Cannot start observer: (UndefinedFunctionError) function :observer.start/0 is undefined

This looks like an issue caused by code path pruning introduced in Elixir 1.15.

if you have an application or dependency that does not specify its dependencies on Erlang and Elixir application, it may no longer compile successfully in Elixir v1.15

The fix is to add :observer and :wx to your list of :extra_applications in mix.exs.

It’s a bit strange that we need to opt-in to something in a mix project that’s already available by default in vanilla iex, though.

Done, but :observer.open() results in the following error:

{:error,
 {:undef,
  [
    {:observer_backend, :sys_info, [], []},
    {:observer_sys_wx, :init, 1, [file: ~c"observer_sys_wx.erl", line: 50]},
    {:wx_object, :init_it, 6, [file: ~c"wx_object.erl", line: 404]},
    {:proc_lib, :init_p_do_apply, 3, [file: ~c"proc_lib.erl", line: 240]}
  ]}}

Any idea why?

Should be :observer.start().

I mistyped the command, I actually used :observer.start() and got the error message…

It looks like you need :observer, :wx, and :runtime_tools in your :extra_applications

saved me hehe, I wonder why after upgrading to 1.15 :observer.start` is undefined now it work

Correct. I also submitted a bug report upstream.

Seems that not only :observer is an issue.
After upgrading to elixir 1.15 and rebuilding project dialyzer stopped working as well.

mix dialyzer now gives this:

Error loading poison, dependency list may be incomplete.
 {~c"no such file or directory", ~c"poison.app"}
Error loading table, dependency list may be incomplete.
 {~c"no such file or directory", ~c"table.app"}
Error loading myxql, dependency list may be incomplete.
 {~c"no such file or directory", ~c"myxql.app"}
Error loading tds, dependency list may be incomplete.
 {~c"no such file or directory", ~c"tds.app"}
Error loading bandit, dependency list may be incomplete.
 {~c"no such file or directory", ~c"bandit.app"}
Error loading ecto_psql_extras, dependency list may be incomplete.
 {~c"no such file or directory", ~c"ecto_psql_extras.app"}
Error loading ecto_mysql_extras, dependency list may be incomplete.
 {~c"no such file or directory", ~c"ecto_mysql_extras.app"}
Error loading ecto_sqlite3_extras, dependency list may be incomplete.
 {~c"no such file or directory", ~c"ecto_sqlite3_extras.app"}
[:arangox, :asn1, :bandit, :benchee, :broadway, :broadway_kafka, :brod, :castore, :certifi, :compiler, :core, :cowboy, :cowboy_telemetry, :cowlib, :crc32cer, :crypto, :db_connection, :decimal, :deep_merge, :ecto, :ecto_mysql_extras, :ecto_psql_extras, :ecto_sql, :ecto_sqlite3, :ecto_sqlite3_extras, :eex, :elixir, :ex_json_schema, :ex_raconteur, :expo, :exqlite, :faker, :fastglobal, :gen_stage, :gettext, :gun, :hackney, :hammox, :hpax, :httpoison, :idna, :jason, :joken, :jose, :json_logic, :kafka_protocol, :kernel, :logger, :metrics, :mime, ...]
Looking up modules in dialyxir_erlang-26.0.2_elixir-1.15.2_deps-dev.plt
** (UndefinedFunctionError) function :dialyzer_plt.plt_kind/1 is undefined (module :dialyzer_plt is not available)
    (dialyzer 5.1) :dialyzer_plt.plt_kind(~c"/*******project_path/_build/dev/dialyxir_erlang-26.0.2_elixir-1.15.2_deps-dev.plt")
    (dialyzer 5.1) dialyzer.erl:294: :dialyzer.plt_info/1
    (dialyxir 1.3.0) lib/dialyxir/plt.ex:243: Dialyxir.Plt.plt_files/1
    (dialyxir 1.3.0) lib/dialyxir/plt.ex:16: Dialyxir.Plt.find_plts/3
    (dialyxir 1.3.0) lib/mix/tasks/dialyzer.ex:254: Mix.Tasks.Dialyzer.check_plt/1
    (dialyxir 1.3.0) lib/mix/tasks/dialyzer.ex:174: Mix.Tasks.Dialyzer.run/1
    (mix 1.15.2) lib/mix/task.ex:447: anonymous fn/3 in Mix.Task.run_task/5
    (mix 1.15.2) lib/mix/cli.ex:92: Mix.CLI.run_task/2

After downgrading back to Elixir 1.14, everything works like a charm.

