So I wrote the following simple Elixir script that uses benchee. I did not give it much thought to be honest. Let me know what you think.

TLDR; Performance for both approaches indeed seem to be equivalent.

Script

defmodule Test do
  @numbers Range.to_list(1..1000)

  def work(input, fun) do
    Enum.map(input, fun)
  end

  for number <- @numbers do
    def do_work_generated(unquote(number)), do: unquote(number)
  end

  def do_work_guards(number) when number in @numbers, do: number
end

Mix.install([:benchee, :benchee_markdown])
list_of_random_numbers = fn -> Enum.map(1..500_000, fn _ -> :rand.uniform(1000) end) end

inputs =
  list_of_random_numbers
  |> Stream.repeatedly()
  |> Stream.take(5)
  |> Stream.with_index()
  |> Stream.map(fn {numbers, idx} -> {"Batch #{idx + 1}", numbers} end)

Benchee.run(
  %{
    "Generated Functions" => fn input -> Test.work(input, &Test.do_work_generated/1) end,
    "Guards" => fn input -> Test.work(input, &Test.do_work_guards/1) end
  },
  memory_time: 60,
  inputs: inputs,
  formatters: [
    {Benchee.Formatters.Markdown, file: "BENCHMARK.md"},
    Benchee.Formatters.Console
  ]
)

Output

Configuration

Benchmark suite executing with the following configuration:

Statistics

Input: Batch 1

Run Time

Run Time Comparison

Memory Usage

Input: Batch 2

Run Time

Run Time Comparison

Memory Usage

Input: Batch 3

Run Time

Run Time Comparison

Memory Usage

Input: Batch 4

Run Time

Run Time Comparison

Memory Usage

Input: Batch 5

Run Time

Run Time Comparison

Memory Usage

in/2 doesn’t compile to an Enum.member?/2 call when used in a guard:

You can use GitHub - michalmuskala/decompile · GitHub to take a look how things will look at lower levels.