Dialyzer and telling me to match a type I AM matching

Hello everyone,

dialyzer and I are fighting again :smiley: Ok so I’ve got the following code:

defimpl DeepMerge.Resolver, for: Any do
  def resolve(original = %{__struct__: struct}, override = %{__struct__: struct}, resolver) do
     implementors = get_implementors(DeepMerge.Resolver.__protocol__(:impls))

    if Enum.member?(implementors, struct) do
      Map.merge(original, override, resolver)

  def resolve(_original, override, _fun), do: override

  defp get_implementors({:consolidated, implementors}), do: implementors
  defp get_implementors(:not_consolidated) do
    IO.warn "Protocols not consolidated and trying to merge two structs of the same type. Not supported!"

    # let the code work with override semantics without being intrusive

This is for my deep_merge library, trying to provide a default derivable behaviour while keeping fallback_to_any in place (which I need). You can check out & play with the real code in this PR.

The warning dialyzer gives me is:

The pattern
{:consolidated, _implementors}

can never match the type

Which I don’t understand because unless I have been having a gross typo for the last 30+ minutes then I am matching on :not_consolidated.

I’m thinking dialyzer might be confused because the elixir definition is:

@spec __protocol__(:impls) :: :not_consolidated | {:consolidated, [module]}
Kernel.def(__protocol__(:impls), do: :not_consolidated)

Where it’s success typing might end up as "this is always :not_consolidated"

Removing the clause with {:consolidated, implementors} funnily enough removes the dialyzer error but makes my tests fail :wink:

Is there anything I can do in my setup/code that isn’t just configuring dialyxir to ignore the error? :grin:



That is… Somewhat disappointing but not entirely unexpected. Thanks for the help!

You can configure dialyzer with an ignore warnings list.

# /mix.exs
defmodule MyProject.MixProject do
  def project do
      app: :my_app,
      version: "0.1.0",
      elixir: "~> 1.5",
      dialyzer: [
        plt_add_apps: [:mix],
        ignore_warnings: "dialyzer.ignore-warnings"

Then you should be able to tell dialyzer to ignore the warning by adding it to /dialyzer.ignore-warnings

See https://www.google.com/search?q=dialyzer.ignore-warning for some examples

Yes, the dialyzer concludes that in the context where it’s invoked the result of __protocol__/1 will always be :not_consolidated, and so the {:consolidated, _} pattern can never match.

Providing a @dialyzer attribute would be one way. Another is to use apply/3: apply(DeepMerge.Resolver, :__protocol__, [:impls]).

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