Gain insights into metaprogramming in Elixir. Delve into macros and discover how to elevate your coding skills, creating powerful libraries with concise, efficient code.

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Get introduced to the concept of metaprogramming. Learn how to level up your programming skills by discovering the full potential of the macro system in Elixir. Understand the ins and outs of metaprogramming at a fundamental level and write incredible libraries by doing more with less code.

Metaprogramming in Elixir

Let’s take a moment to appreciate that Elixir makes the world our playground. Rules are made to be broken! So, here are some ways to  misapply macros in Elixir and some interesting ways to bend Elixir’s syntax.



# Misusing valid Elixir syntax
Rewriting the AST to change the meaning of valid Elixir expressions probably sounds wrong to most people. In some cases, it’s actually a powerful tool. Consider Elixir’s [__Ecto2__](https://github.com/elixir-ecto/ecto) library, which is a database wrapper and Language Integrated Query system. 

Let’s take a look at what an Ecto query looks like to understand how it misuses Elixir’s syntax. We don’t have to be familiar with Ecto, so just be mindful of the query syntax in the following example:
```
query = from user in User,
      where: user.age > 21 and user.enrolled == true,
     select: user
```
Internally, Ecto converts this completely valid Elixir expression into a string of SQL. It misuses operators such as `in`, `and`, `==`, and `>` to form SQL expressions out of valid Elixir code. This is an elegant use of macros. Ecto lets us build queries in Elixir’s natural syntax, using bound variables that are typecasted appropriately in SQL. Other languages with a Language Integrated Query feature require an entirely new syntax on top of the language. With Elixir, we can use macros to change regular Elixir code into a SQL representation.

Ecto is a large project worthy of its own book, but let’s imagine how we could implement a similar library. Let’s see what the quoted form of our query above looks like.
 

Let’s try a few variations out in `iex`. While working, think about ways we could use ourAST tricks, such as Macro.postwalk from the lesson [Chapter 6 : Generate Less Code by Walking the AST](https://www.educative.io/courses/metaprogramming-elixir/generating-less-code-by-walking-the-ast).


Execute the following commands in the terminal:
``` elixir
iex(1)> quote do                                           
...(1)>   from user in User,                               
...(1)>     where: user.age > 21 and user.enrolled == true,
...(1)>    select: user                                    
...(1)> end
# Output:
# {:from, [],
# [
#   {:in, [context: Elixir, import: # Kernel],
#    [{:user, [], Elixir}, {:__aliases__, [alias: false], [:User]}]},
   [
 #    where: {:and, [context: Elixir, import: Kernel],
#      [
#        {:>, [context: Elixir, import: # Kernel],
 #        [{{:., [], [{:user, [], Elixir}, :age]}, [], []}, 21]},
 #       {:==, [context: Elixir, import: Kernel],
#         [{{:., [], [{:user, [], Elixir}, :enrolled]}, [], []}, true]}
 #     ]},
 #    select: {:user, [], Elixir}
#   ]
# ]} 
```


Let's explore some gray areas where misusing macros in Elixir is a worthy trade-off.

Introduction

The Language of Macros

Extending Elixir with Metaprogramming

Advanced Compile-Time Code Generation

Code Generation from Git API

How to Test Macros

Create an HTML Domain-Specific Language

With Great Power Comes Great Responsibility (and Fun!)

Bending the Rules

Misusing valid Elixir syntax