13 04 parser states 2

A parser state is declared with the following syntax:

parserState
: optAnnotations STATE name
  "{" parserStatements transitionStatement "}"
;

Each state has a name and a body. The body consists of a sequence of statements that describe the processing performed when the parser transitions to that state, including:

• Local variable declarations,
• Assignment statements,
• Method calls, which serve several purposes:
  • Invoking functions (e.g., using verify to check the validity of data already parsed), and
  • Invoking methods (e.g., extracting data out of packets or computing checksums) and other parsers (see Section Sub-parsers), and
• Conditional statements,
• Transitions to other states (discussed in Section Transition statements).

The syntax for parser statements is given by the following grammar rules:

parserStatements
: /* empty */
| parserStatements parserStatement
;

parserStatement
: assignmentOrMethodCallStatement
| directApplication
| emptyStatement
| variableDeclaration
| constantDeclaration
| parserBlockStatement
| conditionalStatement
;

parserBlockStatement
: optAnnotations "{" parserStatements "}"
;

Architectures may place restrictions on the expressions and statements that can be used in a parser—e.g., they may forbid the use of operations such as multiplication or place restrictions on the number of local variables that may be used.

In terms of the ParserModel, the sequence of statements in a state are executed sequentially.