Sum Product extend example

extend.js

const {Tuple} = require('fantasy-tuples')
const Either = require('data.either') // missing extend, but could pr

Tuple(current_user, age)
.map(age => age + 1)
.extend(({_1: user, _2: age}) =>
  Object.assign({}, user, {age}))
// Tuple(current_user, current_user_with_age)

// ^ notice this only ever changes the _2, but you can see _1 while you work.


Either.of([1,2,3])
.map(arr => arr.slice(1))
.extend(either =>
  either.concat(Either.fromNullable([4])))
// Right([2,3,4])

// ^ notice this only ever changes the Right, but you get the whole Right.

// Untested, but ya know...