Console

Flix provides Console as a library effect for terminal I/O. The Console effect has a default handler, so no explicit runWithIO call is needed in main. The key module is Sys.Console.

The Console Effect

The Console effect supports reading from standard input and writing to standard output and standard error:

pub eff Console {
    /// Reads a single line from the console.
    def readln(): String

    /// Prints the given string `s` to the standard out.
    def print(s: String): Unit

    /// Prints the given string `s` to the standard err.
    def eprint(s: String): Unit

    /// Prints the given string `s` to the standard out followed by a new line.
    def println(s: String): Unit

    /// Prints the given string `s` to the standard err followed by a new line.
    def eprintln(s: String): Unit
}

The Console Module

The Console module provides several higher-level functions built on the Console effect:

mod Sys.Console {
    /// Prints prompt `p`, reads a line, and returns `default` if the input is empty.
    def readlnWithDefault(p: a, default: String): String \ Console

    /// Prints prompt `p`, reads a line, and applies `f` to the input.
    /// Re-prompts on `Err(msg)`, returns `v` on `Ok(v)`.
    def readlnWith(p: a, f: String -> Result[String, b]): b \ Console

    /// Prints prompt `p` with a yes/no hint and reads a boolean answer.
    /// Empty or unrecognized input returns `default`.
    def confirm(p: a, default: {default = Bool}): Bool \ Console

    /// Prints prompt `p` with a numbered list of choices and reads a selection.
    /// Returns `None` if the input is invalid.
    def pick(p: a, choices: List[b]): Option[b] \ Console

    /// Like `pick`, but re-prompts until the user makes a valid selection.
    def pickWith(p: a, choices: List[b]): b \ Console
}

Basic Console I/O

The simplest use of Console is to print a prompt, read input, and respond:

use Sys.Console

def main(): Unit \ Console =
    Console.print("What is your name? ");
    let name = Console.readln();
    Console.println("Hello ${name}!")

Confirmed Input

The Console.confirm function asks a yes/no question and returns a Bool. You can supply a default value that is used when the user presses Enter without typing anything:

use Sys.Console

def main(): Unit \ Console =
    let proceed = Console.confirm("Deploy to production?", default = true);
    if (proceed)
        Console.println("Deploying...")
    else
        Console.println("Aborted.")

Validated Input

The Console.readlnWith function repeatedly prompts the user until the input passes a validator. The validator returns Ok(value) on success or Err(message) to re-prompt:

use Sys.Console

def main(): Unit \ Console =
    let n = Console.readlnWith("Enter a number (1-10): ", s ->
        match Int32.fromString(s) {
            case Some(i) if i >= 1 and i <= 10 => Ok(i)
            case _ => Err("Please enter a number between 1 and 10.")
        }
    );
    Console.println("You entered: ${n}")