Scannerless parsing

In computer science, scannerless parsing (also called lexerless parsing) refers to performing both tokenization (breaking a stream of characters into words) and parsing (arranging the words into phrases) in a single step, rather than breaking it up into a pipeline of a lexer followed by a parser, executing concurrently. It also refers to the associated grammar: using a single formalism to express both the lexical (word level) grammar and phrase level grammar used to parse a language.

Dividing processing up into a lexer followed by a parser is generally viewed as better design because it is more modular, and scannerless parsing is primarily used when a clear lexer–parser distinction is unneeded or unwanted. Examples of when this is appropriate include TeX, most wiki grammars, makefiles, simple per application scripting languages, and Perl 6.

Advantages

• Only one metalanguage is needed
• Non-regular lexical structure is handled easily
• "Token classification" is unneeded which removes the need for design accommodations such as "the lexer hack" and language reserved words (such as "while" in C)
• Grammars can be compositional (can be merged without human intervention)

Disadvantages

• Since the lexical scanning and syntactic parsing are combined, the resulting parser tends to be harder to understand and debug for more-complex languages

Required extensions

Eelco Visser identified five key extensions to classical context-free syntax which handle almost all common non-context-free constructs arising in practice:

• Follow restrictions, a limited form of "longest match"
• Reject productions, a limited form of negative matching (as found in boolean grammars)
• Preference attributes to handle the dangling else construct in C-like languages
• Per-production transitions rather than per-nonterminal transitions in order to facilitate:
  • Associativity attributes, which prevent a self-reference in a particular production of a nonterminal from producing that same production
  • Precedence/priority rules, which prevent self-references in higher-precedence productions from producing lower-precedence productions

Implementations

• SGLR is a parser for the modular Syntax Definition Formalism SDF, and is part of the ASF+SDF Meta-Environment and the Stratego/XT program transformation system.
• JSGLR, a pure Java implementation of SGLR, also based on SDF.
• TXL supports character-level parsing.
• dparser generates ANSI C code for scannerless GLR parsers.
• Spirit allows for both scannerless and scanner-based parsing.
• SBP is a scannerless parser for boolean grammars (a superset of context-free grammars), written in Java.
• Laja is a two phase scannerless parser generator with support for mapping the grammar rules into objects, written in Java.
• The Perl 6 Grammars feature of the general purpose programming language Perl 6.
• PyParsing is a scannerless parser written in pure Python.
• META II Has built in token parsers functions.
• TREE-META Like META II also is scannerless having builtin lexer functions.
• CWIC Compiler for Writing and Implementing Compilers. Has token rules as part of its language. Rules in CWIC were compiled into boolean functions returning success or failure.

Notes

• ^ This is because parsing at the character level makes the language recognized by the parser a single context-free language defined on characters, as opposed to a context-free language of sequences of strings in regular languages. Some lexerless parsers handle the entire class of context-free languages, which is closed under composition.

Further reading

• Visser, E. (August 1997). Scannerless Generalized-LR Parsing. The Netherlands: University of Amsterdam. Retrieved 22 November 2012.