Call to undefined function :erlang.'++'/2 in OTP 22

[japhib]

Hi, just want to say how awesome this project is. I'm hoping to use it on a 300k+ LOC Elixir project for work -- just want to work out some issues first. Note that because I'm using Elixir with Gradient, the error message in the title is probably formatted differently -- but I've narrowed this down to 100% being an issue with Gradualizer, not Gradient.

The issue is that using the ++ operator on OTP 22 generates the error message Call to undefined function :erlang.'++'/2. This issue does not happen with OTP 24. I was able to narrow this down to gradualizer_db:import_module/2 loading specs from the erlang BEAM file, and on OTP 22 that file doesn't contain a spec for ++.

I verified this by disassembling the BEAM files using:

iex> {:ok, forms} = Gradient.ElixirFileUtils.get_forms_from_beam('/Users/myusername/.asdf/installs/erlang/22.3.4.22/lib/erts-10.7.2.14/ebin/erlang.beam')
iex> IO.puts(:erl_prettypr.format(:erl_syntax.form_list(forms)))

Again, sorry for using Elixir here rather than Erlang, but it's probably pretty simple to do the same thing from Erlang using :erl_prettypr.format(:erl_syntax.form_list(forms)). Anyways, I logged out what file it's loading the erlang module from, and when it loads from .asdf/installs/erlang/22.3.4.22/lib/erts-10.7.2.14/ebin/erlang.beam, the ++ spec is not present, but when it loads from .asdf/installs/erlang/24.2.1/lib/erts-12.2.1/ebin/erlang.beam, the ++ spec IS present.

(I guess it's possible that in the very latest version of OTP 22, since 22.3.4.22, this spec has been added. But that seems unlikely. I can double check on this.)

What seems like the easiest fix is to add a thing somewhere inside gradualizer_db when we import the erlang module, which is only enabled for OTP version 22 and earlier. (Maybe also OTP 23 -- I'll have to check.) At that point we'd import some hard-coded function specs for missing functions, such as ++, and any others we find. (I'm assuming there are quite a few more.)

But, I'm open to other ways to fix this. I'm happy to submit an MR for this sometime in the next few days/weeks, just wanted to agree on the method of solving the issue before I start coding it. What do you think?

[erszcz]

Hi, @japhib!

Thanks for your interest!

In general, the notion of overriding OTP specs already has a solution in Gradualizer - it's the prelude. It's also possible to specify locations of other such *.specs.erl in a project setting. Please see the following example:

$ cat plusplus.erl
-module(plusplus).

-export([f/0,
         g/0]).

-spec f() -> [integer()].
f() ->
    [3] ++ [1].

-spec g() -> [integer()].
g() ->
    plusplus_helper:concat([3], [1]).
$ gradualizer plusplus.erl
plusplus.erl: Call to undefined function plusplus_helper:concat/2 on line 12 at column 20
$ gradualizer --specs_override_dir gradualizer_overrides/ -- plusplus.erl
$ ls gradualizer_overrides/
plusplus_helper.specs.erl
$ cat gradualizer_overrides/plusplus_helper.specs.erl
-module(plusplus_helper).

-spec plusplus_helper:concat([T], [T]) -> [T].

specs_override_dir could also be used from a rebar.config file to apply to an entire Rebar3 Erlang project. So in general, there's already a way provide the external specs for functions defined without them.

Now, things get tricky when we're considering the above for Elixir - I think it's not yet possible to do something similar with Gradient. I've created a ticket to track it - esl/gradient#122.

For this particular case, though, I see no harm in including the specs directly in Gradualizer's prelude - #440.

Thanks for reaching out and good luck with Gradient and your project!

[japhib]

Thanks for all the information and the quick response! I’ll test the proposed changes with my project to make sure it fixes it as intended.