Constants and Enumerated Values for Elixir

Overview

This module adds support for a const macro that exports single constants and an enum macro that exports enumerated constant values from a module. These values can be used in guards, match expressions or within normal expressions, as the macro takes care of expanding the reference to the constant or enumerated value to its corresponding literal value or function call, depending on the context where it was used.

A module using const or enum macros can be defined in the following way:

defmodule Settings
  use Const
  import Bitwise, only: [bsl: 2]

  @ar "AR"
  @it "IT"
  @us "US"

  const version, do: "1.0"

  const base_path, do: System.cwd()

  const country_codes, do: [@ar, @it, @us]

  enum country_code, do: [argentina: @ar, italy: @it, usa: @us]

  enum color do
    red   bsl(0xff, 16)
    green bsl(0xff, 8)
    blue  bsl(0xff, 0)
  end

  enum color_tuple do
    red   {255, 0, 0}
    green {0, 255, 0}
    blue  {0, 0, 255}
  end
end

As you can see, the constants can be assigned both literal values or expressions that will be resolved at compile-time.

Single Constants

You can create single constant values by using the const macro with the following syntax:

const <name>, do: <value>

e.g.

const version, do: "1.0"

The macro invocation will create and export another macro with the name that was set in the const declaration (e.g. version/0) and replace each reference to it with the value that was assigned to it (e.g. "1.0").

You can use any expression that can be resolved at compile-time as the value for the const.

The single constants can be accessed with a nomal function invocation:

require Settings
Settings.version

As the reference to the const will be replaced by its literal value, you can even use them in match expressions or in guards. e.g.

require Settings
Settings.version = "1.0"

Enumerated Values

You can create enumerated values by using the enum macro with the compact syntax:

enum <name>, do: [<key_1>: <value_1>, <key_2>: <value_2>, ...]

Or with the expanded syntax:

enum <name> do
  <key_1> <value_1>
  <key_2> <value_2>
  [...]
end

e.g.

enum country_code, do: [argentina: "AR", italy: "IT", usa: "US"]

Or:

enum country_code do
  argentina "AR"
  italy     "IT"
  usa       "US"
end

For each enum instance, the macro will create the following additional macros and functions in the module where it was invoked:

1. Macro with the name that was assigned to the enum. This macro will replace every reference to itself with its literal value (if it was called with a literal atom as key or was referenced from a match expression) or with a call to the fallback function.
2. Fallback function with a name formed by appending the string _enum to the name of the enum (e.g. country_code_enum/1).
3. Function that will retrieve the key corresponding to a value in the enum. If there are is more than one key with the same value, the first in the enum will be used and the other ones will be disregarded.

e.g.

defmacro country_code(atom) :: String.t
def country_code_enum(atom) :: String.t
def from_country_code(String.t) :: atom

The enumerated values can be accessed with a function call:

require Settings
Settings.color(:blue)

And can also be used in match expressions or guards:

require Settings
import Settings
value = "AR"
case value do
  country_code(:argentina) ->
    {:ok, "Argentina"}
  country_code(:italy) ->
    {:ok, "Italy"}
  code when code == country_code(:usa) ->
    {:ok, "United States"}
  _ ->
    {:error, {:must_be_one_of, country_codes()}}
end

As the expressions assigned to constants will be resolved at compile-time, the previous function would be equivalent to the following one:

value = "AR"
case value do
  "AR" -> {:ok, "Argentina"}
  "IT" -> {:ok, "Italy"}
  code when code == "US" -> {:ok, "United States"}
  _ -> {:error, {:must_be_one_of, ["AR", "IT", "US"]}}
end

Sometimes, when an enum is referenced in the code, the key to its value is passed as an expression that cannot be resolved at compile-time. In those cases the expression will be expanded to a function invocation instead of to a literal value:

require Settings
key = :green
Settings.color_tuple(key)

This works because the macro replaces the reference to itself with a call to the fallback function. The name of the function is that of the enum with the _enum string appended to it. For example, for an enum named country the function will be country_enum/1. You have to keep this in mind when you import the module where the enum was defined and restrict the functions that are imported.