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function setArity (arity, fn) {
  if (typeof setArity.cache === 'undefined') {
    setArity.cache = {}
  }
  if (typeof setArity.cache[arity] === 'undefined') {
    setArity.cache[arity] = (fn) => {
      switch (arity) {
        case 0: return function () { return fn.apply(this, arguments) }
        case 1: return function (a) { return fn.apply(this, arguments) }
        case 2: return function (a, b) { return fn.apply(this, arguments) }

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// this file is just so I can reference the idea behind placeholders - if you want a maintained version, check out:
//   - https://github.com/thisables/curry
//   - http://ramdajs.com

const __ = Symbol

const ARITIES = {}

const setArity = (arity, fn) => {
  if (!ARITIES[arity]) {

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const users = {
  1: { name: 'bob' },
  2: { name: 'sally' }
}

const posts = [
  { author: 'sally', title: 'Hello World' },
  { author: 'bob', title: 'How to do stuff' }
]

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function sequentialCallback (...fns) {
  return (...args) => {
    const done = args.pop()
    const next = (error, ...args) => {
      if (error) return done(error)
      if (fns.length) {
        const fn = fns.shift()
        return fn(...args, next)
      }
      return done(null, ...args)

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const validateTransformation = sequentialCallback(
  transformSomething,
  validateTransformed,
  (validated, callback) => callback(null, validated, 'data is valid:')
)

validateTransformation({ foo: 'bar' }, (error, validated, label) => {
  if (error) {
    console.error(new Error(error))
  } else {

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const validateTransformation = sequentialCallback(
  transformSomething,
  validateSomething
)

const something = { foo: 'bar' }

validateTransformation(something, (error, validated) => {
  if (error) {
    console.error(new Error(error)) 

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const getValidatedTransformation = sequentialCallback(
  findSomething,
  transformSomething,
  validateTransformed
)

getValidatedTransformation((error, validated) => {
  if (error) {
    console.error(new Error(error))
  } else {

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findSomething()
  .then(transformSomething)
  .then(validateTransformed)
  .then((validated) => console.log('data is valid:', validated))
  .catch((error) => console.error(new Error(error))

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findSomething()
  .then((something) => transformSomething(something))
  .then((transformed) => validateTransformed(transformed))
  .then((validated) => console.log('data is valid:', validated)
  .catch((error) => console.error(new Error(error))

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findSomething((error, something) => {
  if (error) {
    throw new Error(error) 
  }
  transformSomething(something, (error, transformed) => {
    if (error) {
      throw new Error(error) 
    }
    validateTransformed(transformed, (error, validated) => {
      if (error) {