piler.js

var stylus = require('stylus')
  , nib = require('nib')
  , uglify = require('uglifyjs')

function processStylus(input, cb) {

  fs.readFile(input.src, function (err, data) {

    if (err) return cb(err)

    stylus
      .use(nib())
      .set('compress', true)
      .set('more options', { foo: 1 })
      .define('my function', function () { /* ... */ })
      .compile(data, function (err, css) {
        if (err) return cb(err)
        fs.writeFile(input.dest, function (err) {
          if (err) return cb(err)
          cb(null, 'yay')
      })
  })

}

function processJS(input, cb) {

  // The string of concated js
  var out = ''

  function write() {
    fs.writeFile(input.dest, out, function (err) {
      if (err) return cb(err)
      cb(null, 'yay')
    })
  }

  // Determine whether to bundle or provide loader
  if (someFlag) {

    // This makes the assumption that the paths in srcs are the
    // the same as the public available web path. (but they are not)
    out += buildLoader(input.srcs)

  } else {

    var done = 0

    input.srcs.forEach(function (src) {
      fs.readFile(src, function (err, data) {
        out += uglify(data)
        if (++done === input.srcs.length) write()
      })
    })

  }

}

// admin bundle - piler.js

module.exports = function(piler) {

  // Define custom processors. min and watch could be built in.
  var processStylus = require('processStylus')
    , processJs = require('processJs')

  var buildCss = piler.task.buildCss = function(done) {
    processStylus(
      { src: 'public/css/screen.styl'
      , dest: 'public/css/screen.css'
      }, done)
  }

  piler.task.buildJavascript = function(done) {
    processJs(
      [ { dest: 'public/js/app.js'
        , srcs: [ 'public/js/a.js', 'public/js/b.js' ]
        }
      ], done)
  }

  piler.task.watchCss = piler.watch.bind(null, 'public/css/**/*.styl', buildCss)
}


// global piler.js

module.exports = function (piler) {
  piler.loadTasks('./bundles/**/piler.js')

  // Define single tasks for build etc
  piler.task('build', function(done) {
    piler.tasks.version(function(error) {
      if (error) {
        return done(error)
      }
      piler.tasks.buildCss(function(error) {
        if (error) {
          return done(error)
        }
        piler.tasks.buildJavascript(done)
      })
    })
  })

  // or possibly more simply
  piler.task.build = function() {
    piler.seq([
      piler.task.version,
      piler.task.buildCss,
      piler.task.buildJavascript
    ])
  }
}

// cli

// Watch for changes and call tasks
$ piler watch

// Run all stylus tasks
$ piler stylus

// Build project
$ piler build

@bengourley there will be a config per bundle so it is quite hard to reference the other build tasks. This is a problem for both string based invocation and anonymous function wrapped functions. I don't really want to assign to a variable before passing them.