Arcsecond

Arcsecond is a zero-dependency, Fantasy Land compliant JavaScript Parser Combinator library largely inspired by Haskell's Parsec.

The arcsecond-binary peer library includes parsers specifically for working with binary data.

---

- Arcsecond

- Release Notes
- Installation
- Tutorial
- Usage
- Running the examples
- API

Click to expand

- Parser Methods
- .run
- .fork
- .map
- .errorMap
- .errorChain
- .chain
- .mapFromData
- .chainFromData
- Functions
- setData
- withData
- mapData
- getData
- coroutine
- char
- anyChar
- str
- digit
- digits
- letter
- letters
- whitespace
- optionalWhitespace
- peek
- anyOfString
- regex
- sequenceOf
- namedSequenceOf
- choice
- lookAhead
- sepBy
- sepBy1
- exactly
- many
- many1
- between
- everythingUntil
- everyCharUntil
- anythingExcept
- anyCharExcept
- possibly
- startOfInput
- endOfInput
- skip
- pipeParsers
- composeParsers
- takeRight
- takeLeft
- recursiveParser
- tapParser
- decide
- mapTo
- errorMapTo
- fail
- succeedWith
- either
- toPromise
- toValue
- parse

- A note on recursive grammars
- Fantasy Land
- Equivalent Operations
- of
- map
- chain
- ap
- Name

Release Notes

Since version 2.0.0, the release notes track changes to arcsecond.

Installation

``bash
npm i arcsecond
`

Tutorials

The tutorials provide a practical introduction to many of the concepts in arcsecond, starting from the most basic foundations and working up to more complex topics.

- 1. Parsing weather data
- 2. Extracting useful information
- 3. Error handling
- 4. Building utility parsers
- 5. Recursive Structures
- 6. Debugging
- 7. Stateful parsing

Usage

You can use ES6 imports or CommonJS requires.

`JavaScript
const {char} = require('arcsecond');

const parsingResult = char('a').fork(
// The string to parse
'abc123',

// The error handler (you can also return from this function!)
(error, parsingState) => {
const e = new Error(error);
e.parsingState = parsingState;
throw e;
},

// The success handler
(result, parsingState) => {
console.log(Result: ${result});
return result;
}
);
`

Running the examples

`bash
git clone git@github.com:francisrstokes/arcsecond.git
cd arcsecond
npm i

json example


node -r esm examples/json/json.js
`

The examples are built as es6 modules, which means they need node to be launched with the -r esm require flag, which allows import and export statements to be used.

API

$3

#### .run

.run is a method on every parser, which takes input (which may be a string, TypedArray, ArrayBuffer, or DataView), and returns the result of parsing the input using the parser.

Example

`JavaScript
str('hello').run('hello')
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }
`

#### .fork

The .fork method is similar to .run. It takes input (which may be a string, TypedArray, ArrayBuffer, or DataView), an _error transforming function_ and a _success transforming function_, and parses the input. If parsing was successful, the result is transformed using the _success transforming function_ and returned. If parsing was not successful, the result is transformed using the _error transforming function_ and returned.

Example

`JavaScript
str('hello').fork(
'hello',
(errorMsg, parsingState) => {
console.log(errorMsg);
console.log(parsingState);
return "goodbye"
},
(result, parsingState) => {
console.log(parsingState);
return result;
}
);
// [console.log] Object {isError: false, error: null, target: "hello", data: null, index: 5, …}
// -> "hello"

str('hello').fork(
'farewell',
(errorMsg, parsingState) => {
console.log(errorMsg);
console.log(parsingState);
return "goodbye"
},
(result, parsingState) => {
console.log(parsingState);
return result;
}
);
// [console.log] ParseError (position 0): Expecting string 'hello', got 'farew...'
// [console.log] Object {isError: true, error: "ParseError (position 0): Expecting string 'hello',…", target: "farewell", data: null, index: 0, …}
// "goodbye"
`

#### .map

.map takes a function and returns a parser does not consume input, but instead runs the provided function on the last matched value, and set that as the new last matched value. This method can be used to apply structure or transform the values as they are being parsed.

Example

`JavaScript
const newParser = letters.map(x => ({
matchType: 'string',
value: x
});

newParser.run('hello world')
// -> {
// isError: false,
// result: {
// matchType: "string",
// value: "hello"
// },
// index: 5,
// data: null
// }
`

#### .chain

.chain takes a function which recieves the last matched value and should return a parser. That parser is then used to parse the following input, forming a chain of parsers based on previous input. .chain is the fundamental way of creating _contextual parsers_.

