Signature of main()

[BruceEckel]

In https://doc.flix.dev/main.html, it says:

The entry point of any Flix program is the main function which must have the signature:
def main(): Unit \ IO

However, to get the following to run I had to add NonDet, so I wonder if there are other effects that can be added to the signature of main().

mod Time {
    pub def ofDay(): Int64 \ Clock =
        Clock.currentTime(TimeUnit.Milliseconds)
}

mod Rand {
    pub def number(): Int64 \ Random =
        Random.randomInt64()
} 

def add(x: a, y: a): a with Add[a] = x + y
def divide(x: a, y: a): a with Div[a] = x / y

def g(a: Int64, b: Int64): Int64 \ {Clock, Random} =
  let x = add(a, Time.ofDay());
  let y = divide(b, Rand.number());
  add(x, y)

def main(): Unit \ {IO, NonDet} =
    run {
        println(g(11i64, 42i64))
    } with Clock.runWithIO with Random.runWithIO

[magnus-madsen]

The signature of main -- frustratingly -- has changed many times.

But I think we are finally converging on something like:

• Main must take zero arguments and return Unit.
• Main can have any combination of the primitive effects: IO, NonDet, Chan.
• Main can have any effect for which there is a default effect handler.

To me, the annoying part is the strangeness around NonDet and Chan.

These are primitive effects (i.e. cannot be handled). I find them ugly. We could merge them into IO but then relative harmless concurrency would suddenly have the dangerous IO effect. I don't really yet have a good answer.

But ignoring NonDet and Chan, I think the above three points are here to stay and is the right way to think of main. It is a function that takes zero arguments, returns Unit, and can have an IO effect and any effect that has a default handler.

This is what the documentation should say.

[magnus-madsen]

Access to program arguments - an indirect input - is provided via the Env effect.
Access to exit the program -- a form of system effect - is provided via the Exit effect.

Both have default handlers, I believe.

[magnus-madsen]

This works:

def main(): Unit \ {Env, Exit} = 
    if (List.isEmpty(Env.getArgs()))
        Exit.exit(0)
    else 
        Exit.exit(1)

[magnus-madsen]

I forgot to talk about your example. The challenge / question / problem is that Clock and Random are pretty harmless effects. We cannot treat them as pure, so we have to give them some effect. Giving the IO would work, but since IO encompasses everything (e.g. deleting files) that seems overly harsh. So we introduced the NonDet effect. But now we have both NonDet and IO which seems to make for an unpleasant experience.

I should also say that NonDet arises when working with channels (e.g. because you can spawn two threads and have them race on whether to send 1 or 2 to a channel.)

[BruceEckel]

I'm OK with NonDet; the problem was that the documentation for main() does not mention it as a possibility. It says that the only thing you can use is IO. *Bruce Eckel* ThinkingInTypes.com MindViewLLC.com <http://mindviewllc.com/> EffectOrientedProgramming.com HappyPathProgramming.com Blog: BruceEckel.com EvolveWork.co WinterTechForum.com <http://www.WinterTechForum.com> SummerTechForum.com <http://summertechforum.com/> OnJava8.com <http://www.OnJava8.com> www.AtomicKotlin.com

…

On Sun, Feb 1, 2026 at 4:16 AM Magnus Madsen ***@***.***> wrote: *magnus-madsen* left a comment (flix/book#285) <#285 (comment)> I forgot to talk about your example. The challenge / question / problem is that Clock and Random are pretty harmless effects. We cannot treat them as pure, so we have to give them some effect. Giving the IO would work, but since IO encompasses everything (e.g. deleting files) that seems overly harsh. So we introduced the NonDet effect. But now we have both NonDet and IO which seems to make for an unpleasant experience. I should also say that NonDet arises when working with channels (e.g. because you can spawn two threads and have them race on whether to send 1 or 2 to a channel.) — Reply to this email directly, view it on GitHub <#285 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAHUTLBXJPUPREYZH4LCRS34JXOAPAVCNFSM6AAAAACTRMZZQGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQMZQHA4TQOBXGM> . You are receiving this because you authored the thread.Message ID: ***@***.***>