Scannerless parsing

In computer science, scannerless parsing (also called lexerless parsing) performs 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. A language grammar is scannerless if it uses a single formalism to express both the lexical (word level) and phrase level structure of the language.

Dividing processing into a lexer followed by a parser is more modular; 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 application-specific scripting languages, and Raku.

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 Raku Grammars feature of the general purpose programming language Raku.
  • 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. CiteSeerX 10.1.1.37.7828.
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