Why does Elixir Plug compile plugs into AST using macro?

Question

As the code https://github.com/elixir-plug/plug/blob/v1.5.0/lib/plug/builder.ex#L183 shows, plugs definition will be compiled to AST at macro expansion phase. But why? Why not just keep the plugs definition and use Enum.reduce_while or recursion to call plugs one by one?

Dogbert Dogbert · Accepted Answer · 2018-04-18T16:03:16

Two reasons I can think of:

Performance. Consider these two snippets that do the same thing but one does it using compiled function calls and the other uses Enum.reduce and apply:

defmodule A do
  def add1(x), do: x + 1
  def sub1(x), do: x - 1

  def compiled(x) do
    x
    |> add1()
    |> sub1()
    |> add1()
    |> sub1()
    |> add1()
    |> sub1()
    |> add1()
    |> sub1()
  end

  @pipeline [
    {A, :add1},
    {A, :sub1},
    {A, :add1},
    {A, :sub1},
    {A, :add1},
    {A, :sub1},
    {A, :add1},
    {A, :sub1}
  ]
  def runtime(x) do
    Enum.reduce(@pipeline, x, fn {module, function}, acc ->
      apply(module, function, [acc])
    end)
  end
end

A simple benchmark shows that the runtime implementation is 5 times slower.

IO.inspect(
  :timer.tc(fn ->
    for _ <- 1..1_000_000, do: A.compiled(123)
    :ok
  end)
  |> elem(0)
)

IO.inspect(
  :timer.tc(fn ->
    for _ <- 1..1_000_000, do: A.runtime(123)
    :ok
  end)
  |> elem(0)
)

Output:

82800
433198

Catching bugs at compile time. If you pass a module to plug that doesn't implement call/2, you get an error at compile time instead of a runtime error you'd normally get if you do everything at runtime.

Why does Elixir Plug compile plugs into AST using macro?

1 Answers