cet-is » fhicl » fhicl-cpp

FHiCL currently provides fhicl::TupleAs<> allowing to create an arbitrary type from a sequence: [ 1.0, 2.0, 1.0, blue ] may become a ColorPoint.
For more complex data structures the sequence fits tight, and the full power of a Table may be desired: { point: [ 1.0, 2.0, 1.0 ] color: blue }.
This allows a better control on the single components and a more precise documentation. It also gives more flexibility (e.g. a default colour value).
I would like an entity fhicl::TableAs (and its fhicl::OptionalTableAs sibling) to be introduced, which accepts some form of function to convert the configuration structure in the table into a single object.
A rough sketch of how the usage I foresee:

struct ColorPoint {
  Point_t point;
  Color_t color;
};

struct ColorPointConfig {
  fhicl::TupleAs<Point_t(double, double, double)> point
    { fhicl::Name{ "Point" }, fhicl::Comment{ "position: [ x, y, z ] in global coordinates [cm]" } };
  fhicl::Atom<Color_t> color
    { fhicl::Name{ "color" }, fhicl::Comment{ "color of the point" }, makeColor("black") /* default */ };

  ColorPoint operator() () const;
}; // struct ColorPointConfig

struct Config {

  fhicl::TableAs<ColorPointConfig, &ColorPointConfig::operator()> point {
    fhicl::Name{ "point" },
    fhicl::Comment{ "reference point (including location and color)" },
    ColorPoint{ Point_t{}, makeColor{ "black" } }
    };

}; // struct Config

void parse(art::EDProducer::Table<Config> const& config) {
  auto pc = config().point(); // this is a ColorPoint object
  // ...
}

(please ignore any "alternative C++" syntax that I may have unintentionally introduced)
The intention is that the conversion function, second template parameter of TableAs, may be any function taking the table structure as argument.
A more restrictive pattern might instead require the Config struct be convertible to the target type, now with

struct ColorPointConfig {
  fhicl::TupleAs<Point_t(double, double, double)> point
    { fhicl::Name{ "Point" }, fhicl::Comment{ "position: [ x, y, z ] in global coordinates [cm]" } };
  fhicl::Atom<Color_t> color
    { fhicl::Name{ "color" }, fhicl::Comment{ "color of the point" }, makeColor("black") /* default */ };

  explicit operator ColorPoint() const; // now with 56% more conversion!
}; // struct ColorPointConfig

struct Config {

  fhicl::TableAs<ColorPointConfig, ColorPoint> point { // ... and we still need to specify the target type but not any more how to get it
    fhicl::Name{ "point" },
    fhicl::Comment{ "reference point (including location and color)" },
    ColorPoint{ Point_t{}, makeColor{ "black" } }
    };

}; // struct Config

or require an operator() in the fashion of the first example, which might make its implementation a more straightforward extension of fhicl::Table.