Interpreting Dialyzer Messages

Questions / Help
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1

I’m using VS Code 1.23.0, Elixir 1.6.4… ElixirLS is giving me messages that don’t seem to make sense to me.

I’m using the ExAws.EC2 library. I’m calling the function describe_instance_status in the following way:

opts = [filters: ["event.code": "*"]]
EC2.describe_instance_status(opts)

ElixirLS Dialyzer reports:

	"severity": 4,
	"message": "The call 'Elixir.ExAws.EC2':describe_instance_status(Vopts@1::[{'filters',[{'event.code',<<_:8>>},...]},...]) breaks the contract (opts::describe_instance_status_opts()) -> 'Elixir.ExAws.Operation.Query':t()",

The spec for that function in the library is:

  @spec describe_instance_status() :: ExAws.Operation.Query.t
  @spec describe_instance_status(opts :: describe_instance_status_opts) :: ExAws.Operation.Query.t

The type is defined up above that spec as:

  @type describe_instance_status_opts :: [
    dry_run: boolean,
    filters: [filter, ...],
    include_all_instances: boolean,
    instance_ids: [binary, ...],
    max_results: integer,
    next_token: binary
  ]

The same problems show up running : mix dialyzer from command line. Can someone help me with interpreting what dialyzer is trying to tell me?

2

The filter is specified as:

 @type filter :: {name :: binary | atom, value :: [binary,...]}

Which means a value of "*" that you use does not match the type for the value. If aws supports using “*” as the filter, this is a bug in the type spec in the library.

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3

OK, that makes sense. Thank you. AWS does support the * to match everything although its hidden pretty well in their doc. I’ll see if I can form in a way that aligns with the current spec and give feedback to the library authors.

4

Found that:

opts = [filters: ["event.code": ["*"]]]

works with AWS EC2 API and that approach matches the spec.

1 Like