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Pattern variables and local scopes
Prerequisite Concepts		Related Syntax

Pattern variables are "declared" in a manner slightly different from other names: they are declared in a pattern assignment embedded in a pattern (using =>), instead of in a variable declaration.

Because pattern variables are declared and can be referenced before the local scope begins, they are treated differently than the variables declared within the local scope. In effect, they are treated as if two local scopes are defined, one within the other. The outer one defines the pattern variables, and the inner can contain local declarations to declare other kinds of shelves.

This means:

• Pattern variables which are defined in a rule header or match pattern can also be referenced in the condition which follows it. (Versions of OmniMark prior to V3 did not permit this.)
• This also means that it is not an error to declare a local variable in the inner local scope whose name is the same as that of a pattern variable defined in the rule header or match pattern. The declared local variable hides the pattern variable.
• The name of a pattern variable cannot be used as a keyword anywhere within its scope: in the pattern, the following condition, if any, or in the following local scope.

The other context in which patterns, and therefore pattern assignments, can occur, is the second argument of the matches operator. This argument defines an entire local scope itself. This means that:

• A pattern variable can have the same name as that of a pattern variable (or any other kind) defined in an outer scope. The new pattern variable hides the one defined in the outer scope.
• A name assigned in such a second argument pattern cannot be used as a keyword within the same second argument pattern. There is no prohibition from using the name as a keyword outside of the second argument pattern, even within the same enclosing expression.

Prerequisite Concepts      Pattern matching    Scope of variables

Related Syntax    pattern

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