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In a lazy (or non-strict) language, the arguments to a function are not evaluated before calling the function.  Instead, the arguments are suspended and only evaluated by the function if needed. <p>
<a href="StandardML">Standard ML</a> is an eager (or strict) language, not a lazy language.  However, it is easy to delay evaluation of an expression in SML by creating a <em>thunk</em>, which is a nullary function.  In SML, a thunk is written <tt>fn&nbsp;()&nbsp;=&gt;&nbsp;e</tt>.  Another essential feature of laziness is <em>memoization</em>, meaning that once a suspended argument is evaluated, subsequent references look up the value.  We can express this in SML with a function that maps a thunk to a memoized thunk. 
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<pre class=code>
<B><FONT COLOR="#0000FF">signature</FONT></B> LAZY =
   <B><FONT COLOR="#0000FF">sig</FONT></B>
      <B><FONT COLOR="#A020F0">val</FONT></B> lazy: (unit -&gt; 'a) -&gt; unit -&gt; 'a
   <B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
 
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<p>
This is easy to implement in SML. 
</p>

<pre class=code>
<B><FONT COLOR="#0000FF">structure</FONT></B> Lazy: LAZY =
   <B><FONT COLOR="#0000FF">struct</FONT></B>
      <B><FONT COLOR="#A020F0">fun</FONT></B> lazy (th: unit -&gt; 'a): unit -&gt; 'a =
         <B><FONT COLOR="#A020F0">let</FONT></B>
            <B><FONT COLOR="#A020F0">val</FONT></B> r: 'a option ref = ref NONE
         <B><FONT COLOR="#A020F0">in</FONT></B>
            <B><FONT COLOR="#A020F0">fn</FONT></B> () =&gt;
            <B><FONT COLOR="#A020F0">case</FONT></B> !r <B><FONT COLOR="#A020F0">of</FONT></B>
               NONE =&gt;
                  <B><FONT COLOR="#A020F0">let</FONT></B>
                     <B><FONT COLOR="#A020F0">val</FONT></B> a = th ()
                     <B><FONT COLOR="#A020F0">val</FONT></B> () = r := SOME a
                  <B><FONT COLOR="#A020F0">in</FONT></B>
                     a
                  <B><FONT COLOR="#A020F0">end</FONT></B>
             | SOME a =&gt; a
         <B><FONT COLOR="#A020F0">end</FONT></B>
   <B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
 
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Last edited on 2005-01-26 20:33:55 by <span title="cfs36.cs.cornell.edu"><a href="MatthewFluet">MatthewFluet</a></span>.
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