p>It is a common mistake to use redundant semicolons in SML code. This\r | |
199 | is probably caused by the fact that in an SML REPL, a semicolon (and\r | |
200 | enter) is used to signal the REPL that it should evaluate the\r | |
201 | preceding chunk of code as a unit. In SML source files, semicolons\r | |
202 | are really needed in only two places. Namely, in expressions of the\r | |
203 | form</p></div>\r | |
204 | <div class="listingblock">\r | |
205 | <div class="content"><div class="highlight"><pre><span class="p">(</span><span class="n">exp</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">exp</span><span class="p">)</span><span class="w"></span>\r | |
206 | </pre></div></div></div>\r | |
207 | <div class="paragraph"><p>and</p></div>\r | |
208 | <div class="listingblock">\r | |
209 | <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">exp</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">exp</span><span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
210 | </pre></div></div></div>\r | |
211 | <div class="paragraph"><p>Note that semicolons act as expression (or declaration) separators\r | |
212 | rather than as terminators.</p></div>\r | |
213 | </div>\r | |
214 | </div>\r | |
215 | <div class="sect1">\r | |
216 | <h2 id="_stale_bindings">Stale bindings</h2>\r | |
217 | <div class="sectionbody">\r | |
218 | <div class="paragraph"><p></p></div>\r | |
219 | </div>\r | |
220 | </div>\r | |
221 | <div class="sect1">\r | |
222 | <h2 id="_unresolved_records">Unresolved records</h2>\r | |
223 | <div class="sectionbody">\r | |
224 | <div class="paragraph"><p></p></div>\r | |
225 | </div>\r | |
226 | </div>\r | |
227 | <div class="sect1">\r | |
228 | <h2 id="_value_restriction">Value restriction</h2>\r | |
229 | <div class="sectionbody">\r | |
230 | <div class="paragraph"><p>See <a href="ValueRestriction">ValueRestriction</a>.</p></div>\r | |
231 | </div>\r | |
232 | </div>\r | |
233 | <div class="sect1">\r | |
234 | <h2 id="_type_variable_scope">Type Variable Scope</h2>\r | |
235 | <div class="sectionbody">\r | |
236 | <div class="paragraph"><p>See <a href="TypeVariableScope">TypeVariableScope</a>.</p></div>\r | |
237 | </div>\r | |
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