traverse.ts

export function traverse<T, U>(obj: Dictionary<T>, f: (t: T) => Array<U>) : Array<Dictionary<U>> {
  return sequence(
    mapValues(obj, t => {
      return f(t)
    })
  )
}

export function sequence<T>(obj: Dictionary<Array<T>>) : Array<Dictionary<T>> {
  function loop<T>(acc: Array<Dictionary<T>>, obj: Dictionary<Array<T>>) : Array<Dictionary<T>> {
    return traverseHead(obj)
      .map(head => loop(acc.concat(head), traverseTail(obj)))
      .orElse(acc) 
  }

  return loop([], obj) 
}

export function traverseHead<T>(obj: Dictionary<Array<T>>) : Maybe<Dictionary<T>> {
  const res: Dictionary<T> = {}

  forEach(obj, (val: T[], key: string) => {
    maybe(val[0]).forEach(val => {
      res[key] = val
    })
  })

  if (Object.keys(res).length > 0) {
    return maybe(res)
  } else {
    return maybe<Dictionary<T>>(null)
  }
}

export function traverseTail<T>(obj: Dictionary<Array<T>>) : Dictionary<Array<T>> {
  return mapValues(obj, (val: T[]) => {
    return val.slice(1)
  })
}