sweet.js: async macro

do.js

let do = macro {
	rule { $vars (,) ... = $fname ... ($params ...); $rest ...} => {
		$fname ... ($params ..., function ($vars (,) ...) { $rest ... })
	}
}

var buffer = new Buffer(1024);

do err, infile = fs.open("/tmp/in.txt");
do err, outfile = fs.open("/tmp/out.txt");
do err, bytesRead, buffer = fs.read(infile, buffer, 0, 1024, 0);
do err, bytesWritten, buffer = fs.write(outfile, buffer, 0, 1024, 0);
console.log("We read " + bytesRead + " bytes and wrote " + bytesWritten + " bytes.");

So, welcome to the unhandled contextless exceptions hell?

Named functions are your bestest friends:

fs.open('/tmp/in.txt', next1);

function next1 (err$190, infile$191) {
    if (err$190)
        throw err$190;
    fs.open('/tmp/out.txt', next2);
}

function next2 (err$193, outfile$194) {
    if (err$193)
        throw err$193;
    fs.read(infile$191, buffer$188, 0, 1024, 0, next3);
}

function next3 (err$196, bytesRead$197, buffer$188) {
    if (err$196)
        throw err$196;
    fs.write(outfile$194, buffer$188, 0, 1024, 0, next4);
}

function next4 (err$200, bytesWritten$201, buffer$188) {
    if (err$200)
        throw err$200;
    console.log('We read ' + bytesRead$197 + ' bytes and wrote ' + bytesWritten$201 + ' bytes.');
}

Then you don't need a library, macros, or any other stuff that screws with stack traces.