Conditional tags

With Active Contents a lot of tags can be dynamic: all tags that can have options support a special option _cnd_ that when present is evaluated as a Perl expression. If the result of the expression is not true the tag is ignored.

As an example one could check an image file for existence, to avoid a PerlPoint parsing error in case it is missed. This would do the trick:

    \IMAGE{_cnd_="-r q(image.gif)" src="image.gif"}

Similarly, an \EMBED part might make no sense unless a certain state is found in the database:

    \EMBED{_cnd_="$dbh->selectrow_array($stateCheckQuery)"}
  
      ...
  
    \END_EMBED

As the _cnd_ tag is standard it is not documented with every tag, the rule is just that if a tag supports options it should support _cnd_.

While in most cases use of _cnd_ it is up to you it is essential with \STOP. \STOP is a tag that produces a fatal parsing error, thus stopping processing of the source file. While this makes no sense in itself, with _cnd_ it allows every document author to define his very own fatal errors. A typical use of this feature is to check for a certain PerlPoint parser version, in order to make sure that newer features are supported (especially useful when sharing document sources with others, or processing them on different systems):

    // check minimal PerlPoint parser version
    \STOP{_cnd_="$PerlPoint::Parser::VERSION>=0.40"}