Shared Common Constants/Errors Across the Application

apoorv-2204

What is the best way to define Elixir App wide constants and errors?.In languages like go lang we export and import constants and errors.

What is the idiomatic or best, the De facto industry standard of Elixir community or generally design pattern for it?

requirements: we should be able to match them in guards and clauses, it should be as similar to normal variables in elixir function.What if we want to have constants for UI and backend at the same time, Like for an atom UI will show a String, in backend a integer.

ps: I found this article from 2016.

And some examples from timex lib, but cant use them in guard or clause
https://github.com/bitwalker/timex/blob/main/lib/timex/constants.ex

10 comments

#design-patterns #constants

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2023-09-13 13:40:35 UTC

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JEG2

Author of Designing Elixir Systems with OTP

I’m also interested in hearing answers to this, especially for options that don’t spread compile time dependencies far and wide.

Post #1

ImNotAVirus

I had replied to your old post and I’m reposting my reply but with a few additions.

For many projects, I’ve often needed to use constants/enums. So I made a lib for it: SimpleEnum.

I’ll let you read the thread for more details, but to sum up:

Enums can be used in guards
There are no dependencies, as with EctoEnum for example.
the introspection system makes it easy to connect SimpleEnum to other libs

Here is an example from the documentation:

iex> defmodule MyEnums do
...>   import SimpleEnum, only: [defenum: 2]
...>
...>   defenum :color, [:blue, :green, :red]
...>   defenum :day, monday: "MON", tuesday: "TUE", wednesday: "WED"
...> end

iex> require MyEnums

iex> MyEnums.color(:blue)
0
iex> MyEnums.color(0)
:blue
iex> MyEnums.day(:monday)
"MON"
iex> MyEnums.day("MON")
:monday

Post #2

apoorv-2204

Thank you @ImNotAVirus ,
I would like to explore macros.

Post #3

dimitarvp

To be fair I’d just do this:

defmodule MyApp.UnixErrors do
  defmacro __using__(_opts) do
    quote do
      @bad_argument :badarg
      @invalid :einval
    end
  end
end

defmodule MyApp.UserModule1 do
  use MyApp.UnixErrors

  def hello(), do: @bad_argument
end

This too is some level of compile-time dependencies but they are flat, there are no cycles or deep graphs involved.

Post #4

apoorv-2204

able to match them in guards and clauses?

Post #5

dimitarvp

Have you tried?

Post #6

apoorv-2204

yes. I have to assign to temporary variables.And also cant have capital letters

defmodule Nat.Constants do
  @moduledoc false

  defmacro __using__(_) do
    quote do
      @st1 "urn:schemas-upnp-org:device:InternetGatewayDevice:1"
      @igd_device_upnp1 "urn:schemas-upnp-org:device:InternetGatewayDevice:1"
      @wan_device_upnp1 "urn:schemas-upnp-org:device:WANDevice:1"
      # @supported_protocols [Upnpv1, Upnpv2, PMP]

      @discover_timeout 5_000
      # :inet.parse_address('239.255.255.250')
      @multicast_ip {239, 255, 255, 250}
      @multicast_port 1900
      @mx_secs "3"
      @default_mapping_lifetime 7200
      @nat_initial_ms 200
      @default_attempts 3

      @st2 "urn:schemas-upnp-org:device:InternetGatewayDevice:2"
      @igd_device_upnp2 "urn:schemas-upnp-org:device:InternetGatewayDevice:2"
      @wan_device_upnp2 "urn:schemas-upnp-org:device:WANDevice:2"

      require Logger

      @nat_tries 3
      @nat_initial_ms 250
      @lifetime 3600

      @pmp_port 5351
    end
  end
end

Post #7

dimitarvp

Why not? For example these:

…are basically atoms.

Post #8

apoorv-2204

Also cant use them in the match and clauses.

Post #9

dimitarvp

Ah, you mean the module attributes can’t start with a capital letter. Can’t help you there, but I don’t see why would that be a hard requirement for you either.

Post #10

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