Turn functions into constants at compile-time. Very useful when following the patterns set forth in the ElixirConf Growing Applications and Taming Complexity talk.

Just add constantizer to your list of dependencies.

def deps do
  [{:constantizer, "~> 0.2.0"}]
end

Constantizer allows you to evaluate a 0-arity function at compile time, instead of repeatedly evaluating it at runtime. This is very helpful when defining a public api module with swappable backends.

defmodule MyApp do
  import Constantizer

  def register_user(params) do
    backend().register_user(params)
  end

  defconstp backend do
    Application.get_env(:my_app, :backend)
  end
end

By default, defconst and defconstp will define a public or private function, respectively, with its return value being set to the result of the block as it exists at compile time.

To allow a constant to be evaluated at runtime (for example, so you can inject a mock backend in your test environment), modify the following setting in your config.

# config/test.exs

config :constantizer, resolve_at_compile_time: false

You can run constantizer's benchmarks with the following command.

mix run benchmarks/general.exs

Here is some sample output from our benchmarks that call the following module

defmodule MyModule do
  import Constantizer

  def runtime_lookup do
    Application.get_env(:constantizer, :foo)
  end

  defconst compile_time_lookup do
    Application.get_env(:constantizer, :foo)
  end
end

Name                          ips        average  deviation         median         99th %
compile_time_lookup       87.43 M      0.0114 μs    ±17.43%      0.0110 μs      0.0130 μs
runtime_lookup             4.62 M        0.22 μs    ±41.15%        0.21 μs        0.40 μs

Comparison:
compile_time_lookup       87.43 M
runtime_lookup             4.62 M - 18.93x slower

The compile time lookup using defconst is 18x faster than using a normal def because Application.get_env/2 is evaluated at compile time instead of runtime.

The docs for this project are available on hexdocs.