Gulpfile for simple elm project

gulfile.js

var concat = require('gulp-concat')
var concatCss = require('gulp-concat-css')
var csso = require('gulp-csso')
var del = require('del')
var elmBins = require('elm')
var glob = require('glob')
var gulp = require('gulp')
var jade = require('gulp-jade')
var jadeConcat = require('gulp-jade-template-concat')
var simpleSpawner = require('simple-spawner')


var paths = {
  dist: 'build',
  css: 'src/styles/*.css',
  elm: 'src/elm/**/*.elm',
  images: 'src/images/*',
  index: 'src/index.html',
  js: 'src/js/*.js',
  templates: 'src/templates/*.jade'
}

function clean () {
  return del(paths.dist)
}

function copyIndex () {
  return gulp.src(paths.index)
  .pipe(gulp.dest(paths.dist))
}

function css () {
  return gulp.src(paths.css)
  .pipe(concatCss('main.css', {
    includePaths: [
      'node_modules/normalize.css/'
    ]
  }))
  .pipe(csso())
  .pipe(gulp.dest(paths.dist))
}

function elm () {
  var elmPaths = glob.sync(paths.elm)
  var args = elmPaths.concat([
    '--yes',
    '--output',
    paths.dist + '/Main.js'
  ])
  return simpleSpawner(elmBins['elm-make'], args)
}

function images () {
  return gulp.src(paths.images)
  .pipe(gulp.dest(paths.dist + '/images'))
}

function js () {
  return gulp.src(paths.js)
  .pipe(concat('vendor.js'))
  .pipe(gulp.dest(paths.dist + '/js'))
}

function templates () {
  return gulp.src(paths.templates)
  .pipe(jade({ client: true }))
  .pipe(jadeConcat('templates.js', { templateVariable: 'templates' }))
  .pipe(gulp.dest(paths.dist + '/js'))
}

function watch () {
  var pathMap = {
    css: css,
    elm: elm,
    images: images,
    index: copyIndex,
    js: js
  }
  Object.keys(pathMap).forEach(function (p) {
    gulp.watch(paths[p], pathMap[p])
  })
}

/* Tasks */

var parallelBuild = [
  copyIndex,
  images,
  js,
  css,
  templates,
  elm
]

// Add a named task for each step
parallelBuild.forEach(function (fn) { gulp.task(fn) })

var buildTasks = [
  clean,
  gulp.parallel(parallelBuild)
]

gulp.task(clean)

gulp.task('default', gulp.series(buildTasks, watch))

gulp.task(watch)