Mix.escript.install and config/config.exs

Questions / Help
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Hi all!

I am fairly new to the elixir ecosystem and I’m working on my first library, weather. It’s a simple library that fetches and displays weather information, and it’s packaged as an escript.

In order to support displaying times in the correct zone for the weather data, I’ve added:

config :elixir, :time_zone_database, Tz.TimeZoneDatabase

to my config/config.exs.

Something strange I’ve noticed: when I install the escript via hex, the times are returned in UTC which leads me to believe the config.exs is not being evaluated.

$ mix escript.install hex weather 0.3.7
$ weather --test clear

🌞 10:17AM | 🌚 1:25AM
# THESE ARE UTC TIMES. THEY SHOULD BE 3PM, 6PM, 9PM,12AM,3AM
76°  ⬇   74°  ⬇   64°  ⬇   60°  ⬇   58°
8PM      11PM     2AM      5AM      8AM

77° | scattered clouds | 37% humidity

However, when installing via github, the times are reported in their correct zone, leading me to believe the config.exs is being evaluated in that case:

$ mix escript.install github spencerolson/weather tag v0.3.7
$ weather --test clear

🌞 5:17AM | 🌚 8:25PM

# THESE TIMES ARE IN THE CORRECT ZONE
76°  ⬇   74°  ⬇   64°  ⬇   60°  ⬇   58°
3PM      6PM      9PM      12AM     3AM

77° | scattered clouds | 37% humidity

Can someone help me understand what’s going on? Is it expected that I’d get different results when installing the escript from different locations? I’ve also tried installing without specifying a tag, and I run into the same issues.

Thanks in advance!

2

To add a bit more: I understand that in general I should avoid using Application configuration for libraries, however I’m not sure how else I’m going to ensure a timezone database is configured for Elixir when this library is run as an escript.

As an alternative, it looks like the Time zone database docs suggests I can call

Calendar.put_time_zone_database(Tz.TimeZoneDatabase)

which I think would solve my problem, however:

  1. I’m not sure where it would make sense to put that call
  2. That method internally calls Application.put_env(:elixir, :time_zone_database, database) and I’m not sure if it’s a good idea to be modifying the Application config at runtime (I’m guessing the answer is “no”).
3

Think I found a solution that I’m happy with, and I can do away with having to tinker with the Application configuration :slight_smile:

I’ll change my call to DateTime.shift_zone/3 here to make use of the third argument I can pass which specifies a Time zone database :slight_smile: DateTime — Elixir v1.17.3

And to answer my own question about the differences between github + hex, it appears that on hex the config.exs file isn’t present (which makes sense, because configs of dependencies are not used so it has no reason to store them)

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4

For posterity, here’s the change I ended up with. If there’s no timezone configured, use Tz.TimeZoneDatabase, otherwise use the already-configured timezone database.

And i was able to simplify installation instructions and get rid of an Application config value :partying_face:

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