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  1. PO
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    <p>I would use explicit lazy evaluation. Here's my implementation of it based on Scheme's take:</p> <pre><code>var delay, lazy, force, promise, promiseForced, promiseRunning; (function () { var getValue = function () { return this.value; }; var RUNNING = {}; var DelayThunk = function (nullaryFunc) { this.value = nullaryFunc; }; DelayThunk.prototype.toString = function () { return "[object Promise]"; }; DelayThunk.prototype.force = function () { if (promiseRunning (this)) { throw new Error ("Circular forcing of a promise."); } var nullaryFunc = this.value; this.value = RUNNING; this.value = nullaryFunc (); this.force = getValue; return this.value; }; var LazyThunk = function (nullaryFunc) { DelayThunk.call (this, nullaryFunc); }; LazyThunk.prototype = new DelayThunk (null); LazyThunk.prototype.constructor = LazyThunk; LazyThunk.prototype.force = function () { var result = DelayThunk.prototype.force.call (this); while (result instanceof LazyThunk) { result = DelayThunk.prototype.force.call (result); } return force (result); }; delay = function (nullaryFunc) { return new DelayThunk (nullaryFunc); }; lazy = function (nullaryFunc) { return new LazyThunk (nullaryFunc); }; force = function (expr) { if (promise (expr)) { return expr.force (); } return expr; }; promise = function (expr) { return expr instanceof DelayThunk; }; promiseForced = function (expr) { return expr.force === getValue || !promise (expr); }; promiseRunning = function (expr) { return expr.value === RUNNING || !promise (expr); }; }) (); </code></pre> <p>Example Syntax:</p> <pre><code>var x = lazy (function () { return expression; }); var y = force (x); var z = delay (function () { return expression; }); var w = force (z); </code></pre> <p>Note values are stored once evaluated, so repeated forcing will not do extra computations.</p> <p>Example usage:</p> <pre><code>function makeThunk (x, y, z) { return lazy (function () { // lots of work done here }); } var thunk = makeThunk (arg1, arg2, arg3); if (condition) { output (force (thunk)); output (force (thunk)); // no extra work done; no extra side effects either } </code></pre>
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    1. POHow do I compute a variable in JavaScript if and only if it is used?
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    1. PTAnswer
    1. USThomas Eding
    1. USThomas Eding
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    1. COThe usage looks promising. Thanks. This is interesting, I'll try to understand it in a bit.
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      1. USfent
 

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