Example

`JavaScript

const lettersThenSpace = sequenceOf([
letters,
char(' ')
]).map(x => x[0]);

const newParser = lettersThenSpace.chain(matchedValue => {
switch (matchedValue) {
case 'number': return digits;

case 'string': return letters;

case 'bracketed': return sequenceOf([
char('('),
letters,
char(')')
]).map(values => values[1]);

default: return fail('Unrecognised input type');
}
});

newParser.run('string Hello')
// -> {
// isError: false,
// result: "Hello",
// index: 12,
// data: null
// }

newParser.run('number 42')
// -> {
// isError: false,
// result: "42",
// index: 9,
// data: null
// }

newParser.run('bracketed (arcsecond)')
// -> {
// isError: false,
// result: "arcsecond",
// index: 21,
// data: null
// }

newParser.run('nope nothing')
// -> {
// isError: true,
// error: "Unrecognised input type",
// index: 5,
// data: null
// }
`

#### .mapFromData

.mapFromData is almost the same as .map, except the function which it is passed also has access to the _internal state data_, and can thus transform values based on this data.

Example

`JavaScript

const parserWithData = withData(letters.mapFromData(({result, data}) => ({
matchedValueWas: result,
internalDataWas: data
})));

parserWithData(42).run('hello');
// -> {
// isError: false,
// result: {
// matchedValueWas: "hello",
// internalDataWas: 42
// },
// index: 5,
// data: 42
// }
`

#### .chainFromData

.chainFromData is almost the same as .chain, except the function which it is passed also has access to the _internal state data_, and can choose how parsing continues based on this data.

Example

`JavaScript
const lettersThenSpace = sequenceOf([
letters,
char(' ')
]).map(x => x[0]);

const parser = withData(lettersThenSpace.chainFromData(({result, data}) => {
if (data.bypassNormalApproach) {
return digits;
}

return letters;
}));

parser({ bypassNormalApproach: false }).run('hello world');
// -> {
// isError: false,
// result: "world",
// index: 11,
// data: { bypassNormalApproach: false }
// }

parser({ bypassNormalApproach: true }).run('hello world');
// -> {
// isError: true,
// error: "ParseError (position 6): Expecting digits",
// index: 6,
// data: { bypassNormalApproach: true }
// }
`

#### .errorMap

.errorMap is like .map but it transforms the error value. The function passed to .errorMap gets an object the _current error message_ (error) , the _index_ (index) that parsing stopped at, and the _data_ (data) from this parsing session.

Example

`JavaScript
const newParser = letters.errorMap(({error, index}) => Old message was: [${error}] @ index ${index});

newParser.run('1234')
// -> {
// isError: true,
// error: "Old message was: [ParseError (position 0): Expecting letters] @ index 0",
// index: 0,
// data: null
// }
`

#### .errorChain

.errorChain is almost the same as .chain, except that it only runs if there is an error in the parsing state. This is a useful method when either trying to recover from errors, or for when a more specific error message should be constructed.

Example

`JavaScript

const parser = digits.errorChain(({error, index, data}) => {
console.log('Recovering...');
return letters;
});

p.run('42');
// -> {
// isError: false,
// result: "42",
// index: 2,
// data: null
// }

p.run('hello');
// [console.log] Recovering...
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }

s = parser.run('');
// [console.log] Recovering...
// -> {
// isError: true,
// error: "ParseError (position 0): Expecting letters",
// index: 0,
// data: null
// }
`

$3

#### setData

setData takes anything that should be set as the _internal state data_, and returns a parser that will perform that side effect when the parser is run. This does not consume any input. If parsing is currently in an errored state, then the data will not be set.

Example

`JavaScript
const parser = coroutine(run=> {
const name = run(letters);

if (name === 'Jim') {
run(setData('The name is Jim'));
}

return name;
});

parser.run('Jim');
// -> {
// isError: false,
// result: "Jim",
// index: 3,
// data: "The name is Jim"
// }
`

If dealing with any complex level of state - such as an object where individual keys will be updated or required, then it can be useful to create utility parsers to assist with updating the _internal state data_.
One possible pattern that could be used is the reducer pattern, famed by redux:

Example

`JavaScript

const createStateReducer = reducer => action => getData.chain(state => setData(reducer(state, action)));

const updateCounterState = createStateReducer((state = 0, action) => {
switch (action.type) {
case 'INC': {
return state + 1;
}
case 'DEC': {
return state - 1;
}
case 'ADD': {
return state + action.payload;
}
case 'RESET': {
return 0;
}
}
});

const parser = coroutine(run=>{
let count = run(updateCounterState({ type: 'RESET' }));
console.log(count);

run(updateCounterState({ type: 'INC' }));
run(updateCounterState({ type: 'INC' }));
run(updateCounterState({ type: 'DEC' }));
count = run(updateCounterState({ type: 'INC' }));
console.log(count);

return run(updateCounterState({ type: 'ADD', payload: 10 }));
});

parser.run('Parser is not looking at the text!');
// [console.log] 0
// [console.log] 2
// -> {
// isError: false,
// result: 12,
// index: 0,
// data: 12
// }
`

#### withData

withData takes a _provided parser_, and returns a function waiting for some _state data_ to be set, and then returns a new parser. That parser, when run, ensures that the _state data_ is set as the _internal state data_ before the _provided parser_ runs.

