Elixir - Macros On Erlang

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"Elixir took the Erlang virtual machine, BEAM, and put a sensible face on it. It gives you all the power of Erlang plus a powerful macro system." — Dave Thomas, author of Programming Elixir, co-author of Pragmatic Programmer

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"Erlang plus a powerful macro system."

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Elixir - Macros On Erlang

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Who?! Thorsten Ball Software Developer thorstenball.com mrnugget / @thorstenball I like programming languages.

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I wrote a book! Get it: interpreterbook.com Coupon code for 20% off: elixirfromthebembel

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interpreterbook.com/lost

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Disclaimer! Use in production at your own risk

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What's a macro?

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Macros — Code that writes code — They allow us to inﬂuence which code gets evaluated

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So, meta-programming? — Short answer: It depends — Long answer: Iiiiitt deeeepeeeeeends — Best answer: Eeeh, maybe, depends on who you ask — Not the Ruby kind of meta-programming — Macros really work with code: not with objects/ classes/methods/functions/etc.

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Two Camps Of Macro Systems — Text-Substitution Macros — Syntactic Macros

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Two Camps Of Macro Systems — Search and Replace Macros — Code as Data Macros

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Text-Substitution Macros — The simplest type of macro system — Example: The C Pre Processor (cpp)

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#deﬁne GREETING "Hello there" int main(int argc, char *argv[]) { #ifdef DEBUG printf(GREETING " Debug-Mode!\n"); #else printf(GREETING " Production-Mode!\n"); #endif return 0; }

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Text-Substitution Macros — You can go a long way with it — But you have to be careful: escaping, scoping, overwriting, ... — Much more like a templating system than a macro system of the second kind

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Syntactic Macros — In the land where "Code Is Data" — Yeah, really, it is weird. It needs a small push to wrap one's head around it

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Code is data?

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Code starts out as text — We write code in our text editor — We look at text diffs on GitHub — We store text in our version control system — We modify source code with search/replace

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But then we pass it to our programming language... ... and you won't believe what happens next.

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Parsers turn code into data! — Parsing: turning strings into data structures — Just like a JSON parser turns a string into objects, arrays, integers, strings, ... — Parsers turn code strings into syntax trees (or Abstract Syntax Trees)

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Why? — Analyze it — "is this valid syntax?" — "has this identifier been defined before?" — "is this code unused?" — Modify it — "Let's remove the commented-out code" — "Let's replace calls to this function with the body of the function itself" — Pass it around — "lets hand this and the code from the other files to the pretty printer"

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Pretty handy, huh?

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In language where "code is data" you can do that in the language itself!

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Allow me to explain... — Ruby — Implemented in C — Parse and modify Ruby in C — Parser in C, AST in C, code that uses AST in C — Elixir — "Code is data" — Implemented in Erlang (doesn't matter) — You can parse and modify Elixir in Elixir — Parse in Elixir, AST in Elixir, code that uses AST in Elixir

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Code is data, data is code — Incredibly powerful — Writing code that writes code — The language becomes self-aware...

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Recursion, baby!

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(! Lisp (baby))

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Code is data — Sounds weird? It is — Sounds abstract? It is — Don't get it? That's okay

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Let's write a macro in Elixir The unless macro. The example for macros - in any language! Should look like this: unless 5 == 3, do: IO.inspect("will be printed") unless 3 == 3, do: IO.inspect("will not be printed")

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Unless - The Naiive Version defmodule FunctionUnless do def unless(clause, do: block) do if !clause, do: block end end

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Looking good there... iex(1)> c "unless.exs" [FunctionUnless] iex(2)> FunctionUnless.unless 5 == 3, do: IO.puts("yay condition false") yay condition false [do: :ok]

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Uh oh iex(3)> FunctionUnless.unless 5 == 5, do: IO.puts("yay condition true!") yay condition true! nil

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Macros to the rescue defmodule MacroUnless do defmacro unless(clause, block) do quote do if !unquote(clause), do: unquote(block) end end end

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Just what the alchemist ordered iex(1)> c "unless.exs" [FunctionUnless, MacroUnless] iex(2)> require MacroUnless MacroUnless iex(3)> MacroUnless.unless 5 == 3, do: IO.puts("condition false") condition false :ok iex(4)> MacroUnless.unless 5 == 5, do: IO.puts("condition true") nil

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How? — Macros are evaluated in the "Macro Expansion Phase" — Macro Expansion: replace calls to macros in the source code with the result of the call — Parsing -> Macro Expansion Phase -> Compilation/ Interpretation — Macros receive AST nodes and return AST nodes

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Inspecting a macro defmodule MacroUnless do defmacro unless(clause, do: block) do IO.inspect(clause) IO.inspect(block) quote do if !unquote(clause), do: unquote(block) end end end

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Inspecting a macro iex(9)> MacroUnless.unless 5 == 5, do: IO.puts("condition true") {:==, [line: 9], [5, 5]} {{:., [line: 9], [{:__aliases__, [counter: 0, line: 9], [:IO]}, :puts]}, [line: 9], ["condition true"]} nil Tuples, keyword lists, lists - code is data!

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Return the code you want to see in the world! — quote — unquote

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quote iex> quote do: 1 + 2 {:+, [context: Elixir, import: Kernel], [1, 2]} iex> quote do: IO.puts("foobar") {{:., [], [{:__aliases__, [alias: false], [:IO]}, :puts]}, [], ["foobar"]} iex> quote do: MyModule.add_two(2) {{:., [], [{:__aliases__, [alias: false], [:MyModule]}, :add_two]}, [], [2]} iex> quote do: [1, 2, 3, 4] [1, 2, 3, 4] iex> quote do: [head | tail] = [1, 2, 3, 4] {:=, [], [[{:|, [], [{:head, [], Elixir}, {:tail, [], Elixir}]}], [1, 2, 3, 4]]}

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unquote — The trusty helper of quote — Punches a hole into quote

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unquote iex> quote do: 1 + 2 + 3 {:+, [context: Elixir, import: Kernel], [{:+, [context: Elixir, import: Kernel], [1, 2]}, 3]} iex> quote do: 1 + unquote(2 + 3) {:+, [context: Elixir, import: Kernel], [1, 5]}

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We need unquote iex(18)> quote do: if !(clause), do: block {:if, [context: Elixir, import: Kernel], [{:!, [context: Elixir, import: Kernel], [{:clause, [], Elixir}]}, [do: {:block, [], Elixir}]]}

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Unquote to access arguments iex(19)> clause = quote do: 5 == 5 {:==, [context: Elixir, import: Kernel], [5, 5]} iex(20)> block = quote do: IO.puts("yay!") {{:., [], [{:__aliases__, [alias: false], [:IO]}, :puts]}, [], ["yay!"]} iex(21)> quote do: if !(unquote(clause)), do: unquote(block) {:if, [context: Elixir, import: Kernel], [{:!, [context: Elixir, import: Kernel], [{:==, [context: Elixir, import: Kernel], [5, 5]}]}, [do: {{:., [], [{:__aliases__, [alias: false], [:IO]}, :puts]}, [], ["yay!"]}]]}

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Our macro again defmodule MacroUnless do defmacro unless(clause, block) do quote do if !unquote(clause), do: unquote(block) end end end

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Boom! Macros! Code is data!

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Elixir - Macros On Top Of Erlang — Why "Erlang" and "a macro system"? — Because it's an incredibly powerful macro system — Elixir itself is built using this macro system