Conditionally included subtemplates - StringTemplate

There are many situations when you want to conditionally include some text or another template. StringTemplate provides simple IF-statements to let you specify conditional includes. For example, in a dynamic web page you usually want a slightly different look depending on whether or not the viewer is "logged in" or not. Without a conditional include, you would need two templates: page_logged_in and page_logged_out. You can use a single page definition with if(expr) attribute actions instead:

<html>
...
<body>
$if(member)$
$gutter/top_gutter_logged_in()$
$else$
$gutter/top_gutter_logged_out()$
$endif$
...
</body>
</html>

where template top_gutter_logged_in is located in the gutter subdirectory of my StringTemplateGroup.

IF actions test the presence or absence of an attribute unless the object is a Boolean/bool, in which case it tests the attribute for true/false. The only operator allowed is "not" and means either "not present" or "not true". For example, "$if(!member)$...$endif$".

You can also use elseif to make a chain of tests:

$if(x)$
...
$elseif(y)$
...
$elseif(z)$
...
$else$
...
$endif$

The first true expression "wins".

Whitespace in conditionals issue

There is a simple, but not perfect rule: kill a single newline after <if>, <<, <else>, and <endif> (but for <endif> only if it's on a line by itself) . Kill newlines before <else> and <endif> and >>. For example,

a <if(foo)>big<else>small<endif> dog

is identical to:

a <if(foo)>
big
<else>
small
<endif>
 dog

It is very difficult to get the newline rule to work "properly" because sometimes you want newlines and sometimes you don't. I
decided to chew up as many as is reasonable and then let you explicitly say <\n> when you need to.