Example

`JavaScript
const parserWithoutData = letters;
const parser = withData(parserWithoutData);

parser("hello world!").run('Jim');
// -> {
// isError: false,
// result: "Jim",
// index: 3,
// data: "hello world!"
// }

parserWithoutData.run('Jim');
// -> {
// isError: false,
// result: "Jim",
// index: 3,
// data: null
// }
`

#### mapData

mapData takes a function that recieves and returns some _state data_, and transforms the _internal state data_ using the function, without consuming any input.

Example

`JavaScript
const parser = withData(mapData(s => s.toUpperCase()));

parser("hello world!").run('Jim');
// -> {
// isError: false,
// result: null,
// index: 0,
// data: "HELLO WORLD!"
// }
`

#### getData

getData is a parser that will always return what is contained in the _internal state data_, without consuming any input.

Example

`JavaScript
const parser = withData(sequenceOf([
letters,
digits,
getData
]));

parser("hello world!").run('Jim1234');
// -> {
// isError: false,
// result: ["Jim", "1234", "hello world!"],
// index: 3,
// data: "hello world!"
// }
`

If dealing with any complex level of state - such as an object where individual keys will be updated or required, then it can be useful to create utility parsers to assist.

Example

`JavaScript

const selectState = selectorFn => getData.map(selectorFn);

const parser = withData(coroutine(run=> {
// Here we can take or transform the state
const occupation = run(selectState(({job}) => job));
const initials = run(selectState(({firstName, lastName}) => ${firstName[0]}${lastName[0]}));

console.log(${initials}: ${occupation});

const first = run(letters);
const second = run(digits);

return ${second}${first};
}));

parser({
firstName: "Francis",
lastName: "Stokes",
job: "Developer"
}).run('Jim1234');
// [console.log] FS: Developer
// -> {
// isError: false,
// result: "1234Jim",
// index: 3,
// data: {
// firstName: "Francis",
// lastName: "Stokes",
// job: "Developer"
// }
// }
`

#### coroutine

coroutine takes a user provided _parser function_, to which is passed a run function. Within the _parser function_, the user can run other parsers, and get immediate access to their results.

coroutine allows you to write parsers in a more imperative and sequential way - in much the same way async/await allows you to write code with promises in a more sequential way.

Inside of the _parser function_, you can use all regular JavaScript language features, like loops, variable assignments, try/catch, and conditional statements. This makes it easy to write very powerful parsers using coroutine, but on the other side it can lead to less readable, more complex code.

Debugging is also much easier, as breakpoints can be easily added, and values logged to the console after they have been parsed.

Example

`JavaScript
const parser = coroutine(run => {
// Capture some letters and assign them to a variable
const name = run(letters);

// Capture a space
run(char(' '));

const age = run(digits.map(Number));

// Capture a space
run(char(' '));

if (age >= 18) {
run(str('is an adult'));
} else {
run(str('is a child'));
}

return { name, age };
});

parser.run('Jim 19 is an adult');
// -> {
// isError: false,
// result: { name: "Jim", age: 19 },
// index: 18,
// data: null
// }

parser.run('Jim 17 is an adult');
// -> {
// isError: true,
// error: "ParseError (position 7): Expecting string 'is a child', got 'is an adul...'",
// index: 7,
// data: null
// }
`

#### char

char takes a character and returns a parser that matches that character exactly one time.

Example

`JavaScript
char ('h').run('hello')
// -> {
// isError: false,
// result: "h",
// index: 1,
// data: null
// }
`

#### anyChar

anyChar matches exactly one utf-8 character.

Example

`JavaScript
anyChar.run('a')
// -> {
// isError: false,
// result: "a",
// index: 1,
// data: null
// }

anyChar.run('😉')
// -> {
// isError: false,
// result: "😉",
// index: 4,
// data: null
// }
`

#### str

str takes a string and returns a parser that matches that string exactly one time.

Example

`JavaScript
str('hello').run('hello world')
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }
`

#### digit

digit is a parser that matches exactly one numerical digit /[0-9]/.

