cet-is » fhicl » fhicl-cpp

Consider the following job configuration:

label : {

  module_type : MyModule

  a1 : <val>
  a2 : <val>

  b1 : <val>
  b2 : <val>

}

There are various use cases within art (and likely in user code) where a1 and a2 are used by class A, and b1 and b2 are used by class B, both of which belong to a module called MyModule. With the existence of fhiclcpp types (see issue #8773), a reasonable configuration would be:

class A {
public:
  struct Config {
    Atom<T> a1 { Key("a1") }:
    Atom<T> a2 { Key("a2") }:
  };
  // ...
};

class B {
public:
  struct Config {
    Atom<T> b1 { Key("b1") }:
    Atom<T> b2 { Key("b2") }:
  };
  // ...
};

There is, unfortunately, no way for users to declare a full configuration for MyModule using fhiclcpp types without resorting to multiple inheritance. An InlineTable class template would be very helpful so that a user could still benefit from C++ encapsulation, but fhiclcpp would expect to see a configuration as shown above. For example:

struct ConfigMyModule {
  TableFragment<A::Config> a;
  TableFragment<B::Config> b;
};

would allow a and b to be passed to the relevant functions/classes, but the FHiCL configuration corresponding to the members of {A,B}::Config be inline, as shown in the above FHiCL configuration. This closely parallels the @table syntax in FHiCL.