How to enable maybe_expr?

Hello everyone,

I’m trying enable maybe feature on my project, but always get the error:

{features_not_allowed, [maybe_expr,maybe_expr]}

I’m using rebar3 and OTP 25.1.2 in my project and tried some things to enable the feature.

I tried to add in rebar.config:

{erl_opts, [debug_info, 
            {parse_transform, lager_transform},
            {feature, maybe_expr, enable}]}.

and add in my_module.erl:

-feature(maybe_expr, enable).
-compile({feature, maybe_expr, enable}).

and add in vm.args:

-enable-feature maybe_expr

But I’but I was not successful with any option. To start I just run rebar2 shell on the project.



I usually do it that way, but you need to set the path to vm.args when calling rebar3 commands.
So, if vm.args is at config/vm.args (relative to current directory), you can run it with: env ERL_FLAGS="-args_file config/vm.args" rebar3 <command> ... (assuming you’re using some linux, not sure this would work on Windows)



Does it interact with {relx,[{release,..,[{vm_args,...}]}]} in any way?

Or is it (as I understand)

  • ERL_FLAGS="-args_file config/vm.args" rebar3 ... dev-only (compile, dialyzer, eunit, etc)
  • {relx,...,{vm_args,...}} production-only (release)?

EDIT: is there any way to force this config in some project-local configuration file? I understand you can’t set it from within rebar.config, because it needs to set a flag on BEAM itself, which is already running when processing rebar.config.

1 Like

Note that starting from OTP 26.0 you don’t need to explicitly enable feature in the runtime via vm.args, it is enough to enable it in the compiler


Yes. Additionally, starting from OTP-27 will be default on (“Approved”) and starting from OTP-29 will be always-on (“Permanent”).

My problem is

> pkg info erlang

and FreeBSD being slowly moving target.

I just globally added '-enable-feature maybe_expr' to ERL_FLAGS. I hope it won’t cause any hiccups.

Thank you for your time.