parser17.js

const always = value => 
  new Parser(stream => new Success(value, stream))

const never = value => 
  new Parser(stream => new Failure(value, stream))

const append = (p1, p2) => 
  p1.chain(vs => p2.map(v => vs.concat([v])))

const concat = (p1, p2) =>
  p1.chain(xs => p2.map(ys => xs.concat(ys)))

const sequence = list =>
  list.reduce((acc, parser) => append(acc, parser), always([]))

sequence([char('a'), char('b'), char('c')])
.run('abc')
.fold(
  v => console.log('success', v)
  e => console.log('error', e)
)
// => success ['a', 'b', 'c']