// Sometimes, you might want to supply *some* config, but
// not necessarily *all*. Maybe your password is really
// secret, so you don't want to risk saving it in a config
// file. In which case, you'll want to ask the user to enter
// it when your script runs.

// This interface provides a flexible approach, where
// the user is prompted for a missing key when it's
// requested - if a particular key isn't needed for a given
// run, the user won't need to enter it. Of course, if the
// particular key exists in the given config, the user isn't
// asked.

const Task = require('data.task')
const { ReaderT } = require('fantasy-readers')
const { traverse } = require('ramda')

const { stdin: input, stdout: output } = process

// Ask the user a question on the command line.
// question :: String -> Task e String
const question = text => new Task((_, res) => {
  const rl = require('readline')
             .createInterface({ input: process.stdin
                              , output: process.stdout
                              , terminal: false })

  return rl.question(
    `${ text }: `,
    data => { rl.close()
              res(data) })
})

// type ReaderTask u e a = ReaderT u (Task e a)
const ReaderTask = ReaderT(Task)

// Get a config key if it exists, or ask the user if not.
// getConfig :: (String, String)
//           -> ReaderTask StrMap e String
const getConfig = key =>
  ReaderTask.ask.chain(
    config => config[key] === undefined
              ? ReaderTask(_ => question(key))
              : ReaderTask.of(config[key]))

// Ask the user for any of the three keys that are missing,
// then return all the values to be used.
traverse(ReaderTask.of, getConfig,
  [ 'host', 'username', 'password' ])

// Do whatever fun thing with the values you need: connect
// to a database, call an API, whatever.
.map(([ host, user, pass ]) => ({ host, user, pass }))

// Pass in a potentially incomplete config array, whose
// missing values will be populated by user input.
.run({ 'host': 'localhost', 'username': 'tom' })

// None of the actions here can fail, so we'll just ignore
// that and log the result!
.fork(_ => undefined,
      x => console.log(x))

// $ node transformer.js
// password: my-password
// { host: 'localhost', user: 'tom', pass: 'my-password' }