Javascript

memoizer.js

// Example taken from Javascript The Good Parts

var memoizer = function(memo, formula) {
    var recur = function(n) {
        var result = memo[n];
        if (typeof result !== 'number') {
            result = formula(recur, n);
            memo[n] = result;
        }
        return result;
    };
    return recur;
};

var fibonacci = memoizer([0, 1], function(recur, n) {
    return recur(n - 1) + recur(n - 2);
});

var factorial = memoizer([1, 1], function(recur, n) {
    return n * recur(n - 1);
});