Cucumber and ElixirIan Dees • @undeesCukeUp! NYC 2013

Why Elixir?The sad state of concurrency

int HandleMessage(MessageType type, unsigned long arg1, unsigned long arg2) { switch (type) { case SOME_MESSAGE: INFO* info = *((INFO*)arg2); doSomethingWith(info.field); return MEANINGFUL_ERROR_CODE; // ... } // ...}

Semaphore& s = resourceSemaphore();s.acquire();laboriouslyCopyDataFromResource();s.release();doSomethingWithData();

• Dangerous and hard-to-use message types

• Corruption- and deadlock-prone locks

It doesn’t have to be this way!

A taste of Elixirhttp://elixir-lang.org

Ruby style, Erlang substance

Pattern matching

defmodule Ackermann do def ack(0, n), do: n + 1 def ack(m, 0), do: ack(m - 1, 1) def ack(m, n), do: ack(m - 1, ack(m, n - 1))end

IO.puts Ackermann.ack(3, 9)

Actor-style concurrency

defmodule ShoutyEcho do def echo_loop do receive do {sender, msg} -> sender <- {:ok, String.upcase(msg)} echo_loop end endend

defmodule ShoutyEcho do def echo_loop do receive do {sender, msg} -> sender <- {:ok, String.upcase(msg)} echo_loop end endend

defmodule ShoutyEcho do def echo_loop do receive do {sender, msg} -> sender <- {:ok, String.upcase(msg)} echo_loop end endend

defmodule ShoutyEcho do def echo_loop do receive do {sender, msg} -> sender <- {:ok, String.upcase(msg)} echo_loop end endend

defmodule ShoutyEcho do def echo_loop do receive do {sender, msg} -> sender <- {:ok, String.upcase(msg)} echo_loop end endend

pid = spawn(ShoutyEcho, :echo_loop, [])pid <- {self, "Your name here"}

receive do {:ok, response} -> IO.puts response after 500 -> IO.puts "Done"end

pid = spawn(ShoutyEcho, :echo_loop, [])pid <- {self, "Your name here"}

receive do {:ok, response} -> IO.puts response after 500 -> IO.puts "Done"end

pid = spawn(ShoutyEcho, :echo_loop, [])pid <- {self, "Your name here"}

receive do {:ok, response} -> IO.puts response after 500 -> IO.puts "Done"end

pid = spawn(ShoutyEcho, :echo_loop, [])pid <- {self, "Your name here"}

receive do {:ok, response} -> IO.puts response after 500 -> IO.puts "Done"end

Cucumberlhttps://github.com/membase/cucumberl

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo"

-module(graycode).

-export([given/3, main/0]).

given([the, leds, read, _Input], State, _) -> {ok, State}.

main() -> cucumberl:run("./features/graycode.feature").

$ cucumberl

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo"

1 scenarios1 steps

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo" When I reset the counter

---------NO-STEP--------

a step definition snippet...'when'([i,reset,the,counter], State, _) -> undefined.

---------NO-STEP--------

a step definition snippet...'when'([i,reset,the,counter], State, _) -> undefined.

-module(graycode).

-export([given/3, 'when'/3, main/0]).

%% ...

'when'([i, reset, the, counter], State, _) -> {ok, State}.

%% ...

$ cucumberl

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo" When I reset the counter

1 scenarios2 steps

I know what you’re thinking...

What happens with the next “When” step?

Feature: Gray Code Scenario: ... ... When I reset the counter

Scenario: ... ... When I press the button

Pattern matching!

'when'([i, reset, the, counter], State, _) -> {ok, State};'when'([i, press, the, button], State, _) -> {ok, State}.

$ find . -type f

./ebin/graycode.beam

./features/graycode.feature

./src/graycode.erl

$ find . -type f

./ebin/graycode.beam

./features/graycode.feature

./src/graycode.erl

Just a .beam file!

And now in Elixir!

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo"

-module(graycode).

-export([given/3, main/0]).

given([the, leds, read, _Input], State, _) -> {ok, State}.

main() -> cucumberl:run("./features/graycode.feature").

defmodule :graycode do def given([:the, :leds, :read, input], _state, _) do {:ok, input} end

def main() do :cucumberl.run("./features/graycode.feature") endend

$ iex

c("src/graycode.ex", "ebin")

$ find . -type f

./ebin/graycode.beam

./features/graycode.feature

./src/graycode.ex

$ cucumberl

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo"

1 scenarios1 steps

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo" When I reset the counter

---------NO-STEP--------

a step definition snippet...'when'([i,reset,the,counter], State, _) -> undefined.

---------NO-STEP--------

a step definition snippet...'when'([i,reset,the,counter], State, _) -> undefined.

def 'when'([:i, :reset, :the, :counter], _state, _) do {:ok, '...'}end

⬇

== Compilation error on file src/graycode.ex ==** (SyntaxError) ./src/graycode.ex:6: syntax error before: ')'

How to translate?

Hacking Cucumberl

%% cucumberl_gen.erl

When = process_clauses('when', lists:reverse( sets:to_list(dict:fetch('when', Dict)))),

⬇

When_ = process_clauses(when_, lists:reverse( sets:to_list(dict:fetch(when_, Dict)))),

%% cucumberl_gen.erl

When = process_clauses('when', lists:reverse( sets:to_list(dict:fetch('when', Dict)))),

⬇

When_ = process_clauses(when_, lists:reverse( sets:to_list(dict:fetch(when_, Dict)))),

%% cucumberl_parser.erl

string_to_atoms(StrWords) -> lists:map(fun (Y) -> list_to_atom(string:to_lower(Y)) end, string:tokens(StrWords, " ")).

⬇

list_to_valid_atom("when") -> list_to_atom("when_");list_to_valid_atom(Str) -> list_to_atom(Str).

string_to_atoms(StrWords) -> lists:map(fun (Y) -> list_to_valid_atom(string:to_lower(Y)) end, string:tokens(StrWords, " ")).

%% cucumberl_parser.erl

string_to_atoms(StrWords) -> lists:map(fun (Y) -> list_to_atom(string:to_lower(Y)) end, string:tokens(StrWords, " ")).

⬇

list_to_valid_atom("when") -> list_to_atom("when_");list_to_valid_atom(Str) -> list_to_atom(Str).

string_to_atoms(StrWords) -> lists:map(fun (Y) -> list_to_valid_atom(string:to_lower(Y)) end, string:tokens(StrWords, " ")).

def when_([:i, :press, :the, :button], state, _) do {:ok, _next(state)}end

$ cucumberl

Feature: Gray Code Scenario: Reset Given the LEDs read "ooo" When I reset the counter

1 scenarios2 steps

def then([:the, :leds, :should, :read, expected], state, _) do {expected === state, state}end

Scenario Outline: Counter Given the LEDs read "<n>" When I press the button Then the LEDs should read "<nplus1>" Examples: | n | nplus1 | | ... | ..o | | ..o | .oo | | .oo | .o. | | .o. | oo. | | oo. | ooo | | ooo | o.o | | o.o | o.. | | o.. | ... |

def when_([:i, :press, :the, :button], state, _) do {:ok, _next(state)}end

defp _next(leds) do case leds do '...' -> '..o' '..o' -> '.oo' '.oo' -> '.o.' '.o.' -> 'oo.' 'oo.' -> 'ooo' 'ooo' -> 'o.o' 'o.o' -> 'o..' 'o..' -> '...' endend

Caveats

1. Cucumberl has no idea about the Elixir runtime

2. Who will do whatabout When?

https://github.com/undees/cukeup