Extract function for generating modules

Fully parameterized, makes it easy to test module options behavior
without testing against the entire default module list.

__tests__/generateModules.test.js

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+import generateModules from '../src/util/generateModules'
+import defineClasses from '../src/util/defineClasses'
+
+function textAlign() {
+  return defineClasses({
+    'text-left': { 'text-align': 'left' },
+    'text-right': { 'text-align': 'right' },
+    'text-center': { 'text-align': 'center' },
+  })
+}
+
+function display() {
+  return defineClasses({
+    'block': { 'display': 'block' },
+    'inline': { 'display': 'inline' },
+    'inline-block': { 'display': 'inline-block' },
+  })
+}
+
+function borderStyle() {
+  return defineClasses({
+    'border-solid': { 'border-style': 'solid' },
+    'border-dashed': { 'border-style': 'dashed' },
+    'border-dotted': { 'border-style': 'dotted' },
+  })
+}
+
+test('an empty variants list generates a @variants at-rule with no parameters', () => {
+  const result = generateModules([
+    { name: 'textAlign', generator: textAlign },
+  ], {
+    textAlign: [],
+  })
+
+  const expected = `
+    @variants {
+      .text-left { text-align: left }
+      .text-right { text-align: right }
+      .text-center { text-align: center }
+    }
+  `
+  expect(result.toString()).toMatchCss(expected)
+})
+
+test('a `false` variants list generates no output', () => {
+  const result = generateModules([
+    { name: 'textAlign', generator: textAlign },
+  ], {
+    textAlign: false,
+  })
+
+  expect(result.toString()).toMatchCss('')
+})
+
+test('specified variants are included in the @variants at-rule', () => {
+  const result = generateModules([
+    { name: 'textAlign', generator: textAlign },
+  ], {
+    textAlign: ['responsive', 'hover'],
+  })
+
+  const expected = `
+    @variants responsive, hover {
+      .text-left { text-align: left }
+      .text-right { text-align: right }
+      .text-center { text-align: center }
+    }
+  `
+  expect(result.toString()).toMatchCss(expected)
+})
+
+test('options must provide variants for every module', () => {
+  expect(() => {
+    generateModules([
+      { name: 'textAlign', generator: textAlign },
+      { name: 'display', generator: display },
+    ], {
+      textAlign: [],
+    })
+  }).toThrow()
+})
+
+test('variants can be different for each module', () => {
+  const result = generateModules([
+    { name: 'textAlign', generator: textAlign },
+    { name: 'display', generator: display },
+    { name: 'borderStyle', generator: borderStyle },
+  ], {
+    textAlign: [],
+    display: false,
+    borderStyle: ['responsive', 'hover', 'focus']
+  })
+
+  const expected = `
+    @variants {
+      .text-left { text-align: left }
+      .text-right { text-align: right }
+      .text-center { text-align: center }
+    }
+
+    @variants responsive, hover, focus {
+      .border-solid { border-style: solid }
+      .border-dashed { border-style: dashed }
+      .border-dotted { border-style: dotted }
+    }
+  `
+  expect(result.toString()).toMatchCss(expected)
+})
+
+test('generators can reference the generatorOptions object', () => {
+  const result = generateModules([{
+    name: 'parameterized',
+    generator: (generatorParams) => {
+      return defineClasses({
+        'foo': { 'color': generatorParams.color },
+      })
+    }
+  }], {
+    parameterized: [],
+  }, {
+    color: 'blue'
+  })
+
+  const expected = `
+    @variants {
+      .foo { color: blue }
+    }
+  `
+  expect(result.toString()).toMatchCss(expected)
+})

src/util/generateModules.js

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+import _ from 'lodash'
+import postcss from 'postcss'
+import wrapWithVariants from '../util/wrapWithVariants'
+
+export default function(modules, moduleOptions, generatorOptions = {}) {
+  modules.forEach(module => {
+    if (! _.has(moduleOptions, module.name)) {
+      throw new Error(`Module \`${module.name}\` is missing from moduleOptions.`)
+    }
+  })
+
+  return postcss.root({
+    nodes: _(modules)
+      .reject(module => moduleOptions[module.name] === false)
+      .flatMap(module => wrapWithVariants(module.generator(generatorOptions), moduleOptions[module.name]))
+      .value()
+  })
+}