Syntax diagram: Difference between revisions

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{{Short description|Visual description of context-free grammar}}

'''Syntax diagrams''' (or '''railroad diagrams''') are a way to represent a [[context-free grammar]]. They represent a graphical alternative to [[Backus–Naur form]], [[EBNF]], [[Augmented Backus–Naur form]], and other text-based grammars as [[metalanguages]]. Early books using syntax diagrams include the "Pascal User Manual" written by [[Niklaus Wirth]]<ref name="Wirth1974">[http://e-collection.library.ethz.ch/eserv/eth:3059/eth-3059-01.pdf Niklaus Wirth: ''The Programming Language Pascal.'' (July 1973)]</ref> (diagrams start at page 47) and the Burroughs [[CANDE]] Manual.<ref name="burroughs71">[http://bitsavers.org/pdf/burroughs/B6500_6700/5000318_B6700_CANDE_Oct72.pdf Burroughs B6700/B7700 ''Command AND Edit (CANDE) Language: Information Manual'']</ref> In the compilation field, textual representations like BNF or its variants are usually preferred. BNF is text-based, and used by compiler writers and parser generators. Railroad diagrams are visual, and may be more readily understood by laypeople, sometimes incorporated into graphic design. The canonical source defining the [[JSON]] [[data exchange|data interchange]] format provides yet another example of a popular modern usage of these diagrams.

== Principle of syntax diagrams ==

The representation of a grammar is a set of syntax diagrams. Each diagram defines a "nonterminal" stage in a process. There is a main diagram which defines the language in the following way: to belong to the language, a word must describe a path in the main diagram.

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DIGIT = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"

</syntaxhighlight>

Note that ABNF also supports ranges, e.g. {{code|2=abnf|1=DIGIT = %x30-39}}, but it is not used here for consistency with the other examples.

[[Red (programming language)]] Parse Dialect:

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digit: ["0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"]

</syntaxhighlight>

Note that thisThis format also supports ranges, e.g. {{code|2=red|digit: charset [#"0" - #"9"]}}, but it is not used here for consistency with the other examples.

One possible syntax diagram for the example grammars is below. Note that whileWhile the syntax for the text-based grammars differs, the syntax diagram for all of them can be the same because it is a [[metalanguage]].

[[Image:Syntax-diagram-example.png|"Railroad" syntax diagram]]

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* [https://karmin.ch/ebnf/index EBNF Parser & Renderer]

* [https://www.sqlite.org/docsrc/finfo?name=art/syntax/bubble-generator.tcl SQLite syntax diagram generator for SQL]

* [https://www.bottlecaps.de/rr Online Railroad Diagram Generator]

* [https://www.yorku.ca/jmason/asdgram.htm Augmented Syntax Diagram (ASD) grammars]

* [http://www.asd-networks.com (ASD) '''Augmented''' Syntax Diagram Application Demo Site ]