doc/guide/localhost/StandardMLGotchas

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  26 <h1>StandardMLGotchas</h1>
  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>
  31 </div>
  32 <div id="content">
  33 <div id="preamble">
  34 <div class="sectionbody">
  35 <div class="paragraph"><p>This page contains brief explanations of some recurring sources of
  36 confusion and problems that SML newbies encounter.</p></div>
  37 <div class="paragraph"><p>Many confusions about the syntax of SML seem to arise from the use of
  38 an interactive REPL (Read-Eval Print Loop) while trying to learn the
  39 basics of the language.  While writing your first SML programs, you
  40 should keep the source code of your programs in a form that is
  41 accepted by an SML compiler as a whole.</p></div>
  42 </div>
  43 </div>
  44 <div class="sect1">
  45 <h2 id="_the_span_class_monospaced_and_span_keyword">The <span class="monospaced">and</span> keyword</h2>
  46 <div class="sectionbody">
  47 <div class="paragraph"><p>It is a common mistake to misuse the <span class="monospaced">and</span> keyword or to not know how
  48 to introduce mutually recursive definitions.  The purpose of the <span class="monospaced">and</span>
  49 keyword is to introduce mutually recursive definitions of functions
  50 and datatypes.  For example,</p></div>
  51 <div class="listingblock">
  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>
  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>
  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>
  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>
  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>
  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>
  58 </pre></div></div></div>
  59 <div class="paragraph"><p>and</p></div>
  60 <div class="listingblock">
  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>
  62 <span class="w">           </span><span class="cm">(* | ... *)</span><span class="w"></span>
  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>
  64 <span class="w">           </span><span class="cm">(* | ... *)</span><span class="w"></span>
  65 </pre></div></div></div>
  66 <div class="paragraph"><p>You can also use <span class="monospaced">and</span> as a shorthand in a couple of other places, but
  67 it is not necessary.</p></div>
  68 </div>
  69 </div>
  70 <div class="sect1">
  71 <h2 id="_constructed_patterns">Constructed patterns</h2>
  72 <div class="sectionbody">
  73 <div class="paragraph"><p>It is a common mistake to forget to parenthesize constructed patterns
  74 in <span class="monospaced">fun</span> bindings.  Consider the following invalid definition:</p></div>
  75 <div class="listingblock">
  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>
  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>
  78 </pre></div></div></div>
  79 <div class="dlist"><dl>
  80 <dt class="hdlist1">
  81 The pattern `h
  82 </dt>
  83 <dd>
  84 <p>
  85 t` needs to be parenthesized:
  86 </p>
  87 </dd>
  88 </dl></div>
  89 <div class="listingblock">
  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>
  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>
  92 </pre></div></div></div>
  93 <div class="paragraph"><p>The parentheses are needed, because a <span class="monospaced">fun</span> definition may have
  94 multiple consecutive constructed patterns through currying.</p></div>
  95 <div class="paragraph"><p>The same applies to nonfix constructors.  For example, the parentheses
  96 in</p></div>
  97 <div class="listingblock">
  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>
  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>
 100 </pre></div></div></div>
 101 <div class="paragraph"><p>are required.  However, the outermost constructed pattern in a <span class="monospaced">fn</span> or
 102 <span class="monospaced">case</span> expression need not be parenthesized, because in those cases
 103 there is always just one constructed pattern.  So, both</p></div>
 104 <div class="listingblock">
 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">=&gt;</span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">Option</span><span class="w"></span>
 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">=&gt;</span><span class="w"> </span><span class="n">x</span><span class="w"></span>
 107 </pre></div></div></div>
 108 <div class="paragraph"><p>and</p></div>
 109 <div class="listingblock">
 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>
 111 <span class="w">                 </span><span class="n">NONE</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">Option</span><span class="w"></span>
 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">=&gt;</span><span class="w"> </span><span class="n">x</span><span class="w"></span>
 113 </pre></div></div></div>
 114 <div class="paragraph"><p>are fine.</p></div>
 115 </div>
 116 </div>
 117 <div class="sect1">
 118 <h2 id="_declarations_and_expressions">Declarations and expressions</h2>
 119 <div class="sectionbody">
 120 <div class="paragraph"><p>It is a common mistake to confuse expressions and declarations.
