Configuration validation and fhiclcpp types¶

• a way to validate a user-provided, run-time configuration against a supported configuration,
• a way to provide a description of a supported configuration for a given module, service, or other plugin library,
• an improvement over using the 'pset.get<T>("key",optional_default)' pattern.

The fhiclcpp type system was designed so that users would not need to interact directly with a fhicl::ParameterSet object, but can do so if they desire.

Table of contents¶

• Introduction
  • fhiclcpp types vs. FHiCL categories
  • Necessary header files
  • Usage pattern
    1. fhiclcpp parameter declaration
    2. ParameterSet validation
    3. Parameter value filling
    4. Parameter retrieval
  • Configuration description
  • For users outside of art
• Examples
• fhiclcpp types in detail
• Auxiliary classes
• Conditional configuration
• System details

Introduction¶

fhiclcpp types vs. FHiCL categories¶

The mapping between the fhiclcpp and FHiCL types is shown in this table:

parameter : value

sequence : [1, 2, 3]

sequence : [1, 2, 3] # bounded

sequence : [1, "two", true]

table : { 
  parameter : value
  sequence  : [4,5,6]
}

Since the FHiCL sequence type is very flexible, three fhiclcpp types have been introduced to improve the semantics of the C++ code. Formally speaking, the types Sequence<T> and Sequence<T, std::size_t> map to the same FHiCL sequence category. However, Sequence<T,std::size_t> is a bounded sequence, which is explained below. The Optional* parameters above are described here.

Necessary header files¶

#include "fhiclcpp/types/Atom.h"               // fhicl::Atom<T>
#include "fhiclcpp/types/DelegatedParameter.h" // fhicl::DelegatedParameter
#include "fhiclcpp/types/Sequence.h"           // fhicl::Sequence<T> & fhicl::Sequence<T,size_t>
#include "fhiclcpp/types/Tuple.h"              // fhicl::Tuple<T...>
#include "fhiclcpp/types/TupleAs.h"            // fhicl::TupleAs<T(U...)>
#include "fhiclcpp/types/Table.h"              // fhicl::Table<T> 
#include "fhiclcpp/types/TableFragment.h"      // fhicl::TableFragment<T> 

#include "fhiclcpp/types/OptionalAtom.h"                // fhicl::OptionalAtom<T>
#include "fhiclcpp/types/OptionalDelegatedParameter.h"  // fhicl::OptionalDelegatedParameter
#include "fhiclcpp/types/OptionalSequence.h"            // fhicl::OptionalSequence<T> & fhicl::OptionalSequence<T,size_t>
#include "fhiclcpp/types/OptionalTuple.h"               // fhicl::OptionalTuple<T...>
#include "fhiclcpp/types/OptionalTupleAs.h"             // fhicl::OptionalTupleAs<T(U...)>
#include "fhiclcpp/types/OptionalTable.h"               // fhicl::OptionalTable<T> 

// Headers that can be included
//  - 'fhicl::Name', 'fhicl::Comment', and 
//    'fhicl::use_if' and 'fhicl::use_unless' are already 
//    provided by each of the headers above
#include "fhiclcpp/types/Name.h" 
#include "fhiclcpp/types/Comment.h" 
#include "fhiclcpp/types/ConfigPredicate.h" 

Usage pattern¶

The order in which the system is used is the following:

1. fhiclcpp parameter declarations: fhiclcpp parameters are declarated
2. ParameterSet validation: fhicl::ParameterSet is validated against the fhiclcpp parameter collection
3. Value filling: Values are filled for fhiclcpp parameters from the ParameterSet
4. Value retrieval: Values are usable in C++ code

Steps 1 and 4 are exposed to the user. Steps 2 and 3 can be exposed to the user, in principle, but we do not encourage the user to do them unless there is a good reason (please consult artists@fnal.gov for guidance).

For art users, please consult here. Users outside of art should consult the link below.

1. fhiclcpp parameter declaration¶

A fhiclcpp parameter declaration follows the pattern:

fhiclcpp_type < arg_type(s) > name { Name("name"), [Optional arguments] };

where fhiclcpp_type is either Atom, Sequence, Tuple, or Table.

arg_type(s)¶

• std::array
• std::pair
• std::tuple
• std::vector

Name("name")¶

All fhiclcpp types must be initialized with a fhicl::Name rvalue as its first argument. The "name" string corresponds to the sequence of characters corresponding to the parameter name in a FHiCL configuration:

name : "some value for name" 

In general, we suggest the same character sequence for the variable name 'name' as the string supplied in the fhicl::Name. For example,

Atom<int> debugValue { Name("threshold") }; // discouraged

Optional arguments¶

All optional arguments must be specified after the fhicl::Name argument. Both options below can be specified with the same fhiclcpp parameter declaration.

fhicl::Comment

Each of the types may include a fhicl::Comment rvalue argument, whose string will be printed whenever the description of the configuration is requested:

Atom<int> param { 
  Name("param"),           // 'Name' is NOT optional
  Comment("A description") // 'Comment' is optional
};

std::function<bool()> - conditional configuration predicate

In some cases, it is desirable to introduce components of a configuration only if the value of a previously-supplied parameter satisfies a given condition (i.e a predicate). This is done by the user providing an std::function object whose function type receives no arguments, and whose return type is bool. Details as to how a user can enable this feature are given here.

