Gulp less not handling includes properly, included variables not defined

Question

I am using less and Grunt, and am moving to gulp.

My less works. When I run:

lessc public/less/myapp.less

I get working output with no errors. All my less, including includes, is in public/less, BTW. Here is my gulpfile:

var gulp = require('gulp');
var prefixer = require('gulp-autoprefixer');
var less = require('gulp-less');

gulp.task('compileLess', function () {
  gulp
    .src('./public/less/*')
    .pipe(less({
      paths: ['./public/less'], // Search paths for imports
      filename: 'myapp.less'
    }))
    .pipe(gulp.dest('./public/css'));
});

// The default task (called when you run `gulp`)
gulp.task('default', function() {
  gulp.run('compileLess');

  // Watch files and run tasks if they change
  gulp.watch('./public/less/*.less', function(event) {
    gulp.run('less');
  });
});

When I run gulp, I get:

~/Documents/myapp: gulp
[gulp] Using file /Users/me/Documents/myapp/gulpfile.js
[gulp] Working directory changed to /Users/me/Documents/myapp
[gulp] Running 'default'...
[gulp] Running 'compileLess'...
[gulp] Finished 'compileLess' in 11 ms
[gulp] Finished 'default' in 41 ms

stream.js:94
      throw er; // Unhandled stream error in pipe.
            ^
Error: variable @brightblue is undefined

@brightblue is defined, in a file imported at the start of myapp.less.

@import "./colors.less";
body {
  background-color: @brightblue;
}

• Why isn't gulp picking up my includes? Specifying 'paths' option for less should make it work, but it isn't fixing it.
• What's a good way to debug this? I don't have a line number, for example.
• Is there a way to make gulp work?

Answer 1

The difference was what I was compiling:

• When I ran lessc myapp.less, I was compiling the main less file and it's dependencies
• When I ran gulp using the gulpfile above, I was compiling each less file individually, because gulp.src was *.less not myapp.less. Since these less files are only ever loaded from the main less file, they didn't have @imports for the things they depend on (because myapp.less depends on them). Eg, there's no point importing, say, 'theme.less' in every individual file rather than just importing it first in myapp.less.

Here's the working version:

// Run 'gulp' to do the important stuff
var gulp = require('gulp');
var prefixer = require('gulp-autoprefixer');
var less = require('gulp-less');
var path = require('path');

gulp.task('less', function () {
  gulp
    .src('./public/less/myapp.less') // This was the line that needed fixing
    .pipe(less({
      paths: ['public/less']
    }))
    .pipe(prefixer('last 2 versions', 'ie 9'))
    .pipe(gulp.dest('./public/css'));
});

// The default task (called when you run `gulp`)
gulp.task('default', function() {
  gulp.run('less');

  // Watch files and run tasks if they change
  gulp.watch('./public/less/*.less', function(event) {
    gulp.run('less');
  });
});

Edit: see @noducks answer below for an improved version that works with the latest gulp.