Example

`JavaScript
digit.run('99 bottles of beer on the wall')
// -> {
// isError: false,
// result: "9",
// index: 1,
// data: null
// }
`

#### digits

digits is a parser that matches one or more numerical digit /[0-9]/.

Example

`JavaScript
digits.run('99 bottles of beer on the wall')
// -> {
// isError: false,
// result: "99",
// index: 2,
// data: null
// }
`

#### letter

letter is a parser that matches exactly one alphabetical letter /[a-zA-Z]/.

Example

`JavaScript
letter.run('hello world')
// -> {
// isError: false,
// result: "h",
// index: 1,
// data: null
// }
`

#### letters

letters is a parser that matches one or more alphabetical letter /[a-zA-Z]/.

Example

`JavaScript
letters.run('hello world')
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }
`

#### whitespace

whitespace is a parser that matches one or more whitespace characters.

Example

`JavaScript
const newParser = sequenceOf ([
str ('hello'),
whitespace,
str ('world')
]);

newParser.run('hello world')
// -> {
// isError: false,
// result: [ "hello", " ", "world" ],
// index: 21,
// data: null
// }

newParser.run('helloworld')
// -> {
// isError: true,
// error: "ParseError 'many1' (position 5): Expecting to match at least one value",
// index: 5,
// data: null
// }
`

#### optionalWhitespace

optionalWhitespace is a parser that matches zero or more whitespace characters.

Example

`JavaScript
const newParser = sequenceOf ([
str ('hello'),
optionalWhitespace,
str ('world')
]);

newParser.run('hello world')
// -> {
// isError: false,
// result: [ "hello", " ", "world" ],
// index: 21,
// data: null
// }

newParser.run('helloworld')
// -> {
// isError: false,
// result: [ "hello", "", "world" ],
// index: 10,
// data: null
// }
`

#### peek

peek matches exactly one _numerical byte_ without consuming any input.

Example

`JavaScript
peek.run('hello world')
// -> {
// isError: false,
// result: 104,
// index: 0,
// data: null
// }

sequenceOf([
str('hello'),
peek
]).run('hello world')
// -> {
// isError: false,
// result: [ "hello", 32 ],
// index: 5,
// data: null
// }
`

#### anyOfString

anyOfString takes a string and returns a parser that matches exactly one character from that string.

Example

`JavaScript
anyOfString('aeiou').run('unusual string')
// -> {
// isError: false,
// result: "u",
// index: 1,
// data: null
// }
`

#### regex

regex takes a RegExp and returns a parser that matches as many characters as the RegExp matches.

Example

`JavaScript
regex(/^[hH][aeiou].{2}o/).run('hello world')
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }
`

#### sequenceOf

sequenceOf takes an array of parsers, and returns a new parser that matches each of them sequentially, collecting up the results into an array.

Example

`JavaScript
const newParser = sequenceOf ([
str ('he'),
letters,
char (' '),
str ('world'),
])

newParser.run('hello world')
// -> {
// isError: false,
// result: [ "he", "llo", " ", "world" ],
// index: 11,
// data: null
// }
`

#### namedSequenceOf

namedSequenceOf takes an array of string/parser pairs, and returns a new parser that matches each of them sequentially, collecting up the results into an object where the key is the string in the pair.

A pair is just an array in the form: [string, parser]

Example

`JavaScript
const newParser = namedSequenceOf ([
['firstPart', str ('he')],
['secondPart', letters],
['thirdPart', char (' ')],
['forthPart', str ('world')],
])

newParser.run('hello world')
// -> {
// isError: false,
// result: {
// firstPart: "he",
// secondPart: "llo",
// thirdPart: " ",
// forthPart: "world"
// },
// index: 11,
// data: null
// }
`

#### choice

choice takes an array of parsers, and returns a new parser that tries to match each one of them sequentially, and returns the first match. If choice fails, then it returns the error message of the parser that matched the most from the string.

Example

`JavaScript
const newParser = choice ([
digit,
char ('!'),
str ('hello'),
str ('pineapple')
])

newParser.run('hello world')
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }
`

#### lookAhead

lookAhead takes _look ahead_ parser, and returns a new parser that matches using the _look ahead_ parser, but without consuming input.

Example

`JavaScript
const newParser = sequenceOf ([
str ('hello '),
lookAhead (str ('world')),
str ('wor')
]);

newParser.run('hello world')
// -> {
// isError: false,
// result: [ "hello ", "world", "wor" ],
// index: 9,
// data: null
// }
`

#### sepBy

sepBy takes two parsers - a _separator_ parser and a _value_ parser - and returns a new parser that matches zero or more values from the _value_ parser that are separated by values of the _separator_ parser. Because it will match zero or more values, this parser will _fail_ if a _value_ is followed by a _separator_ but NOT another _value_. If there's no _value_, the result will be an empty array, not failure.

