parseltongue

Table of contents

• Introduction
• Installation
• Reference
  • Symbols
  • Numbers
  • Strings
  • Booleans
  • Dotted pairs
  • Lists
  • Quotations
• Partial Parsing
• Error Handling

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). Exponential notation is also not supported at this time.

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 ] ]

Partial Parsing

Parseltoungue has built-in support for partial parsing. That is, the parse function asks for more characters if the input string has to potential to result in a well-defined S-expresssion.

import { Partial, parse } from 'parseltongue';

parse(`(1 2 `);
// Partial {}

parse(`(1 2 `).complete(`3)`);
// [ 1, 2, 3 ]

A partial parsing can be turned into a failure using the .fail() method. When invoked, .fail() will throw a ParseError error.

import { Partial, ParseError, parse } from 'parseltongue';

parse(`(1 2`).fail();
/* throws
   ParseError {
     at: 4,
     expected: {
       oneOf: [
         '0', '1',     '2',
         '3', '4',     '5',
         '6', '7',     '8',
         '9', '/\\s/', ')',
         '.'
       ]
     }
   }
*/

Error Handling

When fed with a malformed input, parse throws a ParseError error detailing what valid characters were instead expected in the input string.

import { ParseError, parse } from 'parseltongue';

parse(`0.12q`);
/* throws
   ParseError {
     at: 4,
     expected: {
       oneOf: [
        '0',
        '1',
        '2',
        '3',
        '4',
        '5',
        '6',
        '7',
        '8',
        '9',
        'end-of-input'
      ]
    }
  }
*/

parse(`0.121`);
// 0.121

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