FHiCL C++ interface for multiple choice values

This issue has been migrated from https://cdcvs.fnal.gov/redmine/issues/22574 (FNAL account required) Originally created by @PetrilloAtWork on 2019-05-09 23:09:29

I would like to request support in FHiCL C++ interface for multiple choice values. I am going with an...

Example

Here unit is a configuration parameter of integral type (int) that may represent three choices: GeV (1), MeV (2) and megajoule (3).

Configuration data structure


struct Config {

  using Name    = fhicl::Name;
  using Comment = fhicl::Comment;
  using Choices = fhicl::Choices;

  fhicl::Atom<double> energy {
    Name    { "energy" },
    Comment { "energy (unit is specified by `unit` parameter)" }
  }; // energy

  fhicl::Choice<int> unit {
    Name    { "unit" },
    Comment { "energy unit" },
    Choices {
      {
        Name { "GeV" },                   // the name of the choice
        Comment{ "energies are in GeV" }, // and its description
        1               // the value of `unit` if this choice is picked
      },
      {
        Name { "MeV" },                   // the name of the choice
        Comment{ "energies are in MeV" }, // and its description
        2               // the value of `unit` if this choice is picked
      },
      {
        Name { "MJ" },                          // the name of the choice
        Comment{ "energies are in megajoule" }, // and its description
        3               // the value of `unit` if this choice is picked
      },
    },
    1 // default: GeV
  }; // unit

}; // Config

If the default value is omitted, the parameter is mandatory. Otherwise, its value must match a supported choice, or else a logic_error type of exception will be thrown at run time.

Configuration FHiCL table

energy: 10
unit:   "MeV"

Description on console screen

  ## energy (unit is specified by `unit` parameter)

  energy: 

  ## energy unit
  ## Choose one among:
  ## "GeV" : energies are in GeV
  ## "MeV" : energies are in MeV
  ## "MJ" : energies are in megajoule

  unit: "GeV" # default

User code to access the values


double energy = config().energy();
int unit = config().unit();
switch (unit) {
  case 1: break;
  case 2: energy *= 1e-3; break;
  case 3: energy *= JouleToGeV * 1e-6; break;
} // switch

Throws a art::error::Configuration type of art::Exception (or equivalent) if the value is not acceptable.

Bonus points

Additional candies:

works with enum and/or enum class types
can be used in a sequence (fhicl::ChoiceSequence)
choice value converted from name by default (would not work in the example, but if the type is std::string instead of int it will save replicating the Name into the value on each entry
different choices may have the same value (e.g. "MJ" and "megajoule" both supported and associated to the same value 3; if 3 is the default value, it will be written in the description as the first of the matching choices)

art-framework-suite / fhicl-cpp