Can MapSet be an attribute?

Background

I have a function that works with MapSets, and I have a set of values (static values) that I need to pass it.
So I have decided to put these values into an attribute, however dialzyer complains and I don’t get why.

Code

defmodule MyApp.Test do

  alias MyApp.MyDependency

  @data MapSet.new(["a", "b", "c"])

  def do_something_with_data(request), do: MyDependency.validate(request, @data)
end

This code gives me the following error:

lib/myapp/test.ex:5: The call ‘Elixir.MyApp.MyDependency’:validate(data@1::any(),#{‘struct’:=‘Elixir.MapSet’, ‘map’:=#{<<:56,:*32>>=>[]}, ‘version’:=2}) does not have an opaque term of type ‘Elixir.MapSet’:t(_) as 2nd argument

Which is rather confusing.

Why does this work?

I have battled with MapSets before, namely with this opaque value issue. So I tried to trick dialyzer and created the following code, which works:

defmodule MyApp.Test do

  alias MyApp.MyDependency

  @data ["a", "b", "c"]

  def do_something_with_data(request), do: MyDependency.validate(request,  MapSet.new(@data))
end

Questions

Why doesn’t the first example work? What am I missing?
Why does the second one work?
Does it have something to do with how the compiler parses the files?

Dialyzer essentially sees this code:

MyDependency.validate(request, %MapSet{map: %{"a" => [], "b" => [], "c" => []}, version: 2})

While this is the same data, this is not like you’re supposed to create opaque types.

The reason for that is that a module attribute does just store plain data at compile time, injecting it everywhere it’s read. Dialyzer being an erlang tool has no idea there even are module attributes, so it cannot track the type from the compile time initialization to being injected, to being used at runtime.

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How interesting!
So, is my workaround the only solution to this issue or would you recommend another one?

There’s a thread on disabling warnings here that may give you some hints…

1 Like