Programming Flix

The Main Function

The entry point of any Flix program is the main function which must have the signature:

def main(): Unit \ IO

That is, the main function:

1. must return Unit, and
2. must be marked as effectful (i.e. have the effect annotation \ IO).

The signature of main does not specify any arguments, but the command line arguments passed to the program can be accessed by calling Environment.getArgs():

def main(): Unit \ IO =
    let args = Environment.getArgs();
    ...

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