1.
Intro to Elixir
metaprogramming
First Kyiv Elixir meetup, June 23, 2016

2.
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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3.
Elixir is Erlang with pretty syntax
and metaprogramming!
What is Elixir?
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4.
▧ Elixir AST
▧ Quote and unquote
▧ Macroses
▧ DSL
▧ Introspection
What will be about
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5.
Why metaprogramming
▧ Extending language
▧ Boilerplate removing
▧ Creating DSL
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6.
Ecto.Schema
Elixir metaprogramming in the wild
Source code
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7.
Phoenix.Router
Elixir metaprogramming in the wild
Source code
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8.
ESpec.Let
Elixir metaprogramming in the wild
Source code
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9.
Initial code parsing
Elixir code
Initial AST
Expansion
Expanded AST
Byte-code generation
Byte-code
Compilation process
Magic is here!
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10.
Compile-time metaprogramming
▧ Code will not be redefined
▧ Static code analysis still works
▧ No performance penalty
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11.
Elixir AST. Quoting
‘quote’ macro gets the representation
of any expression.
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12.
Elixir AST. Quoting
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13.
▧ 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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14.
Elixir AST string representation
‘Macro.to_string’ converts the given
expression to a binary
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15.
Elixir AST evaluation
‘Code.eval_string’ and ‘Code.eval_quoted’
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16.
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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17.
Elixir AST. Unquoting
Trying to evaluate ‘final_expr’
I want to build AST which adds 1 + 2
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18.
Elixir AST. Unquoting
‘unquote’ helps to inject AST fragment
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19.
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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20.
▧ ‘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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21.
Macros. Example
Calculator
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22.
Macros. Example
The result is expect. Nothing special.
Because we see completely compiled
code in the iex console
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23.
Macros. Example
Remember, macros receive quoted
expression and return them
Let’s do what compiler did step by step
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24.
Macros. Dynamic function definition example
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25.
Macros. ‘require’ and ‘import’
We should tell the compiler which module
should be compiled before we can use it.
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26.
‘use’ macro
The simplest way to bring external
functionality into the current lexical scope
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27.
‘Use’ for DSL
‘Use Ecto.Migration’
‘Ecto.Migration.__using__’
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28.
‘Use’ for DSL
‘use GenServer’
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29.
‘Use’ for DSL
‘GenServer.__using__’
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30.
‘Use’ for DSL
‘use ESpec’
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31.
‘Use’ for DSL
‘ESpec.__using__’
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32.
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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33.
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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34.
Elixir introspection
‘__ENV__’ in iex
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35.
And don’t forget about help
‘h IEx.Helpers.b’
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36.
What is next?
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37.
With great power comes
great responsibility!
Thank you!
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