So I've added a watch task to my gulpfile.js that looks like this:

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

    gulp.watch('src/style/*.less', ['css']);
});

but when I run it (gulp watch) I get the following error thrown in my face:

PS C:\Projects\web\basic> gulp watch
[21:28:42] Using gulpfile C:\Projects\web\basic\gulpfile.js
[21:28:42] Starting 'watch'...
[21:28:42] 'watch' errored after 226 μs
[21:28:42] TypeError: Object #<Object> has no method 'watch'
    at Gulp.watch (C:\Projects\web\basic\node_modules\gulp\index.js:40:14)
    at Gulp.<anonymous> (C:\Projects\web\basic\gulpfile.js:37:7)
    at module.exports (C:\Projects\web\basic\node_modules\gulp\node_modules\orchestrator\lib\runTask.js:34:7)
    at Gulp.Orchestrator._runTask (C:\Projects\web\basic\node_modules\gulp\node_modules\orchestrator\index.js:273:3)
    at Gulp.Orchestrator._runStep (C:\Projects\web\basic\node_modules\gulp\node_modules\orchestrator\index.js:214:10)
    at Gulp.Orchestrator.start (C:\Projects\web\basic\node_modules\gulp\node_modules\orchestrator\index.js:134:8)
    at C:\Users\Magnus\AppData\Roaming\npm\node_modules\gulp\bin\gulp.js:129:20
    at process._tickCallback (node.js:415:13)
    at Function.Module.runMain (module.js:499:11)
    at startup (node.js:119:16)

As I've come to understand watch is a built-in function in gulp so what am I doing wrong here? The only other solutions to this dilemma I've seen is Linux specific (and presumable so is the cause for the problem). It is described in this SO-thread, but I'm using Windows and I can't see how that problem could be showing up in a Windows environment.

Also, I've looked through the Gulp index.js-file for project installation and it does contain a watch prototype, but the problems seems (according to the error message) to come from the final line in the watch function:

Gulp.prototype.watch = function(glob, opt, fn) {
  if (typeof opt === 'function' || Array.isArray(opt)) {
    fn = opt;
    opt = null;
  }

  // Array of tasks given
  if (Array.isArray(fn)) {
    return vfs.watch(glob, opt, function() {
      this.start.apply(this, fn);
    }.bind(this));
  }

  return vfs.watch(glob, opt, fn);
};

It's the return vfs.watch(glob, opt, fn);-line that causes the crash (it's line 40). vfs in this case is a variable that contains vinyl-fs and looks like this:

var vfs = require('vinyl-fs');

But even after manually installing it both in my project and globally it still doesn't work. Anyone that has any idea what might be causing this crash?

EDIT: The full gulpfile.js:

var gulp = require('gulp'),
    less = require('gulp-less'),
    concat = require('gulp-concat'),
    uglify = require('gulp-uglify');

var cssPath = 'src/style/*.less';
var jsPath = 'src/js/*.js';
var htmlPath = 'src/index.htm';
var assetPath = 'src/assets/*';

gulp.task('css', function() {
    return gulp.src(cssPath)    //Get all less files from src/style/
        .pipe(less())                   //Pass them on to less          
        .pipe(concat('style.css'))      //Concat all files to one big style.css-file
        .pipe(gulp.dest('dist/style'))  //Pass the less result to gulp so it can be moved to dist/css
});

gulp.task('js', function(){
    return gulp.src(jsPath) //Get all the javascript files from src/js/
        .pipe(uglify())             //Pass them on to uglify
        .pipe(concat('all.js'))     //Concat all the files into one javascript file
        .pipe(gulp.dest('dist/js')) //Pass the result to gulp so it can be moved to dist/js
});

gulp.task('html', function(){
    return gulp.src(htmlPath)
        .pipe(gulp.dest('dist'))
});

gulp.task('assets', function(){
    return gulp.src(assetPath)
        .pipe(gulp.dest('dist/assets'))
});

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

    gulp.watch(cssPath, ['css']);
    gulp.watch(jsPath, ['js']);
    gulp.watch(htmlPath, ['html']);
    gulp.watch(assetPath, ['assets']);
});

gulp.task('default', ['css', 'js', 'html', 'assets']);