Dialyzer error when checking Mix task

ijdickinson

I have a Mix task that extracts the version number of the project (for use in a build pipeline):

defmodule Mix.Tasks.VersionNumber do
  @moduledoc "Mix task to print the current version number to stdout"
  use Mix.Task

  @shortdoc "Print the current application version number"
  def run(_) do
    MyApp.MixProject.project |> Keyword.fetch!(:version) |> IO.puts
  end
end

This all works fine. However, when I run mix dialyzer on my project, I get an error:

$ mix dialyzer
Compiling 1 file (.ex)
Finding suitable PLTs
Checking PLT...
[:asn1, :castore, :certifi, :compiler, :connection, :cowboy, :cowboy_telemetry, :cowlib, 
:crypto, :dart_sass, :db_connection, :decimal, :ecto, :ecto_sql, :eex, :elixir, :esbuild, 
:expo, :file_system, :finch, :geo, :geo_postgis, :gettext, :hackney, :heroicons, :hpax, 
:httpoison, :idna, :jason, :kernel, :logger, :metrics, :mime, :mimerl, :mint, :mix, 
:nimble_options, :nimble_pool, :parse_trans, :phoenix, :phoenix_copy, :phoenix_ecto, 
:phoenix_html, :phoenix_live_dashboard, :phoenix_live_reload, :phoenix_live_view, 
:phoenix_pubsub, :phoenix_template, :plug, :plug_cowboy, ...]
PLT is up to date!
No :ignore_warnings opt specified in mix.exs and default does not exist.

Starting Dialyzer
[
  check_plt: false,
  init_plt: '/home/ian/projects/verna/myapp/_build/dev/dialyxir_erlang-25.1.2_elixir-1.14.2_deps-dev.plt',
  files: ['/home/ian/projects/verna/myapp/_build/dev/lib/myapp/ebin/Elixir.MyApp.LocationSearch.SearchResult.beam',
   '/home/ian/projects/verna/myapp/_build/dev/lib/myapp/ebin/Elixir.MyApp.beam',
   '/home/ian/projects/verna/myapp/_build/dev/lib/myapp/ebin/Elixir.MyApp.Application.beam',
   '/home/ian/projects/verna/myapp/_build/dev/lib/myapp/ebin/Elixir.MyApp.ReferenceData.LandRegistry.beam',
   '/home/ian/projects/verna/myapp/_build/dev/lib/myapp/ebin/Elixir.MyApp.Gettext.beam',
   ...],
  warnings: [:unknown]
]
Total errors: 1, Skipped: 0, Unnecessary Skips: 0
done in 0m4.56s
lib/mix/tasks/version_number.ex:7:unknown_function
Function MyApp.MixProject.project/0 does not exist.
________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2

At a guess, the problem is maybe related to project/0 being defined in an .exs file rather than .ex? Is there a way to either ignore the mix task in the Dialyzer analysis, or, better, help it to find the definition of project/0?

4 comments

#mix #dialyzer

1 1673 4

2024-10-24 20:42:49 UTC

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Billzabob

I’m having the same problem, did you ever find a solution?

Post #1

ijdickinson

We switched approach, so we dropped this task from our project.exs.

However, we did find ourselves having to ignore some other Dialyzer errors. The pattern for that is to create a ./.dialyzer-ignore.exs file, using the pattern:

[
{path, error},
]

So in theory, this should work:

# .dialyzer-ignore.exs
[
{"./project.exs", :unknown_function}
]

But I’ve not tested it, so ymmv.

Good luck!
Ian

Post #2

al2o3cr

Dialyzer depends on the compiler, so it’s definitely not going to see modules defined in .exs files.

That file will be loaded by Mix at runtime before running the task, so it doesn’t actually fail.

A better approach might be to use the built-in Mix machinery, specifically Mix.Project.config/0:

defmodule Mix.Tasks.VersionNumber do
  @moduledoc "Mix task to print the current version number to stdout"
  use Mix.Task

  @shortdoc "Print the current application version number"
  def run(_) do
    Mix.Project.config() |> Keyword.fetch!(:version) |> IO.puts
  end
end

Post #3

kenny-evitt

This answer on the following Stack Overflow question seems to have worked to fix similar errors for one of my own projects:

elixir - How to fix Dialyzer “Callback info about the ‘…’ behaviour is not available” error for new Mix.Tasks - Stack Overflow

The answer states that you can add the following to (the return value of) the project/0 function in your project Mix module file (i.e. mix.exs) – or add :mix to the list if the key-value pair already exists:

      dialyzer: [plt_add_apps: [:mix]]

Example project/0 function:

  def project do
    [
      app: :some_app,
      version: "0.1.0",
      elixir: "~> 1.6",
      start_permanent: Mix.env() == :prod,
      deps: deps(),
      # Added following line:
      dialyzer: [plt_add_apps: [:mix]]
    ]
  end

Post #4

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