main and Exit Codes

main is a program’s entry point and comes in four forms, chosen by whether the program needs command-line arguments, a return value, or both: no return value, an exit code, command-line arguments, and the combination.

Form	Signature	Use
Simplest	`func main()`	Just runs; returns nothing, takes nothing
Returns an exit code	`func main() -> int`	Returns a result the caller can check
Takes arguments	`func main(args: string[])`	Reads command-line arguments
Combined	`func main(args: string[]) -> int`	Both at once

The entry function needs no visibility modifier; public changes nothing either way.

Exit Codes

In the -> int form, the integer returned by return becomes the process exit code: a program whose main runs return 7 exits with code 7. 0 means all is well; non-zero values carry whatever meaning you assign them. Exit codes let you check whether a program ended successfully without parsing its printed output.


$ mush run app.ms
$ echo $?
0

In PowerShell, read the exit code from $LASTEXITCODE; in bash, from $?.

Command-Line Arguments

Add an args: string[] parameter to main and the program receives command-line arguments. Running mush run args.ms a b c puts "a", "b", "c" in args, in order; args.ms itself is not included.

args.ms


func main(args: string[]) -> int
{
    if args.Length == 0
    {
        print("no args");
        return 1;
    }
    print($"hello, {args[0]}");
    return 0;
}


$ mush run args.ms ada
hello, ada
$ echo $?
0
$ mush run args.ms
no args
$ echo $?
1

The type of args, string[], is a string array: args.Length gives the count and args[0] indexes an element; the syntax unfolds across later pages. Declaring it as list<string> is equally valid — the two forms are equivalent, and the language passes the argument in the declared type: write string[] and you get an array, write list<string> and you get a list. Script entry points default to string[].

Notes

The exit code is an int; returning non-zero is not a compile error — it just hands information to the caller.
Without an explicit -> int, the program exits with 0.
args may be declared as string[] or list<string>; entry-point validation accepts only these two shapes and rejects any other parameter form at compile time.

Last updated on October 11, 2026