Checking PLT...
[:arangox, :asn1, :benchee, :broadway, :broadway_kafka, :brod, :castore, :certifi, :compiler, :core, :cowboy, :cowboy_telemetry, :cowlib, :crc32cer, :crypto, :db_connection, :decimal, :deep_merge, :ecto, :ecto_sql, :ecto_sqlite3, :eex, :elixir, :ex_json_schema, :ex_raconteur, :expo, :exqlite, :faker, :fastglobal, :gen_stage, :gettext, :hackney, :hammox, :hpax, :httpoison, :idna, :jason, :joken, :jose, :json_logic, :kafka_protocol, :kernel, :logger, :metrics, :mime, :mimerl, :mint, :mox, :nanoid, :nimble_options, ...]
Looking up modules in dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
Looking up modules in dialyxir_erlang-26.0_elixir-1.14.5.plt
Looking up modules in dialyxir_erlang-26.0.plt
Finding applications for dialyxir_erlang-26.0.plt
Finding modules for dialyxir_erlang-26.0.plt
Creating dialyxir_erlang-26.0.plt
Looking up modules in dialyxir_erlang-26.0.plt
Removing 3 modules from dialyxir_erlang-26.0.plt
Checking 18 modules in dialyxir_erlang-26.0.plt
Adding 190 modules to dialyxir_erlang-26.0.plt
done in 0m7.59s
Finding applications for dialyxir_erlang-26.0_elixir-1.14.5.plt
Finding modules for dialyxir_erlang-26.0_elixir-1.14.5.plt
Copying dialyxir_erlang-26.0.plt to dialyxir_erlang-26.0_elixir-1.14.5.plt
Looking up modules in dialyxir_erlang-26.0_elixir-1.14.5.plt
Checking 208 modules in dialyxir_erlang-26.0_elixir-1.14.5.plt
Adding 253 modules to dialyxir_erlang-26.0_elixir-1.14.5.plt
done in 0m7.08s
Finding applications for dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
Finding modules for dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
Copying dialyxir_erlang-26.0_elixir-1.14.5.plt to dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
Looking up modules in dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
Checking 461 modules in dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
Adding 2046 modules to dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt
done in 0m36.32s
No :ignore_warnings opt specified in mix.exs. Using default: .dialyzer_ignore.exs.

Starting Dialyzer
[
  check_plt: false,
  init_plt: '/Users/konstantinzolotarev/Coding/squillo/ex_raconteur_umbrella/_build/dev/dialyxir_erlang-26.0_elixir-1.14.5_deps-dev.plt',
  files: ['/Users/konstantinzolotarev/Coding/squillo/ex_raconteur_umbrella/_build/dev/lib/ex_raconteur/ebin/Elixir.ExRaconteur.Struct.CollectorStep.beam',
   '/Users/konstantinzolotarev/Coding/squillo/ex_raconteur_umbrella/_build/dev/lib/ex_raconteur/ebin/Elixir.ExRaconteur.Topic.BroadwayConsumer.State.beam',
   '/Users/konstantinzolotarev/Coding/squillo/ex_raconteur_umbrella/_build/dev/lib/spec/ebin/Elixir.ExRaconteur.Spec.Struct.Consumer.beam',
   '/Users/konstantinzolotarev/Coding/squillo/ex_raconteur_umbrella/_build/dev/lib/ex_raconteur_web/ebin/Elixir.ExRaconteurWeb.Endpoint.beam',
   '/Users/konstantinzolotarev/Coding/squillo/ex_raconteur_umbrella/_build/dev/lib/ex_raconteur/ebin/Elixir.ExRaconteur.Explainer.beam',
   ...],
  warnings: [:unknown]
]
Total errors: 3, Skipped: 0, Unnecessary Skips: 9
done in 0m3.45s
lib/ex_raconteur/file_service/adapter/behaviour.ex:11:unknown_type
Unknown type: Core.child_spec/0.
________________________________________________________________________________
lib/ex_raconteur/file_service/adapter/behaviour.ex:11:unknown_type
Unknown type: FileService.t/0.
________________________________________________________________________________
lib/mix/tasks/arango.create.collections.ex:1:callback_info_missing
Callback info about the Mix.Task behaviour is not available.
________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2

That’s a Dialyxir issue and there are open PRs under consideration!

After adding :observer, :wx and :runtime_tools to :extra_application I get:

[notice] Application runtime_tools exited: :stopped
** (Mix) Could not start application observer: could not find application file: observer.app