Parameter value default

All fhiclcpp types can receive a default parameter value except for fhicl::Table objects. If a FHiCL configuration does not provide the parameter value, then the default value will be used. For example,

Atom<int> param { Name("param"), 4 };

auto param = pset.get<int>("param", 4);

2. Configuration validation¶

• missing parameters as determined by the declared fhiclcpp parameters in the C++ code
• extra parameters in a user's configuration file that are not supported by any corresponding declared fhiclcpp parameters

For fhiclcpp parameters that receive a default value, a corresponding FHiCL parameter that is absent in a configuration file is not an error. Any misspelled FHiCL parameters -- i.e. those that do not match the string supplied in the fhicl::Name argument of the fhiclcpp parameter constructor -- are classified as extra parameters and are thus caught.

Consider the following allowed configuration:

struct Config {
  Atom<std::string> name { Name("name"), "Gustav" };
};

Table<Config> pset { Name("pset") };

An attempt is made to provide a configuration to the file that uses this configuration. However the file FHiCL file is misconfigured:

# config.fcl
pset : {
   nam  : Mahler # attempt to override default value of "Gustav" 
   flag : false
}

The parameter nam is misspelled with respect to the allowed configuration name "name", and flag is not supported. When the configuration validation step is performed, an exception is thrown and the following error message is printed to STDOUT:

Unsupported parameters:

 + flag                           [ ./config.fcl:3 ]
 + nam                            [ ./config.fcl:2 ]

The characters in brackets denote the location of the unsupported parameter [ file-name:line-number ]. The listing order of the unsupported parameters is based on the std::string ordering operator, which is lexicographical.

3. Parameter value filling¶

If the validation step (2) is successful -- i.e. the fhicl::ParameterSet object conforms to the supported configuration as declared in the C++ code -- then the values of the parameters are filled using the specified values in the configuration file, or the default values as specified in the C++ code if the corresponding FHiCL parameter has not been provided. After this step, the fhiclcpp parameter now has a value corresponding to the user-provided ones.

The parameter-value-filling step is done internally, the details of which are not necessary to understand for the user.

4. Parameter retrieval¶

Standard parameters¶

For a standard fhiclcpp parameter declared as:

Atom<int> param { Name("param"), 4 };

auto param_value = param(); // 'param_value' is int

Optional parameters¶

For an optional fhiclcpp parameter declared as:

OptionalAtom<int> optParam { Name("optParam") };

int j{};
if ( optParam(j) ) {
   // 'j' can now be used
}

If 'optParam' is present with a non-nil value in a configuration file, the value of optParam(j) will return true, and j will be filled with the configuration-set value. Otherwise, j retains its initial value ('0' in this case). Note that optional parameters can receive no default value.

General structure¶

The fhiclcpp-typed system has been designed in such a way that the parameter declaration and retrieval syntaxes are similar. For example, the FHiCL configuration:

# FHiCL parameter declarations
settings : {
  verbosity : true
}

# FHiCL value retrieval
verbosity : @local::settings.verbosity

// fhiclcpp parameter declarations
struct Settings{
  Atom<bool> verbosity { Name("verbosity") };
};
Table<Settings> settings { Name("settings") };

// fhiclcpp value retrieval
auto verbosity = settings().verbosity();

Configuration description¶

Consider the following fhiclcpp parameter configuration:

struct G4Settings {
  Atom<double> energyCutoff { Name("energyCutoff") };
  Atom<string> physicsList  { Name("physicsList"), "QGSP_BERT" };
};

struct Config {
  Atom<bool> flag       { Name("flag"), false };
  Atom<int>  threshold  { Name("threshold"), Comment("ADC count threshold") };
  OptionalTable<G4Settings> g4Settings { Name("g4Settings") }; 
  Sequence<string,2u>       particles  { Name("particles")  };
};
Table<Config> config { Name("config") };

Formation¶

The description of the allowed FHiCL configuration (as shown above) is formed based on the order of initialization of the class members. In addition to registering the expected key, the expected type and any user-provided defaults are also registered with the fhiclcpp system. The below table shows the order of initialization and the corresponding FHiCL keys and types for each fhiclcpp parameter.

The asterisk '*' means that the order of initialization of the individual Sequence elements is unknown and therefore the key assignment needs to be handled specially.

Print-out¶

For users that have access to the 'config' object, a print-out of the allowed description can be produced using:

Table<Config> config { Name("config") };
config.print_allowed_configuration(std::cout); // Any 'std::ostream' object can be provided

The printed description is sent to STDOUT:

   config: {

      flag: false  # default

      ## ADC count threshold

      threshold: <int>

    # g4Settings: {
    #
    #    energyCutoff: <double>
    #
    #    physicsList: "QGSP_BERT"  # default
    # }

      particles: [
         <string>,
         <string>
      ]
   }

The pound signs ('#') preceding the g4Settings table indicate that it is an optional table, which the user is not required to specify in the configuration file.

For art users, printing out the allowed configuration is described here.

Non-art users¶

For users outside of art, it is possible to use the fhiclcpp types system. The following pattern is recommended:

namespace {

  struct MyConfig {
    // fhiclcpp parameter declarations ...
  };

  fhicl::Table<MyConfig>
  retrieveConfiguration( fhicl::ParameterSet const & pset )
  {
     std::set<std::string> ignorable_keys {}; // keys that should be ignored 
                                              // by the validation system (can be empty)

     fhicl::Table<MyConfig> const result { pset, ignorable_keys }; // performs validation and value setting

     return result;
  }

} // anon. namespace

void some_func( fhicl::ParameterSet const & pset )
{
  auto config = retrieveConfiguration( pset );
  // 'config' now usable
}