Programming Perl, 4th Edition

Generating Other Languages in Perl

Perl is (among other things) a text-processing language, and most computer languages are textual. Beyond that, Perl’s lack of arbitrary limits, together with the various quoting and interpolation mechanisms, make it easy to visually isolate the code of the other language you’re spitting out. For example, here is a small chunk of s2p, the sed-to-perl translator:

print &q(<<"EOT");
:       #!$bin/perl
:       eval 'exec $bin/perl –S \$0 \${1+"\$@"}'
:               if \$running_under_some_shell;
:
EOT

Here the enclosed text happens to be legal in two languages, Perl and sh. We’ve used an idiom right off the bat that will preserve your sanity in the writing of a program generator: the trick of putting a “noise” character and a tab on the front of every quoted line, which visually isolates the enclosed code, so you can tell at a glance that it’s not the code that is actually being executed. One variable, $bin, is interpolated in the multiline quote in two places, and then the string is passed through a function to strip the colon and tab.

Of course, you aren’t required to use multiline quotes. One often sees CGI scripts containing millions of print statements, one per line. It seems a bit like driving to church in a Formula 1 car, but hey, if it gets you there… (We will admit that a column of print statements has its own form of visual distinctiveness.)

When you are embedding a large, multiline quote containing some other language (such as HTML), it’s often helpful to pretend you’re ...