Template proces­sors and engines are one of those pieces of soft­ware where it seems every devel­op­er wants to rein­vent the wheel. Goodness knows I’ve done it ear­li­er in my career. Tell me if this sounds familiar:

1. You need to mix data into a doc­u­ment so you start with Perl’s string inter­po­la­tion in "dou­ble quotes" or sprintf for­mats. (Or maybe you inves­ti­gate formats, but the less said about them the bet­ter.)
2. You real­ize your doc­u­ments need to dis­play things based on cer­tain con­di­tions, or you want to loop over a list or some oth­er structure.
3. You add these fea­tures via key­word pars­ing and escape char­ac­ters, think­ing it’s OK since this is just a small bespoke project.
4. Before you know it you’ve invent­ed anoth­er domain-​specific lan­guage (DSL) and have to sup­port it on top of the appli­ca­tion you were try­ing to deliv­er in the first place.

Stop. Just stop. Decades of oth­ers who have walked this same path have already done this for you. Especially if you’re using a web frame­work like Dancer, Mojolicious, or Catalyst, where the tem­plate proces­sor is either built-​in or plug­gable from CPAN. Even if you’re not devel­op­ing a web appli­ca­tion there are sev­er­al general-​purpose options of var­i­ous capa­bil­i­ties like Template Toolkit and Template::Mustache. Investigate the alter­na­tives and deter­mine if they have the fea­tures, per­for­mance, and sup­port you need. If you’re sure none of them tru­ly meet your unique require­ments, then maybe, maybe con­sid­er rolling your own.

Whatever you decide, real­ize that as your appli­ca­tion or web­site grows your invest­ment in that selec­tion will only deep­en. Porting to a new tem­plate proces­sor can be as chal­leng­ing as port­ing any source code to a new pro­gram­ming language.

Unfortunately, there are about as many opin­ions on how to choose a tem­plate proces­sor as there are tem­plate proces­sors. For exam­ple, in 2013 Roland Koehler wrote a good Python-​oriented arti­cle on sev­er­al con­sid­er­a­tions and the dif­fer­ent approach­es avail­able. Although he end­ed up devel­op­ing his own (quelle sur­prise), he makes a good case that a tem­plate proces­sor ought to at least pro­vide var­i­ous log­ic con­structs as well as embed­ded expres­sions, if not a full pro­gram­ming lan­guage. Koehler specif­i­cal­ly warns against the lat­ter, though, as a tem­plate devel­op­er might change an application’s data mod­el, to say noth­ing of the pos­si­bil­i­ty of exe­cut­ing arbi­trary destruc­tive code.

I can appre­ci­ate this rea­son­ing. I’ve suc­cess­ful­ly used Perl tem­plate proces­sors like the afore­men­tioned Template::Toolkit (which has both log­ic direc­tives and an option­al facil­i­ty for eval­u­at­ing Perl code) and Text::Xslate (which sup­ports sev­er­al tem­plate syn­tax­es includ­ing a sub­set of Template::Toolkit​’s, but with­out the abil­i­ty to embed Perl code). We use the lat­ter at work com­bined with Text::Xslate::Bridge::TT2Like​’s emu­la­tion of var­i­ous Template::Toolkit vir­tu­al meth­ods and it’s served us well.

But using those mod­ules’ DSLs means more sophis­ti­cat­ed tasks need extra time and effort find­ing the cor­rect log­ic and expres­sions. This also assumes that their designer(s) have antic­i­pat­ed my needs either through built-​in fea­tures or exten­sions. I’m already writ­ing Perl; why should I switch to anoth­er, more lim­it­ed lan­guage and envi­ron­ment pro­vid­ed I can remain dis­ci­plined enough to avoid issues like those described above by Koehler?

So for my per­son­al projects, I favor tem­plate proces­sors that use the full pow­er of the Perl lan­guage like Mojolicious’ embed­ded Perl ren­der­er or the ven­er­a­ble Text::Template for non-​web appli­ca­tions. It saves me time and I’ll like­ly want more than any DSL can pro­vide. This may not apply to your sit­u­a­tion, though, and I’m open to counter-arguments.

What’s your favorite tem­plate proces­sor and why? Let me know in the comments.