== Your (HTML) template language should have conditionals

You could call this a war story, or just learning from my painful
experience.

Back when I was first writing DWiki and didn't have enough experience
to know better, I decided that it should have a quite simply template
language because it would be easy to implement and it would avoid the
temptation to do lots of things in templates. As part of this simplicity
I decided that there would be no 'if' conditional operator.

(There is a slightly subtle reason for avoiding this. If you have no
'if' and no looping, you can do template expansion one operator at a
time with text substitution because you need no lookahead; you just find
the next operator, expand it on the spot, and insert the expansion. If
you add 'if' or loops, you now have a pattern of '<if ...> text <else>
other text <endif>' and you need some parsing.)

Of course I wound up needing conditional stuff anyways, so I invented
conditional template inclusion and some other tricks. This preserved
the simplicity of my template expansion code but had another drawback.
Instead of having templates with conditional logic, I instead wound up
with an explosion of templates; every separate 'if' clause basically
had to become a separate (conditionally included) template.

(You don't want to know how the actual conditions are implemented.
I swear, it made sense at the time.)

An explosion of templates is not a usability improvement over
conditional logic in templates. Rather the contrary, because it's much
harder to see and follow things across five or six separate files than
it would be if everything was in one place. In the end almost any
templating system is going to have some form of conditionality; the
only question is where it emerges and how ugly the result is. I've come
around to the view that having actual 'if' conditional operators is the
least bad way even if they do complicate various things and risk your
templates becoming increasingly hard to follow.

(And I know, real templating systems have dealt with all of these issues
long ago and for any real project you should almost certainly not try
to write your own templating system but adopt an existing one, which
renders all of this moot. DWiki was a special case for all sorts of
reasons.)