capnp.erl

% struct FooBar {
%   union {
%     foo @0 :Foo
%     bar @1 :Bar
%   }
% }
% struct Foo {
%   foo @0 :Text
% }
% struct Bar {
%   bar @0 :Text
% }

{ok, R} = ecapnp:set_root(foobar_capnp:'FooBar'()),
Foo = ecapnp:set(foo, R),
ok = ecapnp:set(foo, <<"Foo">>, Foo).

%** exception error: no function clause matching ecapnp_ref:ptr(0,{ref,0,2,0,0,null,{builder,<0.341.0>}}) (src/ecapnp_ref.erl, line 120)
%     in function  ecapnp_set:set_field/3 (src/ecapnp_set.erl, line 127)