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<title>Overloading - MLton Standard ML Compiler (SML Compiler)</title>
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      Overloading
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In <a href="StandardML">Standard ML</a>, constants (like <tt>13</tt>, <tt>0w13</tt>, <tt>13.0</tt>) are overloaded, meaning that they can denote a constant of the appropriate type as determined by context.  SML defines the overloading classes Int, Real, and Word, which denote the sets of types that integer, real, and word constants may take on.  In MLton, these are defined as follows. 
    <ul>


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<td>
 Int </td>
<td>
 <tt>Int2.int</tt>, <tt>Int3.int</tt>, ... <tt>Int32.int</tt>, <tt>Int64.int</tt>, <tt>LargeInt.int</tt>, <tt>IntInf.int</tt> </td>
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<td>
 Real </td>
<td>
 <tt>Real32.real</tt>, <tt>Real64.real</tt>, <tt>LargeReal.real</tt> </td>
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 <tr>
<td>
 Word </td>
<td>
 <tt>Word2.word</tt>, <tt>Word3.word</tt>, ... <tt>Word32.word</tt>, <tt>Word64.int</tt>, <tt>LargeWord.word</tt> </td>
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    </ul>


<p>
The <a href="DefinitionOfStandardML">Definition</a> allows flexibility in how much context is used to resolve overloading.  It says that the context is <em>no larger than the smallest enclosing structure-level declaration</em>, but that <em>an implementation may require that a smaller context determines the type</em>.  MLton uses the largest possible context allowed by SML in resolving overloading.  If the type of a constant is not determined by context, then it takes on a default type.  In MLton, these are defined as follows. 
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<td>
 Int </td>
<td>
 Int32.int </td>
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 <tr>
<td>
 Real </td>
<td>
 Real64.real </td>
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 <tr>
<td>
 Word </td>
<td>
 Word64.word </td>
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    </ul>


<p>
Other implementations may use a smaller context or different default types. 
</p>
<h2 id="head-a4bc8bf5caf54b18cea9f58e83dd4acb488deb17">Also see</h2>

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    <li>
<p>
 <a class="external" href="http://mlton.org/basis/top-level-chapter.html"><img src="moin-www.png" alt="[WWW]" height="11" width="11">discussion of overloading in the Basis Library</a> 
</p>
</li>

    </ul>


<h2 id="head-eb01bf04c9a0e8a71c45816513df424f1c7ffedb">Examples</h2>

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    <li>
<p>
 The following program is rejected. 
<pre class=code>
<B><FONT COLOR="#0000FF">structure</FONT></B> S: 
   <B><FONT COLOR="#0000FF">sig</FONT></B> 
      <B><FONT COLOR="#A020F0">val</FONT></B> x: Word8.word 
   <B><FONT COLOR="#0000FF">end</FONT></B> =
   <B><FONT COLOR="#0000FF">struct</FONT></B>
      <B><FONT COLOR="#A020F0">val</FONT></B> x = <B><FONT COLOR="#5F9EA0">0w0</FONT></B>
   <B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
  The smallest enclosing structure declaration for <tt>0w0</tt> is   <tt>val&nbsp;x&nbsp;=&nbsp;0w0</tt>.  Hence, <tt>0w0</tt> receives the default type for  words, which is <tt>Word32.word</tt>. 
</p>
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Last edited on 2005-12-02 01:19:19 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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