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(): String
    def print(s: String): Unit
    def eprint(s: String): Unit
    def println(s: String): Unit
    def eprintln(s: String): Unit
}