Example

`JavaScript
const newParser = sepBy (char (',')) (letters)

newParser.run('some,comma,separated,words')
// -> {
// isError: false,
// result: [ "some", "comma", "separated", "words" ],
// index: 26,
// data: null
// }

newParser.run('')
// -> {
// isError: false,
// result: [],
// index: 0,
// data: null
// }

newParser.run('12345')
// -> {
// isError: false,
// result: [],
// index: 0,
// data: null
// }
`

#### sepBy1

sepBy1 is the same as sepBy, except that it matches one or more occurence.

Example

`JavaScript
const newParser = sepBy1 (char (',')) (letters)

newParser.run('some,comma,separated,words')
// -> {
// isError: false,
// result: [ "some", "comma", "separated", "words" ],
// index: 26,
// data: null
// }

newParser.run('1,2,3')
// -> {
// isError: true,
// error: "ParseError 'sepBy1' (position 0): Expecting to match at least one separated value",
// index: 0,
// data: null
// }
`

#### exactly

exactly takes a positive number and returns a function. That function takes a parser and returns a new parser which matches the given parser the specified number of times.

Example

`JavaScript
const newParser = exactly (4)(letter)

newParser.run('abcdef')
// -> {
// isError: false,
// result: [ "a", "b", "c", "d" ],
// index: 4,
// data: null
// }

newParser.run('abc')
// -> {
// isError: true,
// error: 'ParseError (position 0): Expecting 4 letter, but got end of input.',
// index: 0,
// data: null
// }

newParser.run('12345')
// -> {
// isError: true,
// error: 'ParseError (position 0): Expecting 4 letter, got '1'',
// index: 0,
// data: null
// }
`

#### many

many takes a parser and returns a new parser which matches that parser zero or more times. Because it will match zero or more values, this parser will always match, resulting in an empty array in the zero case.

Example

`JavaScript
const newParser = many (str ('abc'))

newParser.run('abcabcabcabc')
// -> {
// isError: false,
// result: [ "abc", "abc", "abc", "abc" ],
// index: 12,
// data: null
// }

newParser.run('')
// -> {
// isError: false,
// result: [],
// index: 0,
// data: null
// }

newParser.run('12345')
// -> {
// isError: false,
// result: [],
// index: 0,
// data: null
// }
`

#### many1

many1 is the same as many, except that it matches one or more occurence.

Example

`JavaScript
const newParser = many1 (str ('abc'))

newParser.run('abcabcabcabc')
// -> {
// isError: false,
// result: [ "abc", "abc", "abc", "abc" ],
// index: 12,
// data: null
// }

newParser.run('')
// -> {
// isError: true,
// error: "ParseError 'many1' (position 0): Expecting to match at least one value",
// index: 0,
// data: null
// }

newParser.run('12345')
// -> {
// isError: true,
// error: "ParseError 'many1' (position 0): Expecting to match at least one value",
// index: 0,
// data: null
// }
`

#### between

between takes 3 parsers, a _left_ parser, a _right_ parser, and a _value_ parser, returning a new parser that matches a value matched by the _value_ parser, between values matched by the _left_ parser and the _right_ parser.

This parser can easily be partially applied with char ('(') and char (')') to create a betweenRoundBrackets parser, for example.

Example

`JavaScript
const newParser = between (char ('<')) (char ('>')) (letters);

newParser.run('')
// -> {
// isError: false,
// result: "hello",
// index: 7,
// data: null
// }

const betweenRoundBrackets = between (char ('(')) (char (')'));

betweenRoundBrackets (many (letters)).run('(hello world)')
// -> {
// isError: true,
// error: "ParseError (position 6): Expecting character ')', got ' '",
// index: 6,
// data: null
// }
`

#### everythingUntil

Note: Between 2.x and 3.x, the definition of the everythingUntil has changed. In 3.x, what was previously everythingUntil is now everyCharUntil.

everythingUntil takes a _termination_ parser and returns a new parser which matches every possible _numerical byte_ up until a value is matched by the _termination_ parser. When a value is matched by the _termination_ parser, it is not "consumed".

Example

`JavaScript
everythingUntil (char ('.')).run('This is a sentence.This is another sentence')
// -> {
// isError: false,
// result: [84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 115, 101, 110, 116, 101, 110, 99, 101],
// index: 18,
// data: null
// }

// termination parser doesn't consume the termination value
const newParser = sequenceOf ([
everythingUntil (char ('.')),
str ('This is another sentence')
]);

newParser.run('This is a sentence.This is another sentence')
// -> {
// isError: true,
// error: "ParseError (position 18): Expecting string 'This is another sentence', got '.This is another sentenc...'",
// index: 18,
// data: null
// }
`

#### everyCharUntil

everyCharUntil takes a _termination_ parser and returns a new parser which matches every possible _character_ up until a value is matched by the _termination_ parser. When a value is matched by the _termination_ parser, it is not "consumed".

