Kotlin simple terminal CLI

Cli.kt

import java.util.*

inline fun <reified T> prompt(text: String): T {
  return prompt(text, null)
}

inline fun <reified T> prompt(text: String, default: T?): T {
  return when (T::class) {
    String::class -> promptString(text, default as String) as T
    Int::class -> promptInt(text, default as Int) as T
    Boolean::class -> promptBoolean(text, default as Boolean) as T
    else -> throw Exception("Invalid type")
  }
}

fun promptString(text: String, default: String?): String {
  val question = if (default == null) text else "$text ($default)"
  print(question)
  val result = readLine()
  if (default == null && (result == null || result.isNotEmpty())) {
    println("Please type your answer")
    return promptString(text, default)
  }
  return result ?: default!!
}

fun promptInt(text: String, default: Int?): Int {
  val question = if (default == null) text else "$text ($default)"
  print(question)
  val result = readLine()
  if (default == null && (result == null || result.toIntOrNull() == null)) {
    println("Please type a valid integer number")
    return promptInt(text, default)
  }
  return result?.toInt() ?: default!!
}

fun parseBoolean(text: String?): Boolean? {
  return if (text == null)
    null
  else
    when (text.lowercase(Locale.getDefault())) {
      "y" -> true
      "true" -> true
      "n" -> false
      "false" -> false
      else -> null
    }
}

fun booleanToString(bool: Boolean?): String? {
  return if (bool == null)
    null
  else
    when (bool) {
      true -> "Y"
      false -> "N"
    }
}

fun promptBoolean(text: String, default: Boolean?): Boolean {
  val stringDefaultValue = booleanToString(default)
  val question = if (stringDefaultValue == null) text else "$text ($stringDefaultValue)"
  print(question)
  val result = readLine()
  val parsed = parseBoolean(result)
  if (default == null && parsed == null) {
    println("Please type a valid option (Y/N)")
    return promptBoolean(text, default)
  }
  return (parsed ?: default) as Boolean
}

object Cli {
  val eraseDatabase: Boolean by lazy {
    prompt("Erase database?", false)
  }

  val fillDatabase: Boolean by lazy {
    prompt("Fill database?", false)
  }

  val eventStream: Boolean by lazy {
    prompt("Enable event stream?", false)
  }
}

You can use it like:

if (Cli.fillDatabase) doSomething()

To create your custom param, just add a new value to the Cli object:

object Cli {
   ...
   
   val myProp: String by lazy {
     prompt("What is your name?", "Rodrigo")
   }
}