The Main Function

The entry point of any Flix program is the main function which must take zero arguments and return Unit:

def main(): Unit \ IO =
    println("Hello World!")

Effects of Main

The main function can use any combination of:

• Primitive effects: IO and NonDet.
• Any effect with a default handler: for example Env, Exit, Clock, Logger, and others.

Effects with default handlers are automatically translated into IO by the Flix compiler. See Default Handlers for details.

For example, main can use the Env and Exit effects:

use Sys.Env
use Sys.Exit

def main(): Unit \ {Env, Exit} =
    let args = Env.getArgs();
    match List.head(args) {
        case None    =>
            println("Missing argument.");
            Exit.exit(1)
        case Some(a) =>
            println("Hello ${a}!")
    }

Accessing Command Line Arguments

The command line arguments passed to the program can be accessed by calling Env.getArgs() through the Env effect:

use Sys.Env

def main(): Unit \ {Env, IO} =
    let args = Env.getArgs();
    println("Arguments: ${args}")

Exiting the Program

The program can be terminated with a specific exit code using Exit.exit:

use Sys.Exit

def main(): Unit \ Exit =
    Exit.exit(0)

Why Must Main Be Effectful?

Flix requires main to be effectful. If main was pure there would be no reason to run the program. Typically this requirement is satisfied because main prints to the console or has another side-effect.