sequentialActions.js

import readline from "readline-sync"

function read () {
  return {
    action: "read",
    next: pure,
  }
}

function print (string) {
  return {
    action: "print",
    string,
    next: pure,
  }
}

// A pure program that simply returns a value.
function pure (value) {
  return {
    action: "pure",
    value,
  }
}

function interpret (program) {
  switch (program.action) {
    case "read":
      const input = readline.question("")
      return interpret(program.next(input))

    case "print":
      console.log(program.string)
      return interpret(program.next(null))

    case "pure":
      return program.value
  }
}

// Bind two programs together. Return a new program which is equivalent to
// running the first program and feed its return value to the second one.
function bind (program, next) {
  switch (program.action) {
    case "pure":
      return next(program.value)

    default:
      return {
        ...program,
        next: (val) => bind(program.next(val), next)
      }
  }
}

const number = 42;
const game =
  bind(print("Enter a guess"), _ => {
    return bind(read(), (input) => {
      const guess = Number(input)
      if (guess === number) {
        return print("Congratulations! You found it.")
      } else if (guess < number) {
        return bind(print("Higher"), _ => game)
      } else if (guess > number) {
        return bind(print("Lower"), _ => game)
      } else {
        return print("Just numbers, please.")
      }
    })
  })

// To execute the program, we would just have to call:
// interpret(game)

// For later
export default game