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>".
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.