Re: What is the difference between the constant construct in Perl and the Readonly construct in Perl?

The author of Readonly has inspired my ire by neglecting to mention the positive reasons to use constant. I wrote a note at AnnoCPAN and I'm reproducing it for you here as well.

You didn't mention that the constant pragma allows the value to get inlined into the optree. You also didn't mention that when constants are used in conditionals, perl is able to optimize away the non-executable branches.
Further you didn't mention that constant.pm tracks perl's preferred implementation for constants. Prior to 5.9.4 constants were implemented as functions like sub FOO () { 1 }. After that perl does something a little bit different which allows for a smaller memory footprint.

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Comment on Re: What is the difference between the constant construct in Perl and the Readonly construct in Perl?

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Re^2: What is the difference between the constant construct in Perl and the Readonly construct in Perl? by tinita (Parson) on Jul 06, 2006 at 08:35 UTC
I agree. that was one of the few things I didn't like while reading PBP. To many people speed doesn't seem to be important. But if I hadn't used `constant` in my HTC module, it would probably be much slower than it is (I use a lot of constants there...) and I think at least in modules speed is a bit more important than in application code.	[reply] [d/l]
Re^3: What is the difference between the constant construct in Perl and the Readonly construct in Perl? by Argel (Prior) on Jul 07, 2006 at 19:30 UTC
From PBP page 58: If you decide not [to] use the Readonly module in production code (for performance or political reasons), then using constant is still better than using literal values. From that quote I get the impression that TheDamian is aware of the performance issues but takes for granted that others are not as familiar. Regardless however, the performance issue is mentioned in PBP (albeit briefly). Update: Fixed typos	[reply]
Re^2: What is the difference between the constant construct in Perl and the Readonly construct in Perl? by esskar (Deacon) on Jul 07, 2006 at 14:57 UTC
well, i think speed is always an issue!	[reply]
Re^3: What is the difference between the constant construct in Perl and the Readonly construct in Perl? by Happy-the-monk (Canon) on Jul 07, 2006 at 19:29 UTC
While you're often right with this, and certainly in this case too (++), be careful not to generalise: In many other places, speed may not be so much of an issue =) Cheers, Sören	[reply]
Re^4: What is the difference between the constant construct in Perl and the Readonly construct in Perl? by revdiablo (Prior) on Jul 10, 2006 at 21:57 UTC
In many other places, speed may not be so much of an issue Not only that, but what "speed" means can vary wildly. Performance is not a simple, single variable that can be tweaked at will. There are so many variables at play that any widely general rule is going to be wrong some time or another. Take this specific thread as an example. This is a classic case of micro-optimization. Certainly, using constant may be way, way faster than using Readonly. But what if the majority of an application's time is spent on network IO? Does the time it takes to resolve a constant really matter? It takes careful consideration and testing in the proper context to know for sure. To be clear, I'm not poo-pooing optimization by any means. I'm just trying to point out that it's important to optimize the right thing.	[reply]