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Lazy
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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 () => 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 -> 'a) -> unit -> 'a
<B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
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This is easy to implement in SML.
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<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 -> 'a): unit -> '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> () =>
<B><FONT COLOR="#A020F0">case</FONT></B> !r <B><FONT COLOR="#A020F0">of</FONT></B>
NONE =>
<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 => a
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#0000FF">end</FONT></B>
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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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