Power Assert Inside in Elixir

self-introduction • Takayuki Matsubara • ma2gedev @github • ma2ge @twitter • Application Engineer @ M3, Inc. • Rails/Java/JavaScript application • Ruby: breadcrumble, chrono_logger, bundle-star • Elixir: power_assert_ex • HR7@MHX

precondition Understood • the basic syntax of Elixir • about Macro

github.com/ma2gedev ⭐

What is Power Assert?

Test code test "Enum.at should return the element at the given index" do array = [1, 2, 3, 4, 5, 6]; index = 2; two = 2 assert array |> Enum.at(index) == two end

in ExUnit 1) test Enum.at should return the elem... test/power_assert_sample_test.exs:5 Assertion with == failed code: array |> Enum.at(index) == two lhs: 3 rhs: 2 stacktrace: test/power_assert_sample_test.exs:7

in Power Assert 1) test Enum.at should return the elem... test/power_assert_sample_test.exs:5 array |> Enum.at(index) == two | | | | | 3 2 2 [1, 2, 3, 4, 5, 6] stacktrace: test/power_assert_sample_test.exs:7

failed results with descriptive message

• Power Assert (Groovy, Spock framework) • power-assert-js (JavaScript) • power_assert (Ruby) • power-assert-rs (Rust) • vital-power-assert (Vim script) • PowerAssert.Net (.Net) • power_assert.cr (Crystal) • PAssert (Swift) • PowerAssertEx (Elixir)

• Power Assert (Groovy, Spock framework) • power-assert-js (JavaScript) • power_assert (Ruby) • power-assert-rs (Rust) • vital-power-assert (Vim script) • PowerAssert.Net (.Net) • power_assert.cr (Crystal) • PAssert (Swift) • PowerAssertEx (Elixir)

PowerAssertEx Concept • Simple - provide only assert method • Reversible - can go back to ExUnit

PowerAssertEx |> expose

Point # test code(call `assert` method same as ExUnit) assert [1, 2, 3] |> Enum.reverse() |> Enum.sum() == 5 # output(need positions and values) [1, 2, 3] |> Enum.reverse() |> Enum.sum() == 5 | | [3, 2, 1] 6

tl;dr • assert • get positions and values • output

tl;dr • assert • get positions and values • output

review about Elixir's macro

how to handle AST defmacro assert(ast) do # receive an elixir's Abstract Syntax Tree end

AST in Elixir iex> quote do: sum(1, 2) {:sum, [], [1, 2]} iex> quote do: x {:x, [], Elixir} iex> quote do: Enum.sum(1, 2) {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :sum]}, [], [1, 2]}

change AST into string of code iex> Macro.to_string(quote do: Enum.sum(1, 2)) "Enum.sum(1, 2)"

Positions and Values

Positions [1, 2, 3] |> Enum.reverse() |> Enum.sum() | | [3, 2, 1] 6

How to detect • traverse AST for collecting position each expression • detect position • convert each expression's AST to string of code • detect position with above string of code

target iex> quote do: [1, 2, 3] |> Enum.reverse() |> Enum.sum() {:|>, [context: Elixir, import: Kernel], [{:|>, [context: Elixir, import: Kernel], [[1, 2, 3], {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]}, [], []}]}, {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :sum]}, [], []}]}

traverse Macro.prewalk(ast, acc, fun) Macro.postwalk(ast, acc, fun) Macro.traverse(ast, acc, pre, post) # traverse/4 enabled after Elixir v1.2.0 `fun` is function and receives AST each expression

Enum.sum(1,2,3)'s AST

prewalk

postwalk

traverse with Macro.postwalk iex> Macro.postwalk(quote(do: [1, 2, 3] |> Enum.reverse() |> Enum.sum()), fn(ast) -> IO.inspect ast end) 1 2 3 [1, 2, 3] :Enum {:__aliases__, [alias: false], [:Enum]} :reverse {:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]} {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]}, [], []} {:|>, [context: Elixir, import: Kernel], [[1, 2, 3], {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]}, [], []}]} :Enum {:__aliases__, [alias: false], [:Enum]} :sum {:., [], [{:__aliases__, [alias: false], [:Enum]}, :sum]} {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :sum]}, [], []} {:|>, [context: Elixir, import: Kernel], [{:|>, [context: Elixir, import: Kernel], [[1, 2, 3], {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]}, [], []}]}, {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :sum]}, [], []}]}

filtered by pattern match # we need function call {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]}, [], []} # |> Macro.to_string # "Enum.reverse()" # disregard [1, 2, 3]

convert AST to string of code and find position with regex code = {{:., [], [{:__aliases__, [alias: false], [:Enum]}, :reverse]}, [], []} |> Macro.to_string # "Enum.reverse()" # find position with `Regex.scan` by `code`

Values [1, 2, 3] |> Enum.reverse() |> Enum.sum() | | [3, 2, 1] 6

How to collect values • traverse AST • inject an AST to collect values • collect values at runtime

inject an AST Macro.postwalk(ast, acc, fun) • fun function can return modified AST

inject an AST to collect values # before Enum.reverse([1, 2, 3]) # after injected an AST ( v = Enum.reverse([1, 2, 3]) Agent.update(buffer, &[[pos, v] | &1]) v )

Agent!!! # after injected an AST {:ok, buffer} = Agent.start_link(fn -> [] end) ... ( v = Enum.reverse([1, 2, 3]) Agent.update(buffer, &[[pos, v] | &1]) v ) ... values = Agent.get(buffer, &(&1)) Agent.stop(buffer)

inject an AST when |> operator # before [1, 2, 3] |> Enum.reverse # after injected an AST [1, 2, 3] |> ( v = Enum.reverse # <= Does not work!!! Agent.update(buffer, &[[pos, v] | &1]) v )

modify an AST when |> operator # before [1, 2, 3] |> Enum.reverse # after injected an AST l_value = [1, 2, 3] ( v = Enum.reverse(l_value) # <= inject `l_value` # into first argument of rhs Agent.update(buffer, &[[pos, v] | &1]) v )

Positions and Values

summary • traverse AST • analyze AST and detect position of expression • inject AST to collect value and get result of expression

⌚ hereafter • please use power assert • welcome any feedback

! Resources - PowerAssertEx • PowerAssertEx • github.com/ma2gedev/powerassertex • hex.pm/packages/power_assert

! Resources - Books • Programming Elixir • pragprog.com/book/elixir/programming- elixir • Metaprogramming Elixir • pragprog.com/book/cmelixir/ metaprogramming-elixir • defmacro • leanpub.com/defmacro

! Resources - Web • macro with elixir • www.slideshare.net/k1complete/elixir- macroinaction1-14083087 • Understanding Elixir Macros, Part 1 - Basics • theerlangelist.com/article/macros_1

! Resources - Web • power-assert, mechanism and philosophy • www.slideshare.net/t_wada/power-assert- nodefest-2014 • Power Assert in Ruby • speakerdeck.com/k_tsj/power-assert-in- ruby

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