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    <p>The best way to find what is going on is to tell the compiler to output its intermediate representation with <code>-v4</code>. The output is voluminous and a bit hard to read, but should allow you to find out exactly what the difference in the generated code is, and how the compiler arrived there.</p> <p>You will probably notice that <code>fA</code> is being moved outside the function (to the "global scope") on your first example. On your second example, it probably is not (meaning it will be recreated on each call).</p> <p>One possible reason for it not being moved outside the function would be because the compiler is thinking it depends on the value of <code>n</code>. On your working example, there is no <code>n</code> for <code>fA</code> to depend on.</p> <p>But the reason I think the compiler is avoiding moving <code>fA</code> outside on your second example is because it is trying to avoid a space leak. Consider what would happen if <code>fA</code>, instead of your array, were an infinite list (on which you used the <code>!!</code> operator). Imagine you called it once with a large number (for instance <code>f 10000</code>), and later only called it with small numbers (<code>f 2</code>, <code>f 3</code>, <code>f 12</code>...). The 10000 elements from the earlier call are still on memory, wasting space. So, to avoid this, the compiler creates <code>fA</code> again every time you call your function.</p> <p>The space leak avoidance probably does not happen on your first example because in that case <code>f</code> is in fact only called once, returning a closure (we are now at the frontier of the pure functional and the imperative worlds, so things get a bit more subtle). This closure replaces the original function, which will never be called again, so <code>fA</code> is only called once (and thus the optimizer feels free to move it outside the function). On your second example, <code>f</code> does not get replaced by a closure (since its value depends on the argument), and thus will get called again.</p> <p>If you want to try to understand more of this (which will help reading the <code>-v4</code> output), you could take a look at the <a href="http://research.microsoft.com/~simonpj/Papers/spineless-tagless-gmachine.ps.gz" rel="noreferrer">Spineless Tagless G-Machine</a> paper (<a href="http://citeseer.ist.psu.edu/peytonjones92implementing.html" rel="noreferrer">citeseer link</a>).</p> <p>As to your final question, I think it is a compiler peculiarity (but I could be wrong). However, it would not surprise me if all compilers did the same thing, even if it were not part of the language.</p>
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