Running a Parser

Parsing with respect to a grammar does not require generating a specialized parser along with the associated overhead of compiling and loading. Essence provides general parsers which accept a grammar as input and parse "right away." This allows incremental debugging and development of attributed grammars to be used with Essence.

Essence actually comes with a number of different implementations of LR parsing. The packages.scm configuration file contains definitions for a range of structures all with the interface parser-interface. The one intended for production use is in the cps-lr structure whose implementation resides in cps/cps-lr.scm. Parser-interface describes only one binding called parse:

(parse  grammar lookahead method input) procedure

• Grammar is a grammar defined by define-grammar.
• Lookahead is a non-negative integer denoting the lookahead the parser uses.
• Method is a symbol, either lr or slr, specifying the parsing method used--either full LR parsing or SLR parsing.
• Input is a stream (as per the stream structure) consisting of enumerands of the terminals of grammar paired with their attribute values. It is the actual input to the parser.

  The stream actually contains the ordinal numbers of the enumerated values. These are easiest obtained via the enumerated structure of Scheme 48. In the absence of an implementation of grammar and enumerated, the list of components describes the mapping between grammar symbols and enumerands; the enumerands are 0-based.

If the grammar contains productions containing $error symbols, the parser will attempt error recovery when possible.

Parsing is not reentrant. (Actually, the cps-attrib-error structure implemented by cps/cps-attrib-error.scm does implement reentrant parsing. The parser generator does not currently use it, however.)