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ParTSing is a decoder combinator library for TypeScript. You can use it to build parsers/decoders from text, tokens or really any input value.

If you want full control over what to decode your should start from partsing/core/decoder. It provides 3 types parameters

  • In for the input. Remember that you will need to decode one portion of your input at the time and your input probably needs to track somehow the current position.
  • Out it's the type of the value if successfully decoded.
  • Err it's the type of the failure returned when the parser fails.

The library provides two additional set of utility functions to decode string values (partsing/text) and native JS values (partsing/value).

Decoder Error

If you adopt the generic Decoder directly, you can define the shape of your error. The library provides a DecoderError type that should fit most decoding needs. The type of errors available are defined in partsing/error.

DecoderError provides a simple method to debug the result of the decoding (toString) but still leaves the flexibility to give granular control on the representation of the error.

Both [Text Decoding] and [Value Decoding] use DecodeError.

Text Decoding

To be able to keep track of the position of the decoding within a string, Text Decoding uses TextInput to track both the entire text input and the current index.

The decodeText function simplifies the inputs and outputs of decoding text. It takes a text decoder (Decoder<TextInput, T, DecoderError>) and return a function that takes a string input and returns a DecodeResult<string, T, string> (where input and error are of type string).

If you intend to write your own regular expressions decoder functions, consider using the y (sticky) flag. When used, there is no need to reallocate slices of the input string saving memory and CPU cycles. The sticky flag is not available for all implementations of JS.

Value Decoding

To be able to keep track of the position of the decoding within any value, Value Decoding uses ValueInput to track both the input value and the current position within it using a path. path is an array of either string (object field name) or number (array/tuple index position).

The decodeValue function simplifies the inputs and outputs of decoding values. It takes a value decoder (Decoder<ValueInput, T, DecoderError>) and return a function that takes any and returns a DecodeResult<any, T, string> (where input is of type any and error is of type string).


A simple decoder combinator to parse color values from strings into class instances.

class RGB {
  constructor(readonly rgb: number) {}
  toString() {
    let s = this.rgb.toString(16)
    while (s.length < 6) s = `0${s}`
    return `#${s}`
class Grey {
  constructor(readonly value: number) {}
  toString() {
    return `grey ${this.value}`
class HSL {
  constructor(readonly hue: number, readonly saturation: number, readonly lightness: number) {}
  toString() {
    return `hsl(${this.hue},${this.saturation},${this.lightness})`
type Color = RGB | Grey | HSL
// Hue in HSL is generally measured as an angle, not a ratio
const ratioDecoder = regexp(/0[.]\d+/y).map(Number)
const rgbDecoder = regexp(/[#]([0-9a-f]{6})/iy, 1)
  .map(v => parseInt(v, 16))
  .map(v => new RGB(v))
const greyDecoder = matchInsensitive('grey')
  .map(v => new Grey(v))
const hslDecoder = ratioDecoder
  .repeatWithSeparator(3, match(','))
  .map(v => new HSL(v[0], v[1], v[2]))
  .surroundedBy(matchInsensitive('hsl('), match(')'))
const colorTextDecoder = decodeText(
  // the `eoi` at the end, makes sure that there is nothing left to decode
  oneOf(rgbDecoder, greyDecoder, hslDecoder).skipNext(eoi)
// all results are wrapped in a DecodeSuccess
// colorTextDecoder('#003355')          == new RGB(0x003355)
// colorTextDecoder('gray 0.3')         == new Grey(0.3)
// colorTextDecoder('gray0.2')          == new Grey(0.2)
// colorTextDecoder('HSL(0.1,0.2,0.3)') == new HSL(0.1,0.2,0.3)

Another scenario where decoding comes in handy is to validate, type and transform payloads from JSON requests. You can decode a value (after being parsed by JSON.parse) into one of the Color types described above.

A few examples of valid JSON payloads:

{ "grey": 0.5 }
{ "kind": "hsl", "h": 0.2, "s": 0.5, "l": 0.8 }

Here is a colorValueDecoder definition that can deal with those cases:

const ratioValue = numberValue.test(v => v >= 0 && v <= 1, DecodeError.expectedWithinRange('0', '1'))
// reuse the rgbDecoder defined above to validate and trasform the string value into an RGB instance
// example: "#003366"
const rgbValue = stringValue.sub(rgbDecoder, input => ({ input, index: 0 }), v => v)
// example: { "grey": 0.5 }
const greyValue = objectValue(
    { grey: ratioValue },
    [] // the empty array means that no fields are optional
  ).map(v => new Grey(v.grey))
// example: { "kind": "hsl", "h": 0.2, "s": 0.5, "l": 0.8 }
const hslValue = objectValue(
      kind: literalValue('hsl'),
      h: ratioValue,
      s: ratioValue,
      l: ratioValue
  ).map(v => new HSL(v.h, v.s, v.l))
const colorValueDecoder = decodeValue(


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