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<title>DefineTypeBeforeUse - MLton Standard ML Compiler (SML Compiler)</title>
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<a href="StandardML">Standard ML</a> requires types to be defined before they are used.  Because of type inference, the use of a type can be implicit; hence, this requirement is more subtle than it might appear.  For example, the following program is not type correct, because the type of <tt>r</tt> is <tt>t&nbsp;option&nbsp;ref</tt>, but <tt>t</tt> is defined after <tt>r</tt>. 
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> r = ref NONE
<B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">A</FONT> </FONT></B>|<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">B</FONT>
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> () = r := SOME A
</PRE>
<p>
 
</p>
<p>
MLton reports the following error, indicating that the type defined on line 2 is used on line 1. 
</p>

<pre>Error: z.sml 1.1.
  Type escapes the scope of its definition at z.sml 2.10.
    type: t
    in: val r = ref NONE
</pre><p>
While the above example is benign, the following example shows how to cast an integer to a function by (implicitly) using a type before it is defined.  In the example, the ref cell <tt>r</tt> is of type  <tt>t&nbsp;option&nbsp;ref</tt>, where <tt>t</tt> is defined <em>after</em> <tt>r</tt>, as a parameter to functor <tt>F</tt>.  This example causes <a href="PolyML">PolyML</a> 4.1.3 to seg fault. 
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> r = ref NONE
<B><FONT COLOR="#0000FF">functor</FONT></B> F (<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t
           </FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> x: t) =
   <B><FONT COLOR="#0000FF">struct</FONT></B>
      <B><FONT COLOR="#A020F0">val</FONT></B> () = r := SOME x
      <B><FONT COLOR="#A020F0">fun</FONT></B> get () = valOf (!r)
   <B><FONT COLOR="#0000FF">end</FONT></B>
<B><FONT COLOR="#0000FF">structure</FONT></B> S1 = F (<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> unit -&gt; unit
                  </FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> x = <B><FONT COLOR="#A020F0">fn</FONT></B> () =&gt; ())
<B><FONT COLOR="#0000FF">structure</FONT></B> S2 = F (<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> int
                  </FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> x = <B><FONT COLOR="#5F9EA0">13</FONT></B>)
<B><FONT COLOR="#A020F0">val</FONT></B> () = S1.get () ()
</PRE>
 
</p>
<p>
MLton reports the following error. 
</p>

<pre>Warning: z.sml 1.1.
  Unable to locally determine type of variable: r.
    type: ??? option ref
    in: val r = ref NONE
Error: z.sml 1.1.
  Type escapes the scope of its definition at z.sml 2.17.
    type: t
    in: val r = ref NONE
</pre></div>



<p>
<hr>
Last edited on 2005-12-01 03:38:39 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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