Quick Start

Inlet’s workflow is broken into the four following steps:

  • Defining the schema of your input file and reading in the user-provided input file

  • Verifying that the user-provided input file is valid

  • Accessing the input data in your program

  • Optional Generating documentation based on defined schema

Defining Schema

The first step in using Inlet is to define the schema of your input file. Inlet defines an input file into two basic classes: Containers and Fields. Basically Fields are individual values and Containers hold groups of Fields and Containers.

Define the schema by using the following functions, on either the main Inlet class, for global Containers and Fields, or on individual Container instances, for Containers and Fields under that Container:




Adds a Container to the input file schema with the given name.


Adds a boolean Field to the global or parent Container with the given name.


Adds a double Field to the global or parent Container with the given name.


Adds a integer Field to the global or parent Container with the given name.


Adds a string Field to the global or parent Container with the given name.

All possible Containers and Fields that are can be found in the input file must be defined at this step. The value of the Field is read and stored into the Sidre datastore when you call the appropriate add function. Use the required class member function on the Container and Field class to indicate that they have to present in the given input file. You can also set a default value to each field via the type-safe Field::defaultValue() member functions. Doing so will populate the corresponding Fields value if the specific Field is not present in the input file. The following example shows these concepts:

  // defines a required global field named "dimensions" with a default value of 2

  // _inlet_verification_container_start
  // defines a required container named vector with an internal field named 'x'
  auto& v = myInlet.addStruct("vector").required(true);
  // _inlet_verification_container_end


This step helps ensure that the given input file follows the rules expected by the code. This should be done after completely defining your schema, which also reads in the values in the input file. This allows you to access any other part of the user-provided input. These rules are not verified until you call Inlet::verify(). Doing so will return true/false and output SLIC warnings to indicate which Field or Container violated which rule.

As shown above, both Containers and Fields can be marked as required. Fields have two additional basic rules that can be enforced with the following Field class member functions:




Indicates the Field can only be set to one of the given values.


Indicates the Field can only be set to inclusively between two values.

Inlet also provides functionality to write your own custom rules via callable lambda verifiers. Fields and Containers can both register one lambda each via their registerVerifier() member functions. The following example adds a custom verifier that simply verifies that the given dimensions field match the length the given vector:

  v.registerVerifier([&myInlet](const axom::inlet::Container& container) -> bool {
    int dim = myInlet["dimensions"];
    bool x_present = container.contains("x") &&
      (container["x"].type() == axom::inlet::InletType::Double);
    bool y_present = container.contains("y") &&
      (container["y"].type() == axom::inlet::InletType::Double);
    bool z_present = container.contains("z") &&
      (container["z"].type() == axom::inlet::InletType::Double);
    if(dim == 1 && x_present)
      return true;
    else if(dim == 2 && x_present && y_present)
      return true;
    else if(dim == 3 && x_present && y_present && z_present)
      return true;
    return false;

  std::string msg;
  // We expect verification to be unsuccessful since the only Field
  // in vector is x but 2 dimensions are expected
  SLIC_INFO("This should fail due to a missing dimension:");
  myInlet.verify() ? msg = "Verification was successful\n"
                   : msg = "Verification was unsuccessful\n";

  // Add required dimension to schema

  // We expect the verification to succeed because vector now contains
  // both x and y to match the 2 dimensions
  SLIC_INFO("After adding the required dimension:");
  myInlet.verify() ? msg = "Verification was successful\n"
                   : msg = "Verification was unsuccessful\n";


Inlet::getGlobalContainer()->registerVerifier() can be used to add a verifier to apply rules to the Fields at the global level.

For a full description of Inlet’s verification rules, see Verification.

Accessing Data

After the input file has been read and verified by the previous steps, you can access the data by name via Inlet::get() functions. These functions are type-safe, fill the given variable with what is found, and return a boolean whether the Field was present in the input file or had a default value it could fall back on. Variables, on the Inlet side, are used in a language-agnostic way and are then converted to the language-specific version inside of the appropriate Reader. For example, Inlet refers to the Lua variable vector={x=3} or vector.x as vector/x on all Inlet function calls.

For example, given the previous verificiation example, this access previously read values:

  // Get dimensions if it was present in input file
  auto proxy = myInlet["dimensions"];
  if(proxy.type() == axom::inlet::InletType::Integer)
    msg = "Dimensions = " + std::to_string(proxy.get<int>()) + "\n";

  // Get vector information if it was present in input file
  bool x_found = myInlet["vector/x"].type() == axom::inlet::InletType::Double;
  bool y_found = myInlet["vector/y"].type() == axom::inlet::InletType::Double;
  if(x_found && y_found)
    msg = "Vector = " + std::to_string(myInlet["vector/x"].get<double>()) +
      "," + std::to_string(myInlet["vector/y"].get<double>()) + "\n";

Generating Documentation

We provide a slightly more complex but closer to a real world Inlet usage example of the usage of Inlet. You can find that example in our repository here.

Once you have defined your schema, call write() on your Inlet class, passing it a concrete instantiation of a Writer class.


We provided a basic Sphinx documentation writing class but you may want to customize it to your own style. The links below show the example output from the documentation_generation.cpp, mfem_coefficient.cpp, and nested_structs.cpp examples:

Inlet also provides a utility for generating a JSON schema from your input file schema. This allows for integration with text editors like Visual Studio Code, which allows you to associate a JSON schema with an input file and subsequently provides autocompletion, linting, tooltips, and more. VSCode and other editors currently support verification of JSON and YAML input files with JSON schemas.

Using the same documentation_generation.cpp example, the automatically generated schema can be used to assist with input file writing: