doc/guide/localhost/SMLNJDeviations

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>SMLNJDeviations</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>Here are some deviations of <a href="SMLNJ">SML/NJ</a> from
  32 <a href="DefinitionOfStandardML">The Definition of Standard ML (Revised)</a>.
  33 Some of these are documented in the
  34 <a href="http://www.smlnj.org/doc/Conversion/index.html">SML '97 Conversion Guide</a>.
  35 Since MLton does not deviate from the Definition, you should look here
  36 if you are having trouble porting a program from MLton to SML/NJ or
  37 vice versa.  If you discover other deviations of SML/NJ that aren&#8217;t
  38 listed here, please send mail to
  39 <a href="mailto:MLton-devel@mlton.org"><span class="monospaced">MLton-devel@mlton.org</span></a>.</p></div>
  40 <div class="ulist"><ul>
  41 <li>
  42 <p>
  43 SML/NJ allows spaces in long identifiers, as in <span class="monospaced">S . x</span>.  Section
  44 2.5 of the Definition implies that <span class="monospaced">S . x</span> should be treated as three
  45 separate lexical items.
  46 </p>
  47 </li>
  48 <li>
  49 <p>
  50 SML/NJ allows <span class="monospaced">op</span> to appear in <span class="monospaced">val</span> specifications:
  51 </p>
  52 <div class="listingblock">
  53 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">FOO</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>
  54 <span class="w">   </span><span class="k">val</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="n">+</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>
  55 <span class="k">end</span><span class="w"></span>
  56 </pre></div></div></div>
  57 <div class="paragraph"><p>The grammar on page 14 of the Definition does not allow it. Recent
  58 versions of SML/NJ do give a warning.</p></div>
  59 </li>
  60 <li>
  61 <p>
  62 SML/NJ rejects
  63 </p>
  64 <div class="listingblock">
  65 <div class="content"><div class="highlight"><pre><span class="p">(</span><span class="k">op</span><span class="w"> </span><span class="n">*</span><span class="p">)</span><span class="w"></span>
  66 </pre></div></div></div>
  67 <div class="paragraph"><p>as an unmatched close comment.</p></div>
  68 </li>
  69 <li>
  70 <p>
  71 SML/NJ allows <span class="monospaced">=</span> to be rebound by the declaration:
  72 </p>
  73 <div class="listingblock">
  74 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">13</span><span class="w"></span>
  75 </pre></div></div></div>
  76 <div class="paragraph"><p>This is explicitly forbidden on page 5 of the Definition. Recent
  77 versions of SML/NJ do give a warning.</p></div>
  78 </li>
  79 <li>
  80 <p>
  81 SML/NJ allows rebinding <span class="monospaced">true</span>, <span class="monospaced">false</span>, <span class="monospaced">nil</span>, <span class="monospaced">::</span>, and <span class="monospaced">ref</span> by
  82 the declarations:
  83 </p>
  84 <div class="listingblock">
  85 <div class="content"><div class="highlight"><pre><span class="k">fun</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>
  86 <span class="k">fun</span><span class="w"> </span><span class="n">false</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>
  87 <span class="k">fun</span><span class="w"> </span><span class="n">nil</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>
  88 <span class="k">fun</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
  89 <span class="k">fun</span><span class="w"> </span><span class="n">ref</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>
  90 </pre></div></div></div>
  91 <div class="paragraph"><p>This is explicitly forbidden on page 9 of the Definition.</p></div>
  92 </li>
  93 <li>
  94 <p>
  95 SML/NJ extends the syntax of the language to allow vector
  96 expressions and patterns like the following:
  97 </p>
  98 <div class="listingblock">
  99 <div class="content"><div class="highlight"><pre><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="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="w"></span>
 100 <span class="k">val</span><span class="w"> </span><span class="p">#[</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span><span class="n">z</span><span class="p">]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">v</span><span class="w"></span>
 101 </pre></div></div></div>
 102 <div class="paragraph"><p>MLton supports vector expressions and patterns with the <a href="SuccessorML#VectorExpsAndPats"><span class="monospaced">allowVectorExpsAndPats</span></a> <a href="MLBasisAnnotations">ML Basis annotation</a>.</p></div>
 103 </li>
 104 <li>
 105 <p>
 106 SML/NJ extends the syntax of the language to allow <em>or patterns</em>
 107 like the following:
 108 </p>
 109 <div class="listingblock">
 110 <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">foo</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Foo</span><span class="w"> </span><span class="k">of</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">Bar</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 111 <span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">Foo</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">Bar</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">Foo</span><span class="w"> </span><span class="mi">13</span><span class="w"></span>
 112 </pre></div></div></div>
 113 <div class="paragraph"><p>MLton supports or patterns with the <a href="SuccessorML#OrPats"><span class="monospaced">allowOrPats</span></a> <a href="MLBasisAnnotations">ML Basis annotation</a>.</p></div>
 114 </li>
 115 <li>
 116 <p>
 117 SML/NJ allows higher-order functors, that is, functors can be
 118 components of structures and can be passed as functor arguments and
 119 returned as functor results.  As a consequence, SML/NJ allows
 120 abbreviated functor definitions, as in the following:
 121 </p>
 122 <div class="listingblock">
 123 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 124 <span class="w">  </span><span class="k">sig</span><span class="w"></span>
 125 <span class="w">    </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 126 <span class="w">    </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 127 <span class="w">  </span><span class="k">end</span><span class="w"></span>
 128 <span class="k">functor</span><span class="w"> </span><span class="n">F</span><span class="w"> </span><span class="p">(</span><span class="k">structure</span><span class="w"> </span><span class="n">A</span><span class="p">:</span><span class="w"> </span><span class="n">S</span><span class="p">):</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 129 <span class="w">  </span><span class="k">struct</span><span class="w"></span>
 130 <span class="w">    </span><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="n">A</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">A</span><span class="p">.</span><span class="n">t</span><span class="w"></span>
 131 <span class="w">    </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">(</span><span class="n">A</span><span class="p">.</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">A</span><span class="p">.</span><span class="n">x</span><span class="p">)</span><span class="w"></span>
 132 <span class="w">  </span><span class="k">end</span><span class="w"></span>
 133 <span class="k">functor</span><span class="w"> </span><span class="n">G</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">F</span><span class="w"></span>
 134 </pre></div></div></div>
 135 </li>
 136 <li>
 137 <p>
 138 SML/NJ extends the syntax of the language to allow <span class="monospaced">functor</span> and
 139 <span class="monospaced">signature</span> declarations to occur within the scope of <span class="monospaced">local</span> and
 140 <span class="monospaced">structure</span> declarations.
 141 </p>
 142 </li>
 143 <li>
 144 <p>
 145 SML/NJ allows duplicate type specifications in signatures when the
 146 duplicates are introduced by <span class="monospaced">include</span>, as in the following:
 147 </p>
 148 <div class="listingblock">
 149 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">SIG1</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 150 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 151 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 152 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"></span>
 153 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 154 <span class="k">signature</span><span class="w"> </span><span class="n">SIG2</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 155 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 156 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 157 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">v</span><span class="w"></span>
 158 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 159 <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>
 160 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 161 <span class="w">      </span><span class="k">include</span><span class="w"> </span><span class="n">SIG1</span><span class="w"></span>
 162 <span class="w">      </span><span class="k">include</span><span class="w"> </span><span class="n">SIG2</span><span class="w"></span>
 163 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 164 </pre></div></div></div>
 165 <div class="paragraph"><p>This is disallowed by rule 77 of the Definition.</p></div>
 166 </li>
 167 <li>
 168 <p>
 169 SML/NJ allows sharing constraints between type abbreviations in
 170 signatures, as in the following:
 171 </p>
 172 <div class="listingblock">
 173 <div class="content"><div class="highlight"><pre><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>
 174 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 175 <span class="w">      </span><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="n">int</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 176 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">u</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>
 177 <span class="w">      </span><span class="k">sharing</span><span class="w"> </span><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="n">u</span><span class="w"></span>
 178 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 179 </pre></div></div></div>
 180 <div class="paragraph"><p>These are disallowed by rule 78 of the Definition.  Recent versions of
 181 SML/NJ correctly disallow sharing constraints between type
 182 abbreviations in signatures.</p></div>
 183 </li>
 184 <li>
 185 <p>
 186 SML/NJ disallows multiple <span class="monospaced">where type</span> specifications of the same
 187 type name, as in the following
 188 </p>
 189 <div class="listingblock">
 190 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 191 <span class="w">  </span><span class="k">sig</span><span class="w"></span>
 192 <span class="w">     </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 193 <span class="w">     </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 194 <span class="w">  </span><span class="k">end</span><span class="w"></span>
 195 <span class="w">  </span><span class="k">where</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 196 </pre></div></div></div>
 197 <div class="paragraph"><p>This is allowed by rule 64 of the Definition.</p></div>
 198 </li>
 199 <li>
 200 <p>
 201 SML/NJ allows <span class="monospaced">and</span> in <span class="monospaced">sharing</span> specs in signatures, as in
 202 </p>
 203 <div class="listingblock">
 204 <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 205 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 206 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 207 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"></span>
 208 <span class="w">      </span><span class="k">type</span><span class="w"> </span><span class="n">v</span><span class="w"></span>
 209 <span class="w">      </span><span class="k">sharing</span><span class="w"> </span><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="n">u</span><span class="w"></span>
 210 <span class="w">      </span><span class="k">and</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">v</span><span class="w"></span>
 211 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 212 </pre></div></div></div>
 213 </li>
 214 <li>
 215 <p>
 216 SML/NJ does not expand the <span class="monospaced">withtype</span> derived form as described by
 217 the Definition.  According to page 55 of the Definition, the type
 218 bindings of a <span class="monospaced">withtype</span> declaration are substituted simultaneously in
 219 the connected datatype.  Consider the following program.
