/defconst

Helper macros for defining constant values in modules

Primary LanguageElixir

defconstant

Helper macros for defining constants in Elixir code

Installation

def deps do
  [
    {:defconstant, "~> 1.0.0"}
  ]
end

Optionally add :defconstant to your .formatter.exs to have defconst formatted without parens:

[
  import_deps: [
    :defconstant,
    ...
  ],
  ...
]

Usage

This library provides 2 macros:

  • defconst - provided body will be evaluated at compile time
  • defonce - provided body will be evaluated at runtime, but only once. After that it will be cached and served from cache.

Both helper macros allows defining only 0-ary functions (functions that take no arguments).

For details see the full docs on hexdocs.pm

Example usage

defmodule Demo.MyConst do
  import Defconstant

  defconst the_answer do
    42
  end

  # NOTE: For real code you'd use `:math.pi` instead
  defconst pi do
    3.14159
  end

  defonce calculated_at do
    NaiveDateTime.utc_now()
  end

  def run_calulations(circumference) do
    circle_radius = circumference / 2 * pi()
    "radius is #{circle_radius} and was first calculated at #{inspect calculated_at()}"
  end
end

And the constants can be used from another module (e.g. Demo.MyConst.the_answer() will return 42)

Why

Elixir and Erlang/OTP don't have true constants. They do have module attributes, which are often used like constants, but module attributes can be modified so they aren't truly constants. For example:

defmodule ModuleAttributeDemo do
  @pi 3.14159
  def pi, do: @pi

  @pi 33
  def radius(circumference), do: circumference / 2 * @pi
end

Calling ModuleAttributeDemo.radius(5) will use 33 as the value for @pi instead of 3.14159. With defconst this can be avoided because you will receive a warning when re-defining a constant.

License

MIT