Example

`JavaScript
everyCharUntil (char ('.')).run('This is a sentence.This is another sentence')
// -> {
// isError: false,
// result: 'This is a sentence',
// index: 18,
// data: null
// }

// termination parser doesn't consume the termination value
const newParser = sequenceOf ([
everyCharUntil (char ('.')),
str ('This is another sentence')
]);

newParser.run('This is a sentence.This is another sentence')
// -> {
// isError: true,
// error: "ParseError (position 18): Expecting string 'This is another sentence', got '.This is another sentenc...'",
// index: 18,
// data: null
// }
`

#### anythingExcept

Note: Between 2.x and 3.x, the definition of the anythingExcept has changed. In 3.x, what was previously anythingExcept is now anyCharExcept.

anythingExcept takes a _exception_ parser and returns a new parser which matches exactly one _numerical byte_, if it is not matched by the _exception_ parser.

Example

`JavaScript
anythingExcept (char ('.')).run('This is a sentence.')
// -> {
// isError: false,
// result: 84,
// index: 1,
// data: null
// }

const manyExceptDot = many (anythingExcept (char ('.')))
manyExceptDot.run('This is a sentence.')
// -> {
// isError: false,
// result: [84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 115, 101, 110, 116, 101, 110, 99, 101, 46],
// index: 18,
// data: null
// }
`

#### anyCharExcept

anyCharExcept takes a _exception_ parser and returns a new parser which matches exactly one _character_, if it is not matched by the _exception_ parser.

Example

`JavaScript
anyCharExcept (char ('.')).run('This is a sentence.')
// -> {
// isError: false,
// result: 'T',
// index: 1,
// data: null
// }

const manyExceptDot = many (anyCharExcept (char ('.')))
manyExceptDot.run('This is a sentence.')
// -> {
// isError: false,
// result: ['T', 'h', 'i', 's', ' ', 'i', 's', ' ', 'a', ' ', 's', 'e', 'n', 't', 'e', 'n', 'c', 'e'],
// index: 18,
// data: null
// }
`

#### possibly

possibly takes an _attempt_ parser and returns a new parser which tries to match using the _attempt_ parser. If it is unsuccessful, it returns a null value and does not "consume" any input.

Example

`JavaScript
const newParser = sequenceOf ([
possibly (str ('Not Here')),
str ('Yep I am here')
]);

newParser.run('Yep I am here')
// -> {
// isError: false,
// result: [ null, "Yep I am here" ],
// index: 13,
// data: null
// }
`

#### startOfInput

startOfInput is a parser that only succeeds when the parser is at the beginning of the input.

Example

`JavaScript
const mustBeginWithHeading = sequenceOf([
startOfInput,
str("# ")
]);
const newParser = between(mustBeginWithHeading)(endOfInput)(everyCharUntil(endOfInput));

newParser.run('# Heading');
// -> {
// isError: false,
// result: "# Heading",
// index: 9,
// data: null
// }

newParser.run(' # Heading');
// -> {
// isError: true,
// error: "ParseError (position 0): Expecting string '# ', got ' #...'",
// index: 0,
// data: null
// }
`

#### endOfInput

endOfInput is a parser that only succeeds when there is no more input to be parsed.

Example

`JavaScript
const newParser = sequenceOf ([
str ('abc'),
endOfInput
]);

newParser.run('abc')
// -> {
// isError: false,
// result: [ "abc", null ],
// index: 3,
// data: null
// }

newParser.run('')
// -> {
// isError: true,
// error: "ParseError (position 0): Expecting string 'abc', but got end of input.",
// index: 0,
// data: null
// }
`

#### skip

skip takes a _skip_ parser and returns a new parser which matches using the _skip_ parser, but doesn't return its value, but instead the value of whatever came before it.

Example

`JavaScript
const newParser = pipeParsers ([
str ('abc'),
str('123'),
skip (str ('def'))
])

newParser.run('abc123def')
// -> {
// isError: false,
// result: "123",
// index: 9,
// data: null
// }
`

#### pipeParsers

pipeParsers takes an array of parsers and composes them left to right, so each parsers return value is passed into the next one in the chain. The result is a new parser that, when run, yields the result of the final parser in the chain.