 220 </p>
 221 <div class="listingblock">
 222 <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="p">;</span><span class="w"></span>
 223 <span class="k">datatype</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 224 <span class="w">    </span><span class="n">A</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 225 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">u</span><span class="w"></span>
 226 <span class="k">withtype</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 227 <span class="k">and</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">u</span><span class="w"></span>
 228 </pre></div></div></div>
 229 <div class="paragraph"><p>According to the Definition, it should be expanded to the following.</p></div>
 230 <div class="listingblock">
 231 <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="p">;</span><span class="w"></span>
 232 <span class="k">datatype</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 233 <span class="w">    </span><span class="n">A</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">u</span><span class="w"></span>
 234 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">;</span><span class="w"></span>
 235 <span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 236 <span class="k">and</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">u</span><span class="w"></span>
 237 </pre></div></div></div>
 238 <div class="paragraph"><p>However, SML/NJ expands <span class="monospaced">withtype</span> bindings sequentially, meaning that
 239 earlier bindings are expanded within later ones. Hence, the above
 240 program is expanded to the following.</p></div>
 241 <div class="listingblock">
 242 <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="p">;</span><span class="w"></span>
 243 <span class="k">datatype</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 244 <span class="w">    </span><span class="n">A</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 245 <span class="w">  </span><span class="p">|</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">;</span><span class="w"></span>
 246 <span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
 247 <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="n">int</span><span class="w"></span>
 248 </pre></div></div></div>
 249 </li>
 250 <li>
 251 <p>
 252 SML/NJ allows <span class="monospaced">withtype</span> specifications in signatures.
 253 </p>
 254 <div class="paragraph"><p>MLton supports <span class="monospaced">withtype</span> specifications in signatures with the <a href="SuccessorML#SigWithtype"><span class="monospaced">allowSigWithtype</span></a> <a href="MLBasisAnnotations">ML Basis annotation</a>.</p></div>
 255 </li>
 256 <li>
 257 <p>
 258 SML/NJ allows a <span class="monospaced">where</span> structure specification that is similar to a
 259 <span class="monospaced">where type</span> specification.  For example:
 260 </p>
 261 <div class="listingblock">
 262 <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="k">struct</span><span class="w"> </span><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="n">int</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 263 <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>
 264 <span class="w">  </span><span class="k">sig</span><span class="w"></span>
 265 <span class="w">     </span><span class="k">structure</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="k">sig</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 266 <span class="w">  </span><span class="k">end</span><span class="w"> </span><span class="k">where</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">S</span><span class="w"></span>
 267 </pre></div></div></div>
 268 <div class="paragraph"><p>This is equivalent to:</p></div>
 269 <div class="listingblock">
 270 <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="k">struct</span><span class="w"> </span><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="n">int</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 271 <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>
 272 <span class="w">  </span><span class="k">sig</span><span class="w"></span>
 273 <span class="w">     </span><span class="k">structure</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="k">sig</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 274 <span class="w">  </span><span class="k">end</span><span class="w"> </span><span class="k">where</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">T</span><span class="p">.</span><span class="n">t</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">t</span><span class="w"></span>
 275 </pre></div></div></div>
 276 <div class="paragraph"><p>SML/NJ also allows a definitional structure specification that is
 277 similar to a definitional type specification.  For example:</p></div>
 278 <div class="listingblock">
 279 <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="k">struct</span><span class="w"> </span><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="n">int</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 280 <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>
 281 <span class="w">  </span><span class="k">sig</span><span class="w"></span>
 282 <span class="w">     </span><span class="k">structure</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="k">sig</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">S</span><span class="w"></span>
 283 <span class="w">  </span><span class="k">end</span><span class="w"></span>
 284 </pre></div></div></div>
 285 <div class="paragraph"><p>This is equivalent to the previous examples and to:</p></div>
 286 <div class="listingblock">
 287 <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="k">struct</span><span class="w"> </span><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="n">int</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 288 <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>
 289 <span class="w">  </span><span class="k">sig</span><span class="w"></span>
 290 <span class="w">     </span><span class="k">structure</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="k">sig</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="k">where</span><span class="w"> </span><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="n">S</span><span class="p">.</span><span class="n">t</span><span class="w"></span>
