tjchambers
Static Type Checking on guards
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.
Most Liked
mudasobwa
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)
josevalim
tjchambers
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