Download to read offline
![How to use
● Easy installation: npm install nearley --save-dev
● Generate a parser: npx nearleyc -o parser.js parser.ne
○ Add to scripts in package.json
● The .ne files contains rules, terminals, non-terminals, and actions
expr -> "(" _ sum _ ")" {% function (d) { return d[2]; }
%}
| value {% function (d) { return d[0]; }
%}
Non-terminal Terminal Whitespace Action The return value
from the sum rule](https://image.slidesharecdn.com/buildingparsersinjavascript-200221073752/85/Building-parsers-in-JavaScript-12-320.jpg)
![How to use
● Easy installation: npm install nearley --save-dev
● Generate a parser: npx nearleyc -o parser.js parser.ne
○ Add to scripts in package.json
● The .ne files contains rules, terminals, non-terminals, and actions
expr -> "(" _ sum _ ")" {% function (d) { return d[2]; }
%}
| value {% function (d) { return d[0]; }
%}
Non-terminal Terminal Whitespace Action The return value
from the sum rule](https://image.slidesharecdn.com/buildingparsersinjavascript-200221073752/75/Building-parsers-in-JavaScript-12-2048.jpg)
Uploaded byKenneth Geisshirt
Building parsers in JavaScript
This document discusses building parsers in JavaScript. It begins with an overview of parsing and grammar. Nearley is introduced as a parser generator that implements Earley's parsing algorithm. Nearley allows defining grammars and generating JavaScript parsers from them. The document then provides an example of building a fraction calculator parser using Nearley. It presents the grammar and source code for the parser and fraction calculation logic. Key advantages of Nearley for JavaScript parsing are its support for left-recursive grammars and ability to generate parsers with semantic actions.
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Building parsers in JavaScript
- 1. Building parsers in JavaScript KennethGeisshirt kneth kgeisshirt
- 2. Agenda ● What isparsing? ● Fractions ● Nearley ● Example: Fraction calculator Rajiv Patel, https://tinyurl.com/yx6rmcdt
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- 4. You know theproblem ● What is syntax? ● How is the syntax defined? ● How do you check if input matches syntax? ● How can you use the syntax in your applications? https://xkcd.com/859/
- 5. Grammar ● The syntaxis defined by a grammar ● Lexical analysis breaks down input into tokens or terminals ○ Keywords, literals, identifiers, operators ● A set of rules connecting non-terminals to tokens ● One non-terminal is the start symbol ● Parsers are software which use a grammar to verify input keyword identifier Body: statements Example ● S → AA ● A → 𝞪 ● A → 𝞫 Matches 𝞪𝞪, 𝞪𝞫, 𝞫𝞪, and 𝞫𝞫 function add2(n) { let r = n + 2; return r; } Ken Whytock, https://tinyurl.com/s9s3eee literal
- 6. Parser generators ● Manywell-documented algorithms exist ○ Hot research topics in 1960s and 1970s ● It’s not a trivial task to write a parser ● Parser generators can speed up development process ○ Yacc (C) - 1975!! ○ ANTLR (mostly Java) - 1989 ○ Nearley (JavaScript) - 2014 Erica Zabowski, https://tinyurl.com/uqbaldv
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- 8. Quick recap ● Afraction is a rational number ○ Numerator and denominator, both natural numbers ○ Broken latin (fractus, broken) ● Fractions are rational numbers Bill Ward, https://tinyurl.com/r3dtp2b
- 9. Arithmetic Greatest Common Divisor ●Original algorithm by Euclid (c. 300 BC) ● Often used to reduce or simplify a fraction ● https://en.wikipedia.org/wiki/Greatest_common_divisor
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- 11. Earley Parsers inJavaScript ● Nearley implements Earley’s parser algorithm ○ Left-recursive (LR) grammars ○ Deterministic parser ○ Worst-case performance O(n3 ) but O(n) for well-behaving grammars ○ https://en.wikipedia.org/wiki/Earley_parser ● Can generate JavaScript, CoffeeScript, and TypeScript ○ Can run in browsers, node.js and probably React Native ● Inclusion of predefined grammars ○ Numbers, white spaces, strings ● Lexer is also included ○ Define tokens using double-quotes ● Rules can have (semantic) actions ○ Plain JavaScript functions
- 12. How to use ●Easy installation: npm install nearley --save-dev ● Generate a parser: npx nearleyc -o parser.js parser.ne ○ Add to scripts in package.json ● The .ne files contains rules, terminals, non-terminals, and actions expr -> "(" _ sum _ ")" {% function (d) { return d[2]; } %} | value {% function (d) { return d[0]; } %} Non-terminal Terminal Whitespace Action The return value from the sum rule
- 13. Additional tools Supported bymany editors ● VS Code, Atom, Emacs, Vim, Sublime Railroad diagrams
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- 15. kalculator ● Simple Fractionclass ○ Basic arithmetic and simplification ● Parser ○ Actions to perform calculation ● Little driver to read input and call parser
- 16. The grammar (noactions) main → sum expr → ( sum ) | value product → product * expr | product / expr | value sum → sum + expr | sum - expr | product value → fraction | int fraction → int / int Start symbol: main Tokens: (, ), +, -, *, / Positive integer: int Important take-aways ● Recursive rules ● Operator precedence
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- 20. Links ● My example:https://github.com/kneth/kalculator ● Nearley: https://nearley.js.org/ ● Earley parsers explained: http://loup-vaillant.fr/tutorials/earley-parsing/ ● An Efficient Context-Free Parsing Algorithm. Jay Earley’s Ph.D. thesis from 1968. http://reports-archive.adm.cs.cmu.edu/anon/anon/usr/ftp/scan/CMU-CS-68-earl ey.pdf
- 21. Building parsers for JavaScriptis easy - and fun
- 22.