Introduction

The pragmatics of using Essence are slightly different from that of using other parser or parser generation packages.

At the heart of parsing is, as usual, a context-free grammar. Essence provides a new syntactic form define-grammar which embeds a language for attributed context-free grammars into Scheme. This is different from other parser generation packages which either represent a context-free grammar as an S-expression object or in some special syntax in a special file.

Given a grammar, parsing can proceed in one of two modes:

• A general parse procedure will accept a grammar, a parsing method (SLR or LR), a lookahead size, and an input, and produce the result of parsing and attribute evaluation. This mode of operation allows instant turnaround, but also parses very slowly. It is good for incremental development, but impractical for production parsers.
• A parser generator (automatically generated from the implementation of parse) produces a specialized parser from a grammar, a parsing method, and a lookahead. The specialized parser only accepts an input as an argument, but is otherwise identical in operation to the general parser. It is highly efficient, and therefore good for production use.