# memoize -- record results of function evaluation for future use

## Description

memoize f -- produces, from a function f, a new function that behaves the same as f, but remembers previous answers to be provided the next time the same arguments are presented.

 i1 : fib = n -> if n <= 1 then 1 else fib(n-1) + fib(n-2) o1 = fib o1 : FunctionClosure i2 : time fib 28 -- used 2.49304 seconds o2 = 514229 i3 : fib = memoize fib o3 = fib o3 : FunctionClosure i4 : time fib 28 -- used 0.000046126 seconds o4 = 514229 i5 : time fib 28 -- used 1.357e-6 seconds o5 = 514229

The function memoize operates by constructing a MutableHashTable in which the argument sequences are used as keys for accessing the return value of the function.

An optional second argument to memoize provides a list of initial values, each of the form x => v, where v is the value to be provided for the argument x.

Warning: when the value returned by f is null, it will always be recomputed, even if the same arguments are presented.

Warning: the new function created by memoize 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 CacheTable.

## Ways to use memoize :

• "memoize(Function)"
• "memoize(Function,List)"

## For the programmer

The object memoize is .