parseltongue

Table of contents

• Introduction
• Installation
• Reference
  • Symbols
  • Numbers
  • Strings
  • Booleans
  • Dotted pairs
  • Lists
  • Quotations
• Shortcomings

Introduction

Parseltongue is an S-expression parser. It provides a single parse function capable of parsing symbols, numbers, booleans and strings — as well as lists and dotted pairs. Expressions may be quoted.

import { parse } from 'parseltongue';

parse(`(address (street "644 Glen Summit")
                (city "St. Charles")
                (state "Missouri")
                (zip 63304))`);
/*
[
  'address',
  [ 'street', '"644 Glen Summit"' ],
  [ 'city', '"St. Charles"' ],
  [ 'state', '"Missouri"' ],
  [ 'zip', 63304 ]
]
*/

Installation

Parseltongue can be installed via npm with the following command:

npm install parseltongue

Reference

Symbols

Atomic symbols are parsed into native JavaScript strings. As usual, symbols cannot start with a number.

import { parse } from 'parseltongue';

parse(`driver-loop`);
// 'driver-loop'

parse(`+`);
// '+'

Numbers

Atomic numbers are parsed into native JavaScript numbers. As such, they are subject to the same rules and limitations. There is no support for fractions (i.e., exact numbers).

import { parse } from 'parseltongue';

parse(`42`);
// 42

parse(`12.8`);
// 12.8

Strings

Atomic strings are parsed into native JavaScript strings. The content is put in quotation marks (").

import { parse } from 'parseltongue';

parse(`"this is a string"`);
// '"this is a string"'

Booleans

Atomic booleans are parsed into native JavaScript booleans.

import { parse } from 'parseltongue';

parse(`#t`);
// true

parse(`#f`);
// false

Dotted pairs

JavaScript does not provide a native tuple data type. For this reason, dotted pairs are parsed to a custom Pair class.

import { parse, Pair } from 'parseltongue';

parse(`(abelson . sussman)`);
// Pair { car: 'abelson', cdr: 'sussman' }

Lists

Symbolic lists are parsed into native JavaScript arrays. Dotted pairs forming a proper list are also parsed to arrays.

import { parse } from 'parseltongue';

parse(`(roger dave nick rick)`);
// [ 'roger', 'dave', 'nick', 'rick' ]

parse(`(roger . (dave . (nick . (rick . ()))))`);
// [ 'roger', 'dave', 'nick', 'rick' ]

Square and curly brackets are also supported:

import { parse } from 'parseltongue';

parse(`{[roger bass] [dave guitar] [nick drums] [rick keyboard]}`);
/*
[
  [ 'roger', 'bass' ],
  [ 'dave', 'guitar' ],
  [ 'nick', 'drums' ],
  [ 'rick', 'keyboard' ]
]
*/

Quotations

S-expressions may be quoted.

import { parse } from 'parseltongue';

parse(`'42`);
// [ 'quote', 42 ]

parse(`'(1 2 3)`);
// [ 'quote', [ 1, 2, 3 ] ]

Shortcomings

In the presence of an error, Parseltongue only reports what part of the input string could not be parsed.

import { parse } from 'parseltongue';

parse(`12 . 4)`);
// Error: Extraneous characters:  . 4)

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