Help wanted: search paths for Erlang and Elixir

[Ivan Sokolov]

Hello, Guixers!

Erlang/OTP has notion of applications [1], they are usually installed in
$PREFIX/lib/erlang/lib in the form of directories named APP-VERSION,
e.g. asn1-5.0.19.

In order for the Erlang runtime to find the installed application it
must appear in the load path.  If there is ERL_ROOTDIR in the
environment, erl(1) will search for application directories in
$ERL_ROOTDIR/lib and will add them to the load path.  ([2] mentions
$OTPROOT but it's probably an error in the documentation.)  The
following will do for Erlang:

(search-path-specification
  (variable "ERL_ROOTDIR")
  (files '("lib/erlang"))
  (separator #f))

However, Elixir usually installs its applications in
$PREFIX/lib/elixir/lib, and $ERL_ROOTDIR cannot contain a list.  Thus,
the environment variable ERL_LIBS [2] seems to be the most fitting way
to compose the load path for our purpose.

Is there a way to write a search-path-specification that will only yield
application directories?  I could not write such.  Listing
$PREFIX/lib/erlang/lib recursively is a bad option, because OTP
application can contain arbitrary resources.  And it will probably slow
down modules resolution.

Thanks in advance!

[1]: https://www.erlang.org/doc/man/application.html
[2]: https://www.erlang.org/doc/man/code.html#code-path