Example

`JavaScript
const newParser = pipeParsers ([
str ('hello'),
char (' '),
str ('world')
]);

newParser.run('hello world')
// -> {
// isError: false,
// result: "world",
// index: 11,
// data: null
// }
`

#### composeParsers

composeParsers takes an array of parsers and composes them right to left, so each parsers return value is passed into the next one in the chain. The result is a new parser that, when run, yields the result of the final parser in the chain.

Example

`JavaScript
const newParser = composeParsers ([
str ('world'),
char (' '),
str ('hello')
]);

newParser.run('hello world')
// -> {
// isError: false,
// result: "world",
// index: 11,
// data: null
// }
`

#### takeRight

takeRight takes two parsers, _left_ and _right_, and returns a new parser that first matches the _left_, then the _right_, and keeps the value matched by the _right_.

Example

`JavaScript
const newParser = takeRight (str ('hello ')) (str ('world'))

newParser.run('hello world')
// -> {
// isError: false,
// result: "world",
// index: 11,
// data: null
// }
`

#### takeLeft

takeLeft takes two parsers, _left_ and _right_, and returns a new parser that first matches the _left_, then the _right_, and keeps the value matched by the _left_.

Example

`JavaScript
const newParser = takeLeft (str ('hello ')) (str ('world'))

newParser.run('hello world')
// -> {
// isError: false,
// result: "hello",
// index: 11,
// data: null
// }
`

#### recursiveParser

recursiveParser takes a function that returns a parser (a thunk), and returns that same parser. This is needed in order to create _recursive parsers_ because JavaScript is not a "lazy" language.

In the following example both the value parser and the matchArray parser are defined in terms of each other, so one must be one must be defined using recursiveParser.

Example

`JavaScript
const value = recursiveParser (() => choice ([
matchNum,
matchStr,
matchArray
]));

const betweenSquareBrackets = between (char ('[')) (char (']'));
const commaSeparated = sepBy (char (','));
const spaceSeparated = sepBy (char (' '));

const matchNum = digits;
const matchStr = letters;
const matchArray = betweenSquareBrackets (commaSeparated (value));

spaceSeparated(value).run('abc 123 [42,somethingelse] 45')
// -> {
// isError: false,
// result: [ "abc", "123", [ "42", "somethingelse" ], "45" ],
// index: 29,
// data: null
// }
`

#### tapParser

tapParser takes a function and returns a parser that does nothing and consumes no input, but runs the provided function on the last parsed value. This is intended as a debugging tool to see the state of parsing at any point in a sequential operation like sequenceOf or pipeParsers.

Example

`JavaScript
const newParser = sequenceOf ([
letters,
tapParser(console.log),
char (' '),
letters
]);

newParser.run('hello world')
// -> [console.log]: Object {isError: false, error: null, target: "hello world", data: null, index: 5, …}
// -> {
// isError: false,
// result: [ "hello", "hello", " ", "world" ],
// index: 11,
// data: null
// }
`

#### decide

decide takes a function that recieves the last matched value and returns a new parser. It's important that the function always returns a parser. If a valid one cannot be selected, you can always use fail.

decide allows an author to create a context-sensitive grammar.

Example

`JavaScript
const newParser = sequenceOf ([
takeLeft (letters) (char (' ')),
decide (v => {
switch (v) {
case 'asLetters': return letters;
case 'asDigits': return digits;
default: return fail(Unrecognised signifier '${v}');
}
})
]);

newParser.run('asDigits 1234')
// -> {
// isError: false,
// result: [ "asDigits", "1234" ],
// index: 13,
// data: null
// }

newParser.run('asLetters hello')
// -> {
// isError: false,
// result: [ "asLetters", "hello" ],
// index: 15,
// data: null
// }

newParser.run('asPineapple wayoh')
// -> {
// isError: true,
// error: "Unrecognised signifier 'asPineapple'",
// index: 12,
// data: null
// }
`

#### mapTo

mapTo takes a function and returns a parser does not consume input, but instead runs the provided function on the last matched value, and set that as the new last matched value. This function can be used to apply structure or transform the values as they are being parsed.

Example

`JavaScript
const newParser = pipeParsers([
letters,
mapTo(x => {
return {
matchType: 'string',
value: x
}
})
]);

newParser.run('hello world')
// -> {
// isError: false,
// result: {
// matchType: "string",
// value: "hello"
// },
// index: 5,
// data: null
// }
`

#### errorMapTo

errorMapTo is like mapTo but it transforms the error value. The function passed to errorMapTo gets the _current error message_ as its first argument and the _index_ that parsing stopped at as the second.

