Dialyzer says "The pattern false can never match the type true"

What happens if You comment those lines?

      not(is_map(payload))  ->
        IO.puts "\nIgnored: " <> trim_path
        IO.puts "Because: Payload is not a Map."
      %{}

You do mention this…

The input tuple is emitted by Toml.decode/2

So it seems payload will always be a map (or an error, but that is handled before in your condition) from Toml.decode spec

decode(bin, opts \\ [])
decode(binary(), opts()) :: {:ok, map()} | error()

not(is_map(payload)) might never get triggered?

As it happens, I had tried that. The result was that it complained about the last clause, instead of the second clause. FYI, the project is available at GitHub - RichMorin/PA_all: Code and Data files for Pete's Alley · GitHub.

On a vaguely related note, I just realized that functions defined in my test files (eg, foo_test.exs) aren’t being checked by Dialyzer. Is there a Best Practice for getting this to happen?

Test files are just plain elixir scripts, which are executed on demand by the test runner. The only thing different to arbitrary elixir scripts is their folder + naming schema (*_test.exs). They‘re not part of your compiled codebase even when running with MIX_ENV=test, which is why dialyzer doesn‘t act on them.

An addition. If you want to validate code you’re using just in tests modify :elixirrc_paths in your mix.exs (e.g. like phoenix does it) and put the to be checked code in .ex files of test/support. Running dialyzer with the proper mix env should result in that code being checked.

Can confirm this is the blessed way to dialyze your test logic. It unfortunately needs to be in .ex files but most of the time that is sufficient in my experience.

My test support function contains assert\1 calls. How should I make these available to it?

You would move actual test helper logic to the .ex files and the asserts etc would remain in the .exs files.

The support function is a sequence of assert/1 calls. Owell.

In case anyone has a clue to offer; I still need one for the original Dialyzer issue.

-r