[erlang-questions] Deriving records from one another?

[Roger Lipscombe]

I'm probably thinking too OO by calling it "deriving", but here's the problem:

OTP handlers (gen_server, etc.) take a State parameter. You can stick
anything in here, but generally I've been using a #state record, and
keeping a bunch of values in it.

Now I'd like to actually start representing *state* with the State[1].
That is, I'd like to be able to match on #init, #ready,
#expecting_key, #whatever records. This will hopefully make the code
easier to read, and means that my record doesn't have to contain
fields for stuff that's not relevant in the given state.

However, some of the fields *are* shared between states, so my
question is (in OO terms) how do I derive record types from
each-other?

I've thought about using a tuple: {Shared, State} when Shared ::
#shared{}, State :: #init{} | #etc{}
That might be hard to read in matches, though, maybe.

I've thought about using a container record:
-record(state { shared :: #shared{}, inner :: #init{} | #etc{} }).

Are there any good solutions[2] to this problem? Or are there better
ways to do what I'm trying to do?

Cheers,
Roger.

[1] No, I don't think that gen_fsm is the answer at this point.
Interesting suggestion, though, thanks.
[2] Still on R16, so I can't use maps, but I'm still interested in how
they might help, if at all.