Printing to Standard Out

The Flix Prelude defines the println function which prints to standard out. For example:

println("Hello World")

The println function can print any value whose type implements the ToString trait and consequently can be converted to a String. For example:

let o = Some(123);
let l = 1 :: 2 :: 3 :: Nil;
println(o);
println(l)

The println function is rightfully effectful, hence it cannot be called from a pure function. To debug a pure function, use the builtin debugging facilities.

The Console Effect

The Console effect defines operations for reading from and writing to the terminal:

eff Console {
    def readln(): Option[String]
    def write(s: String): Unit
    def writeLine(s: String): Unit
    def writeErr(s: String): Unit
    def writeErrLine(s: String): Unit
}

The operations work on plain Strings. The Console effect is the companion of the Sys.Console module, which defines four printing functions on top of the operations:

mod Sys.Console {
    def print(x: a): Unit \ Console with ToString[a]
    def println(x: a): Unit \ Console with ToString[a]
    def eprint(x: a): Unit \ Console with ToString[a]
    def eprintln(x: a): Unit \ Console with ToString[a]
}

Like the println of the Prelude, these accept any value with a ToString instance, but they have the Console effect rather than IO. The eprint and eprintln functions write to standard error. For example:

use Sys.Console

def main(): Unit \ Console =
    let o = Some(123);
    let l = 1 :: 2 :: 3 :: Nil;
    Console.println(o);
    Console.println(l);
    Console.eprintln("Printed two values.")

See the Console chapter for reading input and for prompts.