Hello, World

Create a file named hello.ow:

main = (Stdout) -> Noop {
    "hello".print(Stdout)
}

Run it:

just run hello.ow

You should see:

hello

That's the whole program. Let's walk through it.

Line by Line

main = (Stdout) -> Noop {

main is the program's entry point. Unlike every other function in Oneway, main is not implemented on a type — it's a top-level binding.

The signature (Stdout) -> Noop says: this function takes one parameter whose type is Stdout, and returns a value of type Noop.

• Stdout is a capability. Real-world capabilities only exist in main, which receives them and threads them down to anything that needs to perform a side effect.
• Noop is a singleton type — a type with exactly one value, named after itself. Returning Noop is the language's way of saying "this function produces nothing useful".

    "hello".print(Stdout)
}

"hello" is sugar for String("hello"). The body of a function is a sequence of expressions separated by newlines; the last one is the return value.

"hello".print(Stdout) is a method call. The method is defined on String:

String.print = (Stdout) -> Noop {
    ...
}

print needs a Stdout capability because writing to standard output is a side effect. A function that does not receive Stdout cannot print, and the type signature is honest about that.

Try Breaking Things

Some small experiments to build intuition:

• Remove Stdout from main's parameters. The compiler will complain when print is called without it.
• Add a comment (// hi). The lexer rejects this — comments are not allowed.
• Return something other than Noop. The body's last expression must match the declared return type.