Error: watching api/swagger/swagger.yaml: watch task has to be a function

i need help pls

var gulp = require("gulp");
var yaml = require("js-yaml");
var path = require("path");
var fs = require("fs");

//converts yaml to js
gulp.task("swagger", function() {
	var doc = yaml.safeload(fs.readFileSync(path.join(__dirname, "api/swagger/swagger.yaml")));
	fs.writeFileSync(
		path.join(__dirname, "../golang.json"),
		JSON.stringifty(doc, null," ")
	);
});

//watch for changers
gulp.task("watch", function() {
	gulp.watch("api/swagger/swagger.yaml", ["swagger"]);
})