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Author: shmink

README.md

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+# Metaprogamming Elixir by Chris McCord
+
+_Write less code, get more done (and have fun!)_ :purple_heart:
+
+## Abstract
+
+Reading this book to learn about how macros work and following some of the relevant
+examples. This README will serve as my notes, therefore you shouldn't take them at face
+value as the notes will make sense for me as I cherry pick a sentence or an analogy.
+Moreover, don't expect direct quotes, changing the sentences vocbulary helps me personally.
+
+## Chapter 1 - The language of macros
+
+-   Macros are code that write code.
+-   Elixir itself is made with macros, as a result you can extend the language itself
+    to include things you think you might need.
+-   Metaprogramming in elixir serves the purpose of extensibility by design.
+-   With this power one can even define languages within elixir. The following is a valid Elixir program.
+
+```elixir
+div do
+    h1 class: "title" do
+        text "Hello"
+    end
+    p do
+        text "Metaprogramming Elixir"
+    end
+end
+"<div><h1 class=\"title\">Hello</h1><p>Metaprogramming Elixir</p></div>"
+```
+
+#### The Abstract Syntax Tree
+
+-   Most languages use AST but you never need to know about them. They are used typically during compilation
+    or interpretation to transform source code into a tree structure before being turned into bytecode
+    or machine code..
+-   José Valim, the creator of Elixir, chose to expose this AST and the syntax to interact with it.
+-   We can now operate at the same level as the compiler.
+-   Metaprogramming in Elixir revolves around manipulating and accessing ASTs.
+-   To access the AST representation we use the `quote` macro.
+
+```elixir
+iex> quote do: 1 + 2
+{:+, [context: Elixir, import: Kernel], [1, 2]}
+```
+
+```elixir
+iex> quote do: div(10, 2)
+{:div, [context: Elixir, import: Kernel], [10, 2]}
+```
+
+-   This is the internals of the Elixir language itself.
+-   This gives you easy options for infering meaning and optimising performance all while being within Elixirs high level syntax.
+-   The purpose of macros is to interact with this AST with the syntax of Elixir.
+-   Macros turn you from language consumer to language creator. You have the same level of power as José when he wrote the standard library.
+
+#### Trying It All Together
+
+"Let's write a macro that can print the spoken form of an Elixir mathematical expression, such as 5 + 2, when calculating a result.
+In most languages, we would have to parse a string expression into something digestible by our program. With Elixr, we can access
+the representation of expressions directly with macros."
+
+[First macro](math.exs)

math.exs

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+defmodule Math do
+  @moduledoc false
+
+  defmacro say({:+, _, [lhs, rhs]}) do
+    quote do
+      lhs = unquote(lhs)
+      rhs = unquote(rhs)
+      result = lhs + rhs
+      IO.puts("#{lhs} plus #{rhs} is #{result}")
+      result
+    end
+  end
+
+  defmacro say({:*, _, [lhs, rhs]}) do
+    quote do
+      lhs = unquote(lhs)
+      rhs = unquote(rhs)
+      result = lhs * rhs
+      IO.puts("#{lhs} times #{rhs} is #{result}")
+      result
+    end
+  end
+end