Example

`JavaScript
const newParser = pipeParsers([
letters,
errorMapTo((message, index) => Old message was: [${message}] @ index ${index})
]);

newParser.run('1234')
// -> {
// isError: true,
// error: "Old message was: [ParseError (position 0): Expecting letters] @ index 0",
// index: 0,
// data: null
// }
`

#### fail

fail takes an _error message_ string and returns a parser that always fails with the provided _error message_.

Example

`JavaScript
fail('Nope').run('hello world')
// -> {
// isError: true,
// error: "Nope",
// index: 0,
// data: null
// }
`

#### succeedWith

succeedWith takes an value and returns a parser that always matches that value and does not consume any input.

Example

`JavaScript
succeedWith ('anything').run('hello world')
// -> {
// isError: false,
// result: "anything",
// data: null
// index: 0,
// }
`

#### either

either takes a parser and returns a parser that will always succeed, but the captured value will be an Either, indicating success or failure.

Example

`JavaScript
either(fail('nope!')).run('hello world')
// -> {
// isError: false,
// result: {
// isError: true,
// value: "nope!"
// },
// index: 0,
// data: null
// }
`

#### toPromise

toPromise converts a ParserResult (what is returned from .run) into a Promise.

Example

`JavaScript
const parser = str('hello');

toPromise(parser.run('hello world'))
.then(console.log)
.catch(({error, index, data}) => {
console.log(error);
console.log(index);
console.log(data);
});
// -> [console.log] hello

toPromise(parser.run('goodbye world'))
.then(console.log)
.catch(({error, index, data}) => {
console.log('Error!');
console.log(error);
console.log(index);
console.log(data);
});
// -> [console.log] Error!
// -> [console.log] ParseError (position 0): Expecting string 'hello', got 'goodb...'
// -> [console.log] 0
// -> [console.log] null
`

#### toValue

toValue converts a ParserResult (what is returned from .run) into a regular value, and throws an error if the result contained one.

Example

`JavaScript
const result = str ('hello').run('hello worbackgroiund

try {
const value = toValue(result);
console.log(value);
// -> 'hello'
} catch (parseError) {
console.error(parseError.message)
}
`

#### parse

parse takes a parser and input (which may be a string, TypedArray, ArrayBuffer, or DataView), and returns the result of parsing the input using the parser.

Example

`JavaScript
parse (str ('hello')) ('hello')
// -> {
// isError: false,
// result: "hello",
// index: 5,
// data: null
// }
`

A note on recursive grammars

If you're parsing a programming language, a configuration, or anything of sufficient complexity, it's likely that you'll need to define some parsers in terms of each other. You might want to do something like:

`JavaScript
const value = choice ([
matchNum,
matchStr,
matchArray
]);

const betweenSquareBrackets = between (char ('[')) (char (']'));
const commaSeparated = sepBy (char (','));

const matchNum = digits;
const matchStr = letters;
const matchArray = betweenSquareBrackets (commaSeparated (value));
`

In this example, we are trying to define value in terms of matchArray, and matchArray in terms of value. This is problematic in a language like JavaScript because it is what's known as an "eager language". Because the definition of value is a function call to choice, the arguments of choice must be fully evaluated, and of course none of them are yet. If we just move the definition below matchNum, matchStr, and matchArray, we'll have the same problem with value not being defined before matchArray wants to use it.

We can get around JavaScript's eagerness by using recursiveParser, which takes a function that returns a parser:

`JavaScript
const value = recursiveParser(() => choice ([
matchNum,
matchStr,
matchArray
]));

const betweenSquareBrackets = between (char ('[')) (char (']'));
const commaSeparated = sepBy (char (','));

const matchNum = digits;
const matchStr = letters;
const matchArray = betweenSquareBrackets (commaSeparated (value));
`

Fantasy Land

This library implements the following Fantasy Land (v3) interfaces:

- Functor
- Apply
- Applicative
- Chain

Every parser, or parser made from composing parsers has a .of, .map, .chain, and .ap method.

$3

#### of

`JavaScript
Parser.of(42)

// is equivalent to

succeedWith (42)
`

#### map

`JavaScript
letters.map (fn)

// is equivalent to

pipeParsers ([ letters, mapTo (fn) ])
`

#### chain

`JavaScript
letters.chain (x => someOtherParser)

// is equivalent to

pipeParsers ([ letters, decide (x => someOtherParser) ])
`

#### ap

`JavaScript
letters.ap (Parser.of (fn))

// is equivalent to

pipeParsers ([
sequenceOf ([ succeedWith (fn), letters ]),
mapTo (([fn, x]) => fn(x))
]);
``

Name

The name is also derived from parsec, which in astronomical terms is an ["astronomical unit [that] subtends an angle of one arcsecond"](https://en.wikipedia.org/wiki/Parsec).