In addition to simply calling parse, Essence also allows the generation of highly efficient specialized parsers with respect to a grammar, lookahead, and parsing method. Essence offers a (Unix) batch version of the parser generator, as well as a Scheme 48 package which allows access from within a REPL.
In order to run, specialized parsers require an implementation of streams as well as an error procedure which accepts one argument, an error message. The specialized parser will call error when an unrecoverable error occurs.