Dialyzer does not catch errors on returned functions

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I deeply respect your participation in this thread, but please forgive me if I am not yet convinced.
I am somewhat familiar with success typing:

From Type Specifications and Erlang:

The other option is then to have a type system that will not prove the absence of errors, but will do a best effort at detecting whatever it can. You can make such detection really good, but it will never be perfect. It’s a tradeoff to be made.

I understand dialyzer is not meant to be perfect. I am not after that here. I am trying to understand what classes of issues I can trust dialyzer.

If I understand (please forgive me if I don’t) you are defending the claim that dialyzer does not check function bodies. This is a falsifiable claim. Let’s see if I can falsify it:

defmodule Test do

  @type option(t) :: some(t) | nothing
  @type some(t) :: [t]
  @type nothing :: []

  @spec validate_name(String.t()) :: option(String.t())
  def validate_name(name) do
    if String.length(name) > 0 do
      [name]
    else
      nil
    end
  end
end

Will return an error if run with mix dialyzer --overspecs.
A very good one also:

Total errors: 1, Skipped: 0, Unnecessary Skips: 0
done in 0m0.88s
lib/test.ex:6:missing_range
The type specification is missing types returned by function.

Function:
Test.validate_name/1

Type specification return types:
[{:some, binary()}]

Missing from spec:
nil

This class of error would only be possible if dialyzer analyses the body of my functions, which is happening (it is checking my function returns nil and my spec does not cover that).

Now, I claim by no means to be an expert in dialzyer, and it may very well be that using different flags completely and totally changes the default algorithm in unimaginable ways.

But running this thread’s original sample with either --underspecs or --overspecs won’t make a difference.

If I can’t understand what I can trust Dialzyer with, my trust in this tool will be very limited. While gradient is a good idea, it is still, IMHO, very green and needs more time in the oven.

That is why I created this thread. I can’t understand what is happening. I need help.

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