is_set (mixed variable) ⇒ boolean
Determines if a variable has already been defined locally, globally or as a parameter. This is particularly useful for determining if an optional parameter has been defined.
is_set function is one of the oldest functions in ZPE and has existed
since one of the first versions. It's underlying implementation has remained
somewhat the same throughout the development until about version 1.5.x where
it had a major overhaul.
It is a very powerful function with one of the most unique capabilities of all predefined functions because it has the ability to assess unevaluated variables.
The function can be used to check if an index of a value exists too, so this can be used to check the arguments of a program:
is_set function can also be used on pure values:
The result when applied to a value will always yield