 291 <span class="w">  </span><span class="k">end</span><span class="w"></span>
 292 </pre></div></div></div>
 293 </li>
 294 <li>
 295 <p>
 296 SML/NJ disallows binding non-datatypes with datatype replication.
 297 For example, it rejects the following program that should be allowed
 298 according to the Definition.
 299 </p>
 300 <div class="listingblock">
 301 <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="p">(</span><span class="n">&#39;a</span><span class="p">,</span><span class="w"> </span><span class="n">&#39;b</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="n">&#39;a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">&#39;b</span><span class="w"></span>
 302 <span class="k">datatype</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 303 </pre></div></div></div>
 304 <div class="paragraph"><p>This idiom can be useful when one wants to rename a type without
 305 rewriting all the type arguments.  For example, the above would have
 306 to be written in SML/NJ as follows.</p></div>
 307 <div class="listingblock">
 308 <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="p">(</span><span class="n">&#39;a</span><span class="p">,</span><span class="w"> </span><span class="n">&#39;b</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="n">&#39;a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">&#39;b</span><span class="w"></span>
 309 <span class="k">type</span><span class="w"> </span><span class="p">(</span><span class="n">&#39;a</span><span class="p">,</span><span class="w"> </span><span class="n">&#39;b</span><span class="p">)</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">(</span><span class="n">&#39;a</span><span class="p">,</span><span class="w"> </span><span class="n">&#39;b</span><span class="p">)</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 310 </pre></div></div></div>
 311 </li>
 312 <li>
 313 <p>
 314 SML/NJ disallows sharing a structure with one of its substructures.
 315 For example, SML/NJ disallows the following.
 316 </p>
 317 <div class="listingblock">
 318 <div class="content"><div class="highlight"><pre><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>
 319 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 320 <span class="w">      </span><span class="k">structure</span><span class="w"> </span><span class="n">S</span><span class="p">:</span><span class="w"></span>
 321 <span class="w">         </span><span class="k">sig</span><span class="w"></span>
 322 <span class="w">            </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
 323 <span class="w">            </span><span class="k">structure</span><span class="w"> </span><span class="n">T</span><span class="p">:</span><span class="w"> </span><span class="k">sig</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 324 <span class="w">         </span><span class="k">end</span><span class="w"></span>
 325 <span class="w">      </span><span class="k">sharing</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">S</span><span class="p">.</span><span class="n">T</span><span class="w"></span>
 326 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 327 </pre></div></div></div>
 328 <div class="paragraph"><p>This signature is allowed by the Definition.</p></div>
 329 </li>
 330 <li>
 331 <p>
 332 SML/NJ disallows polymorphic generalization of refutable
 333 patterns. For example, SML/NJ disallows the following.
 334 </p>
 335 <div class="listingblock">
 336 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="p">[</span><span class="n">x</span><span class="p">]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">[[]]</span><span class="w"></span>
 337 <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="mi">1</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;one&quot;</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"></span>
 338 </pre></div></div></div>
 339 <div class="paragraph"><p>Recent versions of SML/NJ correctly allow polymorphic generalization
 340 of refutable patterns.</p></div>
 341 </li>
 342 <li>
 343 <p>
 344 SML/NJ uses an overly restrictive context for type inference.  For
 345 example, SML/NJ rejects both of the following.
