You might not be wrong about this function, it just isn’t super obvious to me that Dialyzer is right either. Either way, check out this line: plug/lib/plug/conn/query.ex at 274e44f9a149b922099bf60029d8267afe494968 · elixir-plug/plug · GitHub

The encode_pair function is a defp and the output of that function is passed immediately to IO.iodata_to_binary. iodata is one of the real world uses for improper lists, because it reduces the overhead of building out a list front to back. The iodata_to_binary is able to handle improper lists and turn them into a single binary.

Dialyzer warns about improper lists because most of the time you do indeed want to warn for them. If the Plug core team used dialyzer they would probably put a no-warn invocation in there to tell dialyzer that this is intentional.

I don’t think this does create an improper list. It’s a recursive function, and that function has termination clauses that return [], which means it’ll construct a proper list.

As to this:

I think it’s better to approach this by understanding that the cons operator | is, at least in erlang, more foundational than even lists. Its most common use is with lists, but it has uses for low memory btrees, and gets use in IOlists as well.

Sorry for resurrecting an old thread, but while looking into this I also stumbled upon this issue which is relevant I think:

The OTP team seems to be considering disabling these warnings, since there are legit use cases for improper lists:

• IO data as already explained above
• as a low-level performance optimization for libraries: [a | b] takes less memory than {a, b}