Intro to Elixir metaprogramming

First Kyiv Elixir meetup, June 23, 2016

Hello!I am Anton Mishchuk from Lviv ▧ Ruby Dev at Matic (and we are hiring)▧ Fan of Elixir programming language▧ ESpec creator & maintainer▧ GitHub: antonmi

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Elixir is Erlang with pretty syntax and metaprogramming!

What is Elixir?

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▧ Elixir AST▧ Quote and unquote▧ Macroses▧ DSL▧ Introspection

What will be about

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Why metaprogramming

▧ Extending language▧ Boilerplate removing▧ Creating DSL

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Ecto.Schema

Elixir metaprogramming in the wild

Source code

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Phoenix.Router

Elixir metaprogramming in the wild

Source code

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ESpec.Let

Elixir metaprogramming in the wild

Source code

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Initial code parsing

Elixir code

Initial AST

Expansion

Expanded AST

Byte-code generation

Byte-code

Compilation process

Magic is here!

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Compile-time metaprogramming

▧ Code will not be redefined▧ Static code analysis still works▧ No performance penalty

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Elixir AST. Quoting

‘quote’ macro gets the representation of any expression.

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Elixir AST. Quoting

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▧ The first element is an atom identifying the operation or another tuple in the same representation

▧ The second element is a keyword list containing metadata, like numbers and contexts

▧ The third element is either a list of arguments for the function call or an atom. When this element is an atom, it means the tuple represents a variable

Elixir AST - 3 element tuple

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Elixir AST string representation

‘Macro.to_string’ converts the given expression to a binary

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Elixir AST evaluation

‘Code.eval_string’ and ‘Code.eval_quoted’

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Elixir AST. Unquoting

Unquoting is a mechanism to inject code chunks into quoted expressions.i.e. inject one AST fragment into another

Let’s try an example:

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Elixir AST. Unquoting

Trying to evaluate ‘final_expr’

I want to build AST which adds 1 + 2

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Elixir AST. Unquoting‘unquote’ helps to inject AST fragment

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Magic functions - Macros

▧ Macro (which stands for "macroinstruction") is a programmable pattern which translates a certain sequence of input into a preset sequence of output

▧ Macros receive the input AST and must produce the output AST

▧ The compiler will then simply replace the macro call with the AST returned from that macro

▧ Works only at compile time

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▧ ‘defmodule’, ‘defmacro’, ‘def’, defstruct ▧ ‘require’, ‘import’, ‘use’▧ ‘If’, ‘unless’, ‘cond’▧ etc, etc

Macros are everywhere

87% of Elixir code written on Elixir

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Macros. ExampleCalculator

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Macros. Example

The result is expect. Nothing special.Because we see completely compiled

code in the iex console

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Macros. ExampleRemember, macros receive quoted

expression and return them

Let’s do what compiler did step by step

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Macros. Dynamic function definition example

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Macros. ‘require’ and ‘import’We should tell the compiler which module should be compiled before we can use it.

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‘use’ macroThe simplest way to bring external functionality into the current lexical scope

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‘Use’ for DSL‘Use Ecto.Migration’

‘Ecto.Migration.__using__’

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‘Use’ for DSL‘use GenServer’

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‘Use’ for DSL‘GenServer.__using__’

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‘Use’ for DSL‘use ESpec’

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‘Use’ for DSL‘ESpec.__using__’

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OMG! How to handle all the stuff?

▧ What function/macro I can use in current scope?

▧ Where does this function/macro come from?

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Elixir introspection

▧ __ENV__ : Returns a Macro.ENV struct containing current environment information

▧ __MODULE__ : Returns the current module name as an atom

▧ __DIR__ : Returns the current directory▧ __CALLER__ : Returns the caller’s

environment information as a Macro.ENV struct

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Elixir introspection‘__ENV__’ in iex

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And don’t forget about help‘h IEx.Helpers.b’

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What is next?

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With great power comes great responsibility!

Thank you!

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