gistfile1.js

// perform a simple mechanical transform from this:

{ req, res } = yield app.get(/^\/([0-9a-f]+)$/);
object_id = new bson.ObjectID(req.params[0]);
{ err, db } = yield db.open();
if(err) { return res.send(500); }
store = new mongo.GridStore(db, object_id, "r");
{ err, store } = yield store.open();
if(err || !store.length) { return res.send(404); }
{ err, buf } = yield store.readBuffer(store.length);
res.send(buf, { "Content-Type": store.contentType, "Content-Length": store.length, }, 200);

// into this:

app.get(/^\/([0-9a-f]+)$/, function(req, res) {
	object_id = new bson.ObjectID(req.params[0]);
	db.open(function(err, db) {
		if(err) { return res.send(500); }
		store = new mongo.GridStore(db, object_id, "r");
		store.open(function(err, db) {
			if(err || !store.length) { return res.send(404); }
			store.readBuffer(store.length, function(err, buf) {
				res.send(buf, { "Content-Type": store.contentType, "Content-Length": store.length, }, 200);
			});
		});
	});
});

// It needs two things; a kind of destructuring assign (converted into callback parameters), and yield (convert the remainder of the function into a callback).
// i.e. a CPS kind of a transform