Programming Perl, 3rd Edition

Program Generation

Almost from the time people first figured out that they could write programs, they started writing programs that write other programs. We often call these program generators. (If you're a history buff, you might know that RPG stood for Report Program Generator long before it stood for Role Playing Game.) Nowadays they'd probably be called "program factories", but the generator people got there first, so they got to name it.

Now, anyone who has written a program generator knows that it can make your eyes go crossed even when you're wide awake. The problem is simply that much of your program's data looks like real code, but isn't (at least not yet). The same text file contains both stuff that does something and similar looking stuff that doesn't. Perl has various features that make it easy to mix Perl together with other languages, textually speaking.

(Of course, these features also make it easier to write Perl in Perl, but that's rather to be expected by now, we should think.)

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; ...