A faster way to compile code out of a system is to hide it behind
an if(CONSTANT) or unless(CONSTANT), as in
use constant DEBUG => 0;
if (DEBUG) {
warn(
"This code is only compiled into the program ",
" when DEBUG is true.\n";
);
}
The bonus is that you don't introduce more delay in the compile time,
which a lot of people apparently dislike.
I discovered this in POE, a project where I gained about 20% runtime
performance with
POE::Preprocessor
by replacing small, commonly used functions with macros. A contrived
example:
macro num_max (x,y) {
((x) > (y) ? (x) : (y))
}
This macro is then used, template-like, in the main body of source as:
print "You owe: \$", {% num_max $total-$paid, 0 %}, "\n";
Back to compile-time inclusion. POE::Preprocessor uses the common
if/elsif/else syntax, tagged with an "# include" marker. That is, if
you comment a construct with "# include", it will be evaluated at compile time (using the CONSTANT trick), and the code in the block will be included (or not)
depending on the condition's outcome.
unless ($expression) { # include
... lines of code ...
} elsif ($expression) { # include
... lines of code ...
} else { # include
... lines of code ...
} # include
Problems with macros and source filters in general:
- They alter your source's line numbers, which interferes with
warnings and error messages. POE::Preprocessor takes great pains to insert "# line" directives that not only preserve
your original line numbers but also indicate where in your macros the
problem may really lie.
- They confound packagers, most notably perlapp and perl2exe. These Perl
"compilers" do not evaluate source filters at runtime. They don't
even evaluate them at "compile" time. Instead, the original, non-Perl
syntax becomes an error when you try to run things.
- Source filtering is slow. I got no end of complaints about slow
startup times, even though POE::Preprocessor attempts to be optimal Perl.
- Any non-Perl syntax, no matter how trivially like any number of
template toolkits, is greeted with shock and confusion. (Heck, people
still don't like @_[CONST1, CONST2], even though
it is standard Perl syntax.)
Liz's solution is much smarter than mine. It addresses all these problems. Very nice!
-- Rocco Caputo - rcaputo@pobox.com - poe.perl.org
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