AST-based macro transforms for Rollup

Functions can improve readability. Inlining expanded constants can improve performance.

Build-time AST macros allow you to take advantge of both.

Example

Virtual DOM Example

Build-time macro for an IJK-like transform for use with hyperapp, superfine, or preact

MACRO({
  H(nodeName, attributes, ...children) {
    const parts = { nodeName, attributes }
    if (0 !== children.length) {
      parts.children = `[${children.join(', ')}]`
    }

    const body = Object.entries(parts)
      .map(e => e.join(': '))
      .join(', ')

    return `({ ${body} })`
  },
})

export const view = count =>
  H("div", {}
    , H("h1", {}, count)
    , H("button", { onclick: () => render(count - 1) }, "-")
    , H("button", { onclick: () => render(count + 1) }, "+") )

Output (with prettier)

export const view = count => ({
  nodeName: 'div',
  attributes: {},
  children: [
    { nodeName: 'h1', attributes: {}, children: [count] },
    {
      nodeName: 'button',
      attributes: { onclick: () => render(count - 1) },
      children: ['-']
    },
    {
      nodeName: 'button',
      attributes: { onclick: () => render(count + 1) },
      children: ['+']
    }
  ]
})

CSS-in-JS Example

MACRO({
  EXPR: expr => new Function(`return (${expr})`).call(null),

  LET(expr) {
    Object.assign(global, this.EXPR(expr))
    return ''
  },

  CSS(strings, ...args) {
    let res = '`' + strings[0]
    for (let i=0; i<args.length; i++)
      res += args[i] + (strings[i+1] || '')
    res += '`'
    return res
  },
})

LET({
  color: 'blue',
  height: 100,
  dayIsOdd: new Date().getDay() & 1,
  buildDate: new Date().toISOString().replace(/:\d\d\..*Z/, ''),
})

const css_styles = CSS`
  .somewhere {
    top: ${ EXPR(20 + height) } px;
    border: ${ EXPR(1 + 2) } px ${ EXPR( dayIsOdd ? 'solid' : 'dashed' ) } ${color};
  }
  .somewhere::before {
    content: "${ EXPR(buildDate) }";
  }
`

Output (with prettier)

const css_styles = `
  .somewhere {
    top: 120 px;
    border: 3 px solid color;
  }
  .somewhere::before {
    content: "2018-08-31T16:05";
  }
`;

Implementation

The heavy lifting for AST transform is provided by the excellent transform-ast and magic-string packages. Macro functions are evaluated in VM2.NodeVM isolation, providing a measure of protection from abuse.

API

MACRO(«object expression»)

Allows defining new macros and variables. A single argument of type object is expected. The expression will be evaluated in a VM2.NodeVM context and the result assigned to the macro namespace. Any functions defined at the root will define new macros.

Defined macro functions

The macro's arguments are the getSource() strings of the AST arguments nodes. (e.g. ast_node.arguments.map(e => e.getSource())

MACRO({
  url: require('url'),

  EXAMPLE_SRC(...args) {
    return JSON.stringify(args)
  },
})

Allowed Builtin modules:

• require('os')
• require('util')
• require('zlib')
• require('assert')
• require('path')
• require('url')
• require('querystring')
• require('punycode')

This can be customized with {vm2: {builtin: []}}

AST transformations

Additionally, this.ast_node is the macro's CallExpression AST Node, enabling sophisticated AST-based transformations.

MACRO({
  EXAMPLE_ARROW_AST() {
    // translated from https://github.com/goto-bus-stop/transform-ast#example

    for (const node of this.ast_node.arguments) {
      if (node.type !== 'ArrowFunctionExpression')
        continue;

      const params = node.params.map(param =>param.getSource())

      if (node.body.type !== 'BlockStatement')
        node.body.edit.update(`{ return ${node.body.getSource()} }`)

      node.edit.update(`function (${params.join(', ')}) ${node.body.getSource()}`)
    }
  },
})

Rollup Plugin Options

import rpi_ast_macros from 'rollup-plugin-ast-macros'

// defaults:
const macros = null // or {}
const sandbox = null // or {}
const builtin = ['os', 'util', 'zlib', 'assert', 'path', 'url', 'querystring', 'punycode']

const plugins = [
  rpi_ast_macros({
    // include: 'code/**',
    exclude: ['node_modules/**'],

    ast_opt: {}, // passed to transform_ast(source, ast_opt)

    macros, // additional default macros & `this` context for macro invocation

    vm2_opt: {
      sandbox, // passed to VM2.NodeVM({sandbox})
      builtin, // passed to VM2.NodeVM({require:{builtin})
    },
  })
]

License

MIT