Retrieving application version number in mix.exs from git describe

mix.exs

defmodule MyApp.Mixfile do
  use Mix.Project

  def project do
    [app: :my_app,
     version: get_version,
     elixir: "~> 1.0",
     elixirc_paths: elixirc_paths(Mix.env),
     compilers: Mix.compilers,
     build_embedded: Mix.env == :prod,
     start_permanent: Mix.env == :prod,
     deps: deps]
  end

  # Configuration for the OTP application
  #
  # Type `mix help compile.app` for more information
  def application do
    [mod: {MyApp, []},
     applications: [:logger]]
  end

  defp get_version do
    version_from_file
    |> handle_file_version
    |> String.split("-")
    |> case do
      [tag] -> tag
      [tag, _num_commits, commit] -> "#{tag}-#{commit}"
    end
  end

  defp version_from_file(file \\ "VERSION") do
    File.read(file)
  end

  defp handle_file_version({:ok, content}) do
    content
  end
  defp handle_file_version({:error, _}) do
    retrieve_version_from_git
  end
  defp retrieve_version_from_git do
    require Logger
    Logger.warn "Calling out to `git describe` for the version number. This is slow! You should think about a hook to set the VERSION file"
    System.cmd("git", ["describe", "--always", "--tags"]) 
    |> Tuple.to_list 
    |> List.first 
    |> String.strip
  end

  # Specifies which paths to compile per environment
  defp elixirc_paths(:test), do: ["lib", "web", "test/support"]
  defp elixirc_paths(_),     do: ["lib", "web"]

  # Specifies your project dependencies
  #
  # Type `mix help deps` for examples and options
  defp deps do
    [
   ]
  end
end

post-checkout

#!/bin/bash
# installed to .git/hooks/post-checkout
`git describe --always --tags > VERSION`

post-commit

#!/bin/bash
# installed to .git/hooks/post-commit
`git describe --always --tags > VERSION`