gulpfile.js

gulp.task('watch', function() {

  gulp.src([paths.stylesheets + "/**/*.styl"])
    .pipe(watch(function() {
        return gulp.src(paths.stylesheets + "/main.styl")
            .pipe(stylus({compress: true}))
            .pipe(gulp.dest(paths.stylesheets))
            .pipe(gulp.dest(targetPaths.stylesheets))
    }))

});