daggy

Daggy

Library for creating tagged constructors a.k.a. "disjoint union types" or "sum types".

Motivation

JavaScript does not have first class support for sum types, but they can be imitated in a handful of different ways. Nevertheless, this imitation leads to excess boilerplate that can lead to extra work and the potential for errors:

const successCase = { success: true, items: [1, 2, 3] }
const failureCase = { success: false, error: 'There was a problem.' }

function handleResult(result) {
  if (result.success) {
    console.log(result.items)
  } else {
    console.error(result.error)
  }
}

Daggy reduces the boilerplate needed to represent sum types in JavaScript:

const Result = daggy.taggedSum('Result', {
  Success: ['items'],
  Failure: ['error']
})

const successCase = Result.Success([1, 2, 3])
const failureCase = Result.Failure('There was a problem.')

function handleResult(result) {
  result.cata({
    Success: message => console.log(message),
    Failure: error => console.error(error)
  })
}

API

daggy.tagged(typeName, fields)

Creates a new constructor with the given field names

const Point3D = daggy.tagged('Point3D', ['x', 'y', 'z'])
Point3D.toString() // 'Point3D'
const a = Point3D(1, 2, 3) // { x: 1, y: 2, z: 3 }
a.x == 1 && a.y == 2 && a.z == 3 // true
a.toString() // 'Point3D(1, 2, 3)'
Point3D.is(a) // true
Point3D.prototype.scale = function(n){
  return Point3D(this.x * n, this.y * n, this.z * n)
}
const b = a.scale(2) // { x: 2, y: 4, z: 6 }
b.toString() // 'Point3D(2, 4, 6)'
const c = Point3D.from({y: 2, x: 1, z: 3}) // { x: 1, y: 2, z: 3 }

daggy.taggedSum(typeName, constructors)

Returns Type Representative containing constructors of for each key in constructors as a property. Allows {TypeRep}.is and {TypeRep}.{Tag}.is checks for values created by constructors.

const Option = daggy.taggedSum('Option', {
  Some: ['x'],
  None: [],
})
const a = Option.Some(1) // { x: 1 }
a.toString() // 'Option.Some(1)'
Option.Some.is(a) // true
Option.is(a) // true
Option.None.is(Option.None) // true
Option.is(Option.None) // true
Option.None.toString() // 'Option.None'
Option.Some.toString() // 'Option.Some'
Option.prototype.map = function (f) {
  return this.cata({
    Some: (x) => Option.Some(f(x)),
    None: () => this,
  })
}
const b = a.map(x => x+1) // { x: 2 }
b.toString() // 'Option.Some(2)'
const c = Option.Some.from({x: 1}) // { x: 1 }