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Concept
Scannerless parsing
Summary
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, s, simple application-specific scripting languages, and Raku.
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)
Since the lexical scanning and syntactic parsing are combined, the resulting parser tends to be more complicated and thus harder to understand and debug. The same will hold for the associated grammar, if a grammar is used to generate the parser.
The resulting parser tends to be significantly less efficient than a lexer-parser pipeline with regard to both time and memory.
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.
Official source
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