 346 </p>
 347 <div class="listingblock">
 348 <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>
 349 <span class="k">struct</span><span class="w"></span>
 350 <span class="w">  </span><span class="k">val</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=&gt;</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>
 351 <span class="w">  </span><span class="k">val</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="p">[</span><span class="n">true</span><span class="p">]</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="n">nil</span><span class="w"></span>
 352 <span class="k">end</span><span class="w"></span>
 353 </pre></div></div></div>
 354 <div class="listingblock">
 355 <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="k">sig</span><span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">bool</span><span class="w"> </span><span class="n">list</span><span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 356 <span class="k">struct</span><span class="w"></span>
 357 <span class="w">  </span><span class="k">val</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=&gt;</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>
 358 <span class="k">end</span><span class="w"></span>
 359 </pre></div></div></div>
 360 <div class="paragraph"><p>These structures are allowed by the Definition.</p></div>
 361 </li>
 362 </ul></div>
 363 </div>
 364 </div>
 365 <div class="sect1">
 366 <h2 id="_deviations_from_the_basis_library_specification">Deviations from the Basis Library Specification</h2>
 367 <div class="sectionbody">
 368 <div class="paragraph"><p>Here are some deviations of SML/NJ from the <a href="BasisLibrary">Basis Library</a>
 369 <a href="http://www.standardml.org/Basis">specification</a>.</p></div>
 370 <div class="ulist"><ul>
 371 <li>
 372 <p>
 373 SML/NJ exposes the equality of the <span class="monospaced">vector</span> type in structures such
 374 as <span class="monospaced">Word8Vector</span> that abstractly match <span class="monospaced">MONO_VECTOR</span>, which says
 375 <span class="monospaced">type vector</span>, not <span class="monospaced">eqtype vector</span>.  So, for example, SML/NJ accepts
 376 the following program:
 377 </p>
 378 <div class="listingblock">
 379 <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">v</span><span class="p">:</span><span class="w"> </span><span class="n">Word8Vector</span><span class="p">.</span><span class="n">vector</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">v</span><span class="w"></span>
 380 </pre></div></div></div>
 381 </li>
 382 <li>
 383 <p>
 384 SML/NJ exposes the equality property of the type <span class="monospaced">status</span> in
 385 <span class="monospaced">OS.Process</span>. This means that programs which directly compare two
 386 values of type <span class="monospaced">status</span> will work with SML/NJ but not MLton.
 387 </p>
 388 </li>
 389 <li>
 390 <p>
 391 Under SML/NJ on Windows, <span class="monospaced">OS.Path.validVolume</span> incorrectly considers
 392 absolute empty volumes to be valid. In other words, when the
 393 expression
 394 </p>
 395 <div class="listingblock">
 396 <div class="content"><div class="highlight"><pre><span class="n">OS</span><span class="p">.</span><span class="n">Path</span><span class="p">.</span><span class="n">validVolume</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">isAbs</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><span class="n">vol</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="s">&quot;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
 397 </pre></div></div></div>
 398 <div class="paragraph"><p>is evaluated by SML/NJ on Windows, the result is <span class="monospaced">true</span>.  MLton, on
 399 the other hand, correctly follows the Basis Library Specification,
 400 which states that on Windows, <span class="monospaced">OS.Path.validVolume</span> should return
 401 <span class="monospaced">false</span> whenever <span class="monospaced">isAbs = true</span> and <span class="monospaced">vol = ""</span>.</p></div>
 402 <div class="paragraph"><p>This incorrect behavior causes other <span class="monospaced">OS.Path</span> functions to behave
 403 differently. For example, when the expression</p></div>
 404 <div class="listingblock">
 405 <div class="content"><div class="highlight"><pre><span class="n">OS</span><span class="p">.</span><span class="n">Path</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="p">(</span><span class="n">OS</span><span class="p">.</span><span class="n">Path</span><span class="p">.</span><span class="n">fromString</span><span class="w"> </span><span class="s">&quot;</span><span class="se">\\</span><span class="s">usr</span><span class="se">\\</span><span class="s">local&quot;</span><span class="p">)</span><span class="w"></span>
 406 </pre></div></div></div>
 407 <div class="paragraph"><p>is evaluated by SML/NJ on Windows, the result is <span class="monospaced">"\\usr\\local"</span>,
 408 whereas under MLton on Windows, evaluating this expression (correctly)
 409 causes an <span class="monospaced">OS.Path.Path</span> exception to be raised.</p></div>
 410 </li>
 411 </ul></div>
 412 </div>
 413 </div>
 414 </div>
 415 <div id="footnotes"><hr></div>
 416 <div id="footer">
 417 <div id="footer-text">
 418 </div>
 419 <div id="footer-badges">
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