DaAnalyst

DaAnalyst

The following is a snippet depicting my use case:

defmodule Abstract do
  @callback foo() :: binary()

  defmacro __using__( _) do
    quote do
      @behavior Abstract
 
      @impl true
      def foo() do
        raise RuntimeError, "shouldn't have been invoked!"
      end

      defoverridable Abstract
    end
  end
end

defmodule SpecificA do
  use Abstract
end

defmodule SpecificB do
  use Abstract

  @impl true
  def foo() do
    "foo"
  end
end

The problem is dialyzer keeps on showing this warning that foo/0 default implementation in SpecificA “has no local return”. Even if I change the callback return value to binary() | no_return(), it still gives the same warning.

Any ideas on how to circumvent the dialyzer warning without providing a dummy implementation of the foo/0 in SpecificA?

Thanks

Marked As Solved

josevalim

josevalim

Creator of Elixir

In your case, it is better to not provide a default implantation and either let the compile warn or mark the callback as optional. :slight_smile:

Also Liked

al2o3cr

al2o3cr

I believe this specific situation is the same as what motivates this code in GenServer:

https://github.com/elixir-lang/elixir/blob/d33e934021cefda220da4b1720e59e996a03aa52/lib/elixir/lib/gen_server.ex#L799-L802

rvirding

rvirding

Creator of Erlang

The basic reason is that raise generates an error so the function foo never returns. Dialyzer recognises this.

Last Post!

snofang

snofang

Applying the trick mentioned by @al2o3cr,

Dialyzer does not complain anymore:

  defmodule Abstract do
    @callback foo() :: {:ok, binary()} | {:error, binary()}

    defmacro __using__(_) do
      quote do
        @behaviour Abstract

        @impl true
        def foo() do
          case :erlang.phash2(1, 1) do
            0 ->
              {:ok, "Evrything is good:~"}

            1 ->
              {:error, "This is not possible to execute"}
          end
        end

        def bar() do
          case foo() do
            {:ok, message} -> IO.puts(message)
            {:error, message} -> IO.puts(message)
          end
        end

        defoverridable foo: 0
      end
    end
  end

  defmodule SpecificA do
    use Abstract
  end

Thank you

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