defconstant - library for defining constants in your module

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1

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.

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:point_up: @jeg2

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Really cool work, thank you!

4

Nice! For anyone super lazy here’s a simple example of using the defined 0-arity functions:

defmodule MyConstants do
  import Defconstant

  defconst the_answer, do: 42

  def my_func do
    the_answer() * 2 # returns 84
  end
end

I’ve also submitted a PR to add more docs and examples.

5 Likes
5

I was puzzled at first by the usefulness of this but after reading the code, I get it and it’s very cool.

I like the doc additions, @axelson. I feel that while getting an actual error over a “won’t match” warning you would get with def is quite useful, I see the bigger value prop of defconst to be that it avoids this problem, so perhaps calling that out as well would be good.

1 Like
6

I also have an idea to provide comptime/const macro that would force compile time evaluation (quite it will be just wrapper on unquote) and once macro. That would allow to write in-function fragments that will have the same semantics as defconst and defonce.

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Ah dang, my bad, I thought it was already doing this. The second part of my previous can be ignored.

8

Top notch. I think this would feel right at home In the standard library.

5 Likes
9

I have added support for private functions (haven’t released version 1.1.0 with that features yet). But it have come to me that there is one behaviour that may be problematic for some people, so I come here with question for anyone interested in this library:

I would like to hear your opinions on that issue.

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FYI a long time ago I had created a similar library for Elixir called ex_const which also included support for enumerated values. There never was much interest, so I didn’t go much further.

7 Likes
11

I guess since the docs pushed to just use module attributes, it’s normal people would just use that, I try to follow conventions over libraries unless there is a clear gap in the std. :slight_smile:
But your library is sound!