It is partially a response to @PragTob’s blogpost

and @michalmuskala’s tweet:

We need proper first-class constants in Erlang that a module can define

It simply allows you to quickly define constants in your modules without resolving to “ugly hacks” in form of module attributes or unquote in “weird places”.

Source:

Hex:

Usage is dumb simple. There are 2 macros provided:

• defconst - for compile time constants
• defonce - for runtime constants - what it means is that the value of this function will be lazily computed during first call and then cached in persistent_term for future uses

Example:

defmodule Foo do
  import Defconstant

  defconst comptime do
    # This will be evaulated at compile time
    Enum.sum([
      0, 2, 3, 4, 5, 6, 8, 12, 30, 32, 33, 34, 35, 36, 38, 40, 42, 43, 44, 45,
      46, 48, 50, 52, 53, 54, 55, 56, 58, 60, 62, 63, 64, 65, 66, 68, 80, 82,
      83, 84, 85, 86, 88
    ])
  end

  defonce runtime do
    2 * 1068 + 1
  end
end

And that is it.

Oh, one (obvious) limitation - it can define only 0-ary functions, as otherwise it would be impossible to have “constants”. If you want to have caching based on arguments, then you need to look up for other libraries.