// Helper functions

const compose = (...fns) => x =>
  fns.reduceRight((y, f) => f(y), x)

const curry = (fn, n) => {
  const arity = n || fn.length
  return (...params) =>
    params.length >= arity
      ? fn(...params)
      : curry(
          (...rest) => fn(...params, ...rest),
          arity - params.length
        )
}

const log = curry((label, value) => {
  console.log(`[${label}]`, value)
  return value
})

const prop = curry((name, obj) =>
  obj[name]
)

// Array functions

const map = curry((mapper, list) =>
  list.map(mapper)
)

const filter = curry((predicate, list) =>
  list.filter(predicate)
)

const reduce = curry((reducer, initial, list) =>
  list.reduce(reducer, initial)
)

// Domain functions

const add = curry((x, y) =>
  x + y
)

const isAdmin = prop('admin')

const getAge = prop('age')

const sum = reduce(add, 0)

const sumAdminAges = compose(
  sum,
  log('Ages'),
  map(getAge),
  log('Admin users'),
  filter(isAdmin),
  log('List of users')
)

// execution

const users = [
  { admin: true, age: 30 },
  { admin: false, age: 45 },
  { admin: true, age: 40 },
  { admin: false, age: 80 },
]

sumAdminAges(users)

// [List of users] [ { admin: true, age: 30 },
//   { admin: false, age: 45 },
//   { admin: true, age: 40 },
//   { admin: false, age: 80 } ]
// [Admin users] [ { admin: true, age: 30 }, { admin: true, age: 40 } ]
// [Ages] [ 30, 40 ]
// 70