Introduction to Metaprogramming

By Pikender Sharma April 14, 2016

The post belongs to NectarCommerce and Extension Framework Awareness

  1. NectarCommerce Vision
  2. Extension Framework Game Plan
  3. Introduction to Metaprogramming
  4. Ecto Model Schema Extension
  5. Ecto Model Support Functions Extension
  6. Phoenix Router Extension
  7. Phoenix View Extension
  8. Running Multiple Elixir Apps Together
  9. Extension Approach Explained
  10. Learning from failures: First Experiment at NectarCommerce Extension Approach
  11. Developing NectarCommerce Extensions
  12. Building an exrm release including NectarCommerce

What will be NectarCommerce

Off-the-shelf Opensource E-commerce application for building an online store.

Provides an Extension Framework to support features not included in core as extensions.

Strives for unobtrusive parallel development of NectarCommerce and Extensions

NectarCommerce is committed to providing a ready-to-use e-commerce solution but the definition of 100% is different under different business domains. It aims to solve common use-cases as part of the project and relying on extension framework to tap the rest.

Metaprogramming in Elixir

Note: If you are already familiar and experienced with Elixir Metaprogramming, you can jump to last section as it lists Metaprogramming resources
and constructs used in upcoming blogs when creating different extension DSL’s.

Elixir docs are excellent resources to get familiar with Metaprogramming:

Check out Understanding Elixir Macros blog series by @sasajuric

Book Metaprogramming Elixir by @chris_mccord is next step for diving deeper into metaprogramming.

Why another tutorial on an already well-documented metaprogramming topic ?

To revise and refresh something that we would refer time and again when reviewing Model, Router, View extension DSLs

Extension DSLs will be using below constructs to get the job done 🙂

  • Module.register_attribute/3
    • Registers an attribute. By registering an attribute, a developer is able to customize how Elixir will store and accumulate the attribute values.
  • Module.put_attribute/3
    • Puts an Erlang attribute to the given module with the given key and value
  • @before_compile
    • A hook __before_compile__/1 that will be invoked before the module is compiled
    • It allows us to inject code into the module when its definition is complete
  • __using__ hook
    • Checkout the title Examples for the usage & context and Best practices in link above
    • use ModuleName looks and invokes __using__ macro defined in ModuleName module
  • bind_quoted
    • Find the documentation under title Options for bind_quoted option in the link above
    • By using bind_quoted, we can automatically disable unquoting while still injecting the desired variables into the tree
  • Code.ensure_loaded?/1
    • Ensures the given module is loaded
  • apply/3
    • Invokes the given function from module with the array of arguments args.
  • Macro.escape/1
    • Recursively escapes a value so it can be inserted into a syntax tree

Metaprogramming pattern as used across extensions

Minimum three parts are needed to create & use an extension effectively:

  • Library Code
  • Service Code
  • Consumer Code

An extension and its use with Nectar can be viewed as Producer / Consumer relationship bound by a communication protocol.

Extension which wants to add a route, say a list of favorites, is a Producer (Service Code).

Nectar Router is a Consumer (Consumer Code) allowing the route additions through a communication protocol (Library Code)

  • Library code defines a macro providing a DSL, say define_route
  • Collect all definitions provided using DSL from Service code in Module attribute, say defined_routes
  • Library code defines a before_compile hook to add a macro extracting all collected definitions from module attribute, as module attributes are cleaned up as compilation completes
  • Inject generated code into targeting module Consumer code through __using__ hook as invoked by use ModuleWithDSLImplementation, say use ExtensionsManager.RouterExtension

Our aim with these posts is to start a dialog with the Elixir community on validity and technical soundness of our approach. We would really appreciate your feedback and reviews, and any ideas/suggestions/pull requests for improvements to our current implementation or entirely different and better way to do things to achieve the goals we have set out for NectarCommerce.

Enjoy the Elixir potion !!

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