 121 Normally an SML source file should only contain declarations.  The
 122 following are declarations:</p></div>
 123 <div class="listingblock">
 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>
 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>
 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>
 127 <span class="k">infix</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 128 <span class="k">infixr</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 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>
 130 <span class="k">nonfix</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 131 <span class="k">open</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 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>
 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>
 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>
 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>
 136 </pre></div></div></div>
 137 <div class="paragraph"><p>Note that</p></div>
 138 <div class="listingblock">
 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>
 140 </pre></div></div></div>
 141 <div class="paragraph"><p>isn&#8217;t a declaration.</p></div>
 142 <div class="paragraph"><p>To specify a side-effecting computation in a source file, you can write:</p></div>
 143 <div class="listingblock">
 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>
 145 </pre></div></div></div>
 146 </div>
 147 </div>
 148 <div class="sect1">
 149 <h2 id="_equality_types">Equality types</h2>
 150 <div class="sectionbody">
 151 <div class="paragraph"><p>SML has a fairly intricate built-in notion of equality.  See
 152 <a href="EqualityType">EqualityType</a> and <a href="EqualityTypeVariable">EqualityTypeVariable</a> for a thorough
 153 discussion.</p></div>
 154 </div>
 155 </div>
 156 <div class="sect1">
 157 <h2 id="_nested_cases">Nested cases</h2>
 158 <div class="sectionbody">
 159 <div class="paragraph"><p>It is a common mistake to write nested case expressions without the
 160 necessary parentheses.  See <a href="UnresolvedBugs">UnresolvedBugs</a> for a discussion.</p></div>
 161 </div>
 162 </div>
 163 <div class="sect1">
 164 <h2 id="_op">(op *)</h2>
 165 <div class="sectionbody">
 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>.
 167 Before SML&#8217;97, <span class="monospaced">*)</span> was considered a comment terminator in SML and
 168 caused a syntax error.  At the time of writing, <a href="SMLNJ">SML/NJ</a> still
 169 rejects the code.  An extra space may be used for portability:
 170 <span class="monospaced">(op * )</span>. However, parenthesizing <span class="monospaced">op</span> is redundant, even though it
 171 is a widely used convention.</p></div>
 172 </div>
 173 </div>
 174 <div class="sect1">
 175 <h2 id="_overloading">Overloading</h2>
 176 <div class="sectionbody">
 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">&lt;</span>, <span class="monospaced">&gt;</span>, &#8230;) and
 178 numeric constants are overloaded for some of the numeric types (<span class="monospaced">int</span>,
 179 <span class="monospaced">real</span>, <span class="monospaced">word</span>).  It is a common surprise that definitions using
 180 overloaded operators such as</p></div>
 181 <div class="listingblock">
 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">&lt;</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>
 183 </pre></div></div></div>
 184 <div class="paragraph"><p>are not overloaded themselves.  SML doesn&#8217;t really support
 185 (user-defined) overloading or other forms of ad hoc polymorphism.  In
 186 cases such as the above where the context doesn&#8217;t resolve the
 187 overloading, expressions using overloaded operators or constants get
 188 assigned a default type.  The above definition gets the type</p></div>
 189 <div class="listingblock">
 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">-&gt;</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 191 </pre></div></div></div>
 192 <div class="paragraph"><p>See <a href="Overloading">Overloading</a> and <a href="TypeIndexedValues">TypeIndexedValues</a> for further discussion.</p></div>
 193 </div>
 194 </div>
 195 <div class="sect1">
 196 <h2 id="_semicolons">Semicolons</h2>
 197 <div class="sectionbody">
 198 <div class="paragraph"><p>It is a common mistake to use redundant semicolons in SML code.  This
 199 is probably caused by the fact that in an SML REPL, a semicolon (and
 200 enter) is used to signal the REPL that it should evaluate the
 201 preceding chunk of code as a unit.  In SML source files, semicolons
 202 are really needed in only two places.  Namely, in expressions of the
 203 form</p></div>
 204 <div class="listingblock">
 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>
 206 </pre></div></div></div>
 207 <div class="paragraph"><p>and</p></div>
 208 <div class="listingblock">
 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>
 210 </pre></div></div></div>
 211 <div class="paragraph"><p>Note that semicolons act as expression (or declaration) separators
 212 rather than as terminators.</p></div>
 213 </div>
 214 </div>
 215 <div class="sect1">
 216 <h2 id="_stale_bindings">Stale bindings</h2>
 217 <div class="sectionbody">
 218 <div class="paragraph"><p></p></div>
 219 </div>
 220 </div>
 221 <div class="sect1">
 222 <h2 id="_unresolved_records">Unresolved records</h2>
 223 <div class="sectionbody">
 224 <div class="paragraph"><p></p></div>
 225 </div>
 226 </div>
 227 <div class="sect1">
 228 <h2 id="_value_restriction">Value restriction</h2>
 229 <div class="sectionbody">
 230 <div class="paragraph"><p>See <a href="ValueRestriction">ValueRestriction</a>.</p></div>
 231 </div>
 232 </div>
 233 <div class="sect1">
 234 <h2 id="_type_variable_scope">Type Variable Scope</h2>
 235 <div class="sectionbody">
 236 <div class="paragraph"><p>See <a href="TypeVariableScope">TypeVariableScope</a>.</p></div>
 237 </div>
 238 </div>
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