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<head><title>memoize -- record results of function evaluation for future use</title>
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<div><h1>memoize -- record results of function evaluation for future use</h1>
<div class="single"><h2>Description</h2>
<div><tt>memoize f</tt> -- produces, from a function <tt>f</tt>, a new function that behaves the same as <tt>f</tt>, but remembers previous answers to be provided the next time the same arguments are presented.<p/>
<table class="examples"><tr><td><pre>i1 : fib = n -> if n <= 1 then 1 else fib(n-1) + fib(n-2)
o1 = fib
o1 : FunctionClosure</pre>
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<tr><td><pre>i2 : time fib 16
-- used 0.001999 seconds
o2 = 1597</pre>
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<tr><td><pre>i3 : fib = memoize fib
--warning: function fib redefined
o3 = fib
o3 : FunctionClosure</pre>
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<tr><td><pre>i4 : time fib 16
-- used 0. seconds
o4 = 1597</pre>
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<tr><td><pre>i5 : time fib 16
-- used 0. seconds
o5 = 1597</pre>
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<p/>
The function <tt>memoize</tt> operates by constructing a <a href="___Mutable__Hash__Table.html" title="the class of all mutable hash tables">MutableHashTable</a> in which the argument sequences are used as keys for accessing the return value of the function.<p/>
An optional second argument to memoize provides a list of initial values, each of the form <tt>x => v</tt>, where <tt>v</tt> is the value to be provided for the argument <tt>x</tt>.<p/>
Warning: when the value returned by <tt>f</tt> is <a href="_null.html" title="the unique member of the empty class">null</a>, it will always be recomputed, even if the same arguments are presented.<p/>
Warning: the new function created by <tt>memoize</tt> will save references to all arguments and values it encounters, and this will often prevent those arguments and values from being garbage-collected as soon as they might have been. If the arguments are implemented as mutable hash tables (modules, matrices and rings are implemented this way) then a viable strategy is to stash computed results in the arguments themselves. See also <tt>CacheTable</tt>.</div>
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<div class="waystouse"><h2>Ways to use <tt>memoize</tt> :</h2>
<ul><li>memoize(Function)</li>
<li>memoize(Function,List)</li>
</ul>
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