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    <p>Here's a running example in pseudo-javascript-like syntax.</p> <pre><code>function f(x) { var y = ...; function g(z) { function h(w) { .... y, z, w .... } .... x, h .... } .... x, g .... } </code></pre> <p>One representation is a linked chain of environments. That is, a closure consists of a code pointer, some slots, and a reference to the enclosing closure or the top-level environment. In this representation,</p> <pre><code>f = [&lt;code&gt;, &lt;top-level-env&gt;] g = [&lt;code&gt;, f, x, y] h = [&lt;code&gt;, g, z] </code></pre> <p>except sometimes it's better to let every function have a direct reference to the top-level environment, since it's used so often:</p> <pre><code>f = [&lt;code&gt;, &lt;top-level-env&gt;] g = [&lt;code&gt;, &lt;top-level-env&gt;, f, x, y] h = [&lt;code&gt;, &lt;top-level-env&gt;, g, z] </code></pre> <p>(There are other variations too.)</p> <p>One advantage of this representation is that you can store mutable variables right in the closure. (Well, maybe, depending on how you represent function activations.) One disadvantage is some variables may take multiple hops to reach, if you have deeply nested closures. Another disadvantage is that if a closure outlives its parent (eg, <code>g</code> returns <code>h</code>) then this representation might prevent the GC from collecting environment frames that are mostly or even completely unreachable.</p> <p>Another representation is "flat closures": each closure contains a code pointer and slots for all of the code's free variables.</p> <pre><code>g = [&lt;code&gt;, x, y] h = [&lt;code&gt;, y, z] </code></pre> <p>This representation fixes the space/GC problem; no closure pins another closure in memory. On the other hand, free-variable slots are copied instead of shared, so if there is a nested closure with many free variables---or many instances of a nested closure---overall memory usage might be higher. Also, this representation typically requires storage for mutable variables to be heap-allocated (but only for variables that are actually mutated and only when the mutation cannot be automatically rewritten).</p> <p>There are also hybrid approaches. For example, you might have mostly-flat closures but treat the top-level environment specially:</p> <pre><code>g = [&lt;code&gt;, &lt;top-level-env&gt;, x, y] </code></pre> <p>Or you might have a "sufficiently clever" (or at least "sufficiently ambitious") compiler that tries to pick between representations based on number of free variables, nesting depth, etc.</p>
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    1. POHow is a referencing environment generally implemented for closures?
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    1. USRyan Culpepper
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    1. POHow is a referencing environment generally implemented for closures?
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    1. COGreat, thanks so much for the explanation! A couple of questions on the matter: 1) Do you know which specific languages use some of the approaches you suggested? The tree structure approach is pretty cool, where one referencing environment is forked off of a parent one, although you might indeed hold onto large chunks of memory that way. 2) Do you know where one could find out more about details like the ones we discussed here? The Internet isn't too ripe of information for rather obscure concepts like these. Any specific books perhaps?
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      1. USAlexandr Kurilin
 

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