Static Type Checking on guards

Questions / Help
1

Erlang. OTP-28
Elixir 1.20.0-rc.3 (9b80ab5) (compiled with Erlang/OTP 28)

In rc.3 I am getting a new type checking warning for functions I have guards on lists to ensure the list contains terms of the proper type.

Example:

def filter(tags, value)
      when is_list(tags) and is_struct(hd(tags)) and is_binary(value) do
    converted_value = to_regex(value)
Enum.filter(tags, fn tag -> tag.tag =~ ~r|^#{converted_value}|i end)
end

  warning: incompatible types given to Kernel.is_map_key/2:

        is_map_key(hd(tags), :__struct__)

    given types:

        term(), :__struct__

    but expected one of:

        #1
        %{..., __struct__: term()}, term()

        #2
        %{..., __struct__: not_set()}, term()

    where "tags" was given the types:

        # type: empty_list() or non_empty_list(term(), term())
        # from: lib/sct/joe/services/tag_matcher.ex:27:12
        is_list(tags)

        # type: non_empty_list(term(), term())
        # from: lib/sct/joe/services/tag_matcher.ex:27:40
        hd(tags)

    type warning found at:
    │
 27 │       when is_list(tags) and is_struct(hd(tags)) and is_binary(value) do

My guard for tags is attempting to ensure that the tags are a list, and that list is non-empty and that it contains structs. I have not specified that type of the structure since there are a number of compatible structs that this function serves.

I do not quite understand why this should result in a type check warning. Any guidance would be useful. I have dozens of similarly constructed function guards which are all emitting this type of warning.

2

I am not sure, why this results in a type error, but I would definitely write it differently (andalso more precisely):

def filter([%s{} | _] = tags, value)
      when s in [Foo, Bar, Baz] and is_binary(value)
1 Like
3

I do like that better. Thank you! Still begs the question about the type error.

5

It is a bug. Can you please file a report? Thank you!

1 Like
6

will do.

1 Like