functional.js

const Right = x =>
  ({
    x,
    map: f => Right(f(x)),
    inspect: _ => `Right(${x})`,
    isLeft: _ => false,
    chain: f => f(x),
  })

const Left = x =>
  ({
    x,
    map: f => Left(x),
    inspect: _ => `Left(${x})`,
    isLeft: _ => true,
    chain: f => Left(x)
  })

const tryCatch = f => {
  try {
    return Right(f())
  } catch (e) {
    return Left('could not parse')
  }
}

const fromNullable = (error, a) =>
  (a === undefined || a === null) ? Left(error) : Right(a)

const first = xs => fromNullable('empty array', xs[0])

const parse = s => tryCatch(() => JSON.parse(s))

const upper = s => s.toUpperCase()

const shout = s => s.concat('!')

const format = s => Right(s).map(upper).map(shout)

const functional = s => Right(s).chain(parse).chain(first).chain(format)


functional('["hello", "world"]')
// Right(HELLO!)