Read command-line arguments

Your program often needs what the user typed after its name: a file to read, a port, a flag. main takes no argc and argv. The arguments are in process.args, from import std.process.

See it work

import std.io
import std.process

fn main() -> Int {
    if (process.args.count < 2) {
        println("usage: echo_args <word>...")
        return 2
    }
    for i in 1..<process.args.count {
        println(process.args[i])
    }
    return 0
}

process.args[0] is the program's name, so the arguments start at index 1. prismio run passes everything after -- to the program exactly as typed, including words that look like compiler flags:

prismio run echo_args.psm -- alpha "two words" --release

Built echo_args
alpha
two words
--release

A quoted argument stays one argument. Without --, prismio run reads the words as its own and stops at the first it does not know (P1050). In a project, prismio run -- alpha runs the project's only executable, and prismio run app -- alpha names the target.

What the program returns is what you get

prismio run exits with the status main returned and prints nothing about it. Run the example with no arguments:

prismio run echo_args.psm
echo $?

Built echo_args
usage: echo_args <word>...
2

A shell script or CI step can therefore branch on it. The same is true of a built executable: ./echo_args a b prints a and b.

Check the count before reading

process.args[i] answers an empty string for an index outside [0, process.args.count), the same as an argument that is empty, so ask count when the difference matters. std.process documents the rest of the surface, including environment variables and running other programs.

Argument zero can differ by platform, since it is whatever the operating system was given. Do not build an invariant on its exact spelling.