Empty attribute value in release

Questions / Help
1

I have a weird issue; I compute a value read form the Application and store it as an attribute.

So I have a function in Tools:

# Module Tools
def current_version(),
    do: Application.spec(:logfwd, :vsn) |> to_string()

The key :vsn obviously exists, or spec() would crash.

In a different module I have:

# module Apis
@version Tools.current_version()

def dump_version(), do: @version

In release mode, on my machine (Max, Elixir 1.13.4) I call dump_version() and I get “0.7.14”, while on a Linux server (release compiled locally, same sources) I get

iex(logfwd@bqm1)7> Apis.dump_version
""
iex(logfwd@bqm1)8> Tools.current_version
"0.7.14"

What gives? I’m puzzled.

Edit: I see that spec() says that it will return nil if the application is not loaded, but why should then return the correct value (and not nil) on my development box?

2

The second one is from an attribute and evaluated at compile-time, think of @ attributes as replacement of the value it contains in the code, while your first variant is at runtime.

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3

Yes that was the idea - evaluate once.

4

It’s not about only evaluate once, it’s about when to evaluate. Attributes are evaluated at compile-time, in this case the application logfwd is not loaded and you cannot get its version, I think you can load it manually though.

2 Likes
5

In the end I was able to fetch it trough:

       Mix.Project.config()[:version]

And this always works - I’m still puzzled as why the applications seems to be loaded in one case an not the other.

6

If logfwd is the name of your app, then it makes perfect sense, it can’t display your application version at compile-time as it is not compiled yet.

One thing you have to be careful with: mix will not be available once you release the application, so the.call has to be evaluated at compile-time!

If you don’t understand this behaviour yet, read about elixir metaprogramming, it is a rather complex subject and I wish there was more accesible material about it, the way I learned and understood it is by reading Metaprogramming by Chris McCord, a very good learning resource.

7

Yes - I wonder why it works at all on my machine, in a release. I would say it was not supposed to work - the application is just not there.

Thanks for the pointer - I did read that book myself and have worked with Clojure for 10 years or so, so I think I’m pretty familiar with macros and macroexpansion.

1 Like
8

Have you used the same elixir/otp versions? maybe there were some internal changes in config loading in newer versions.

9

In fact it works on 1.13.4/OTP25 (Mac) but it does not on 1.13.4/OTP24 (Linux)

1 Like