Faster alternative to eval? - performance

Question

I'm dealing with a web app that uses a home-grown templating system that lets Perl code be embedded in HTML. These statements are executed by the template parser at run-time using eval EXPR.
This is very flexible, but these statements are scattered everywhere, and get executed a lot. eval EXPR (as opposed to eval BLOCK) requires Perl to fire up the interpreter each time, and my profiling reveals that they're a reasonably significant source of slowdown.
Many of the embedded Perl statements are very simple. For example, a template might have a line like this:
<p>Welcome, <!--E: $user->query('name') -->.
Or:
<p>Ticket number <!--E: $user->generate_ticket_number() --> has been generated.
That is, they're just calling object methods. There are more complicated ones, too, though.
I'm hoping to optimize this, and so far have two ideas, both of which are terrible. The first is to rewrite all templates to replace simple calls with tokens like USER:NAME and USER:GENERATETICKETNUMBER, which the parser could then scan for and invoke the appropriate object method. But then instead of dealing with templates that mix HTML and Perl, I would have templates that mix HTML, Perl, and tokens.
The second idea is to try to parse the embedded Perl, figure out what the statement wants to do, and, if it's simple enough, call the appropriate object method via a symbolic reference. This is obviously insane.
Is there some logical solution I'm overlooking?

Answer 1

Try taking an approach similar to the one that mod_perl uses to compile CGIs:
Convert the template into Perl code. For instance, your first example might convert to something like:
print "<p>Welcome, ";
print $user->query('name');
print ".\n";
Wrap a sub { ... } around that code, along with some code to unpack arguments (e.g, for things like $user in the sample).
eval that code. Note that it returns a coderef.
Call that coderef repeatedly. :)

Answer 2

You might take a look at Mojolicious. It has a templating engine which allow a syntax close to what you are using. You could possibly switch to use it or look at its source (click source on left of previous link) to see if you can draw some ideas.
FYI the Mojolcious templating engine's syntax allows the following forms intermixed with HTML appropriately
<% Perl code %>
<%= Perl expression, replaced with result %>
<%== Perl expression, replaced with XML escaped result %>
<%# Comment, useful for debugging %>
<%% Replaced with "<%", useful for generating templates %>
% Perl code line, treated as "<% line =%>"
%= Perl expression line, treated as "<%= line %>"
%== Perl expression line, treated as "<%== line %>"
%# Comment line, treated as "<%# line =%>"
%% Replaced with "%", useful for generating templates

Answer 3

You might want to look at the guts of Text::MicroTemplate. Realistically, you might want to use Text::MicroTemplate, as it likely fits your needs. It builds a subroutine that concatenates strings as needed, much like duskwuff suggested. Here's the result of build_mt('hello, <?= $_[0] ?>') in re.pl:
$CODE1 = sub {
package Devel::REPL::Plugin::Packages::DefaultScratchpad;
use warnings;
use strict 'refs';
local $SIG{'__WARN__'} = sub {
print STDERR $_mt->_error(shift(), 4, $_from);
}
;
Text::MicroTemplate::encoded_string(sub {
my $_mt = '';
local $_MTREF = \$_mt;
my $_from = '';
$_mt .= 'hello, ';
$_from = $_[0];
$_mt .= ref $_from eq 'Text::MicroTemplate::EncodedString' ? $$_from : do {
$_from =~ s/([&><"'])/$Text::MicroTemplate::_escape_table{$1};/eg;
$_from
};
return $_mt;
}
->(#_));
};

Answer 4

You should not be using 'eval' to call methods in your template. Sorry to sound harsh but the point of a separated view is to remove the processing code from the view layer. The template systems described above along with Template Toolkit just pass in an object / hash so you can access it.
why not pass $user as a hashref like:
$user = {
'name' => 'John',
'id' => '3454'
};
this will allow you to access 'name' with:
$user->{'name'};
Otherwise, it's likely that you have that you're doing something like:
template calls $user->query();
method calls DB to get value
method returns value
That's soooo much more expensive to make a database query than to pass the hash/object reference to the template. You may want to check out some code profiling tools like Devel::NYTProf to see what part of the code execution is really slowing you down. I'm skeptical that the eval is bogging down your program so much that you need to optimize out eval. Sounds like the code inside eval is what is slowing you down.