Sweet.js Not Outputing Anything

I have a very simple sweet.js file I'm trying to compile:

macro @ {
    rule {
        $exp
    }=>{
        + $exp +
    }
}

...using this command:

sjs -o out.js my_file.js

But nothing is getting output; the out.js file is created, but it doesn't contain anything. At first I thought it may be any issue with the sweet.js file itself, but when I test it using the sweet.js online editor, it works as expected.

Am I not using the 'sjs' command properly? Is there something else I'm overlooking?

I was able to compile after adding some lines of code outside of the macro definition:

macro @ {
rule {
    $exp
}=>{
    + $exp +
   }
}

var msg = "Hello";
console.log("message is: " @msg ".");

I misunderstood the documentation I've been reading, and thought just the macro code itself could be compiled into a JavaScript file, but the compiler spits out a JavaScript file based on the defined macro and the code that utilizes the defined macro.