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I think performance optimization should not be the principle determining factor in deciding which technique to use. Unless your application is running unacceptably slowly and you've identified the multi-branch if/else section as a bottleneck, it's ridiculous to change that to something else just for a performance gain.

Maintainability, and design — as in consistency of — those are of primary importance.

I like dispatch tables. They're clever. But at the same time, they raise a red flag for me — the same one I see whenever hashes are used to store a static set of entities in an application: It completely undermines the safety of strict. (There are steps you can take to regain some of this safety, such as using Tie::StrictHash.)

The solution is to make the subs named class methods. I recently did this on a project at work.

I had two dispatch tables, one for the "real" functions and one for a set of stubs, to be used when debugging.

Old way:

my %real_functions = ( wipe_system => sub { system "rm -rf /" }, ); my %debug_stubs = ( wipe_system => sub { warn "wiping system (no, not really)\n" }, ); my $funcs = $debug ? \%debug_stubs : \%real_functions; $funcs->{'wipe_system'}->();

New way:

{ package SystemFunctions::Real; sub wipe_system { shift; # will be the class name system "rm -rf"; } } { package SystemFunctions::Debug; sub wipe_system { shift; # will be the class name warn "wiping system (no, not really)\n"; } } my $funcs = $debug ? 'SystemFunctions::Real' : 'SystemFunctions::Debug'; $funcs->wipe_system();

Now, it sometimes won't be convenient to do this, due to the complication of packages. In my case, it was; in fact, it was a significant improvement, since it gave me a convenient place to encapsulate all my "system functions", which hitherto had lived in the main namespace.

In the very simplest cases, you don't need to worry about packages at all:

my %dispatch = ( incr => sub { $x++ }, decr => sub { $x-- }, ); $dispatch{$op}->();
becomes
sub incr { shift; $x++ } sub decr { shift; $x-- } main->$op();

Note: I only left the shift in as a reminder that it will always be necessary unless the sub takes no arguments (or, as I sometimes do, you pop the args rather than shifting them).

Finally, what about default cases? One possibility is to use AUTOLOAD.

We're building the house of the future together.

In reply to Re: When should I use a dispatch table? by jdporter
in thread When should I use a dispatch table? by Limbic~Region

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