failure to fetch the substitution value.

        `value`

        :   The name of the variable to use for
            substitution.

        See upstream documentation for semantics
        behind the substitution functionality.

        `Example usage:`

        ```nix
          let
            format = pkgs.formats.hocon { };
          in {
            a = 1;
            b = format.lib.mkSubstitution "a";
            c = format.lib.mkSubstitution "SOME_ENVVAR";
            d = format.lib.mkSubstitution {
              value = "SOME_OPTIONAL_ENVVAR";
              optional = true;
            };
          }
        ```

    `Implementation notes:`

    - classpath includes are not implemented in pyhocon,
      which is used for validating the HOCON output. This
      means that if you are using classpath includes,
      you will want to either use an alternative validator
      or set `doCheck = false` in the format options.

`pkgs.formats.libconfig` { *`generator`* ? `<derivation>`, *`validator`* ? `<derivation>` }

:  A function taking an attribute set with values

    `generator`

    :   A derivation used for converting the JSON output
        from the nix settings into libconfig. This might be
        useful if your libconfig variant is slightly different
        from the original one, or for testing purposes.

    `validator`

    :   A derivation used for verifying that the libconfig
        output is correct and parsable. This might be
        useful if your libconfig variant is slightly different
        from the original one, or for testing purposes.

    It returns an attrset with a `type`, `generate` function,
    and a `lib` attset, as specified [below](#pkgs-formats-result).
    Some of the lib functions will be best understood if you have
    read the reference specification. You can find this
    specification here:

    <https://hyperrealm.github.io/libconfig/libconfig_manual.html#Configuration-Files>

    Inside of `lib`, you will find these functions

    `mkHex`, `mkOctal`, `mkFloat`

    :   Use these to specify numbers in other formats.

        `Example usage:`

        ```nix
          let
            format = pkgs.formats.libconfig { };
          in {
            myHexValue = format.lib.mkHex "0x1FC3";
            myOctalValue = format.lib.mkOctal "0027";
            myFloatValue = format.lib.mkFloat "1.2E-3";
          }
        ```

    `mkArray`, `mkList`

    :   Use these to differentiate between whether
        a nix list should be considered as a libconfig
        array or a libconfig list. See the upstream
        documentation for the semantics behind these types.

        `Example usage:`

        ```nix
          let
            format = pkgs.formats.libconfig { };
          in {
            myList = format.lib.mkList [ "foo" 1 true ];
            myArray = format.lib.mkArray [ 1 2 3 ];
          }
        ```

    `Implementation notes:`

    - Since libconfig does not allow setting names to start with an underscore,
      this is used as a prefix for both special types and include directives.

    - The difference between 32bit and 64bit values became optional in libconfig
      1.5, so we assume 64bit values for all numbers.

`pkgs.formats.json` { }

:   A function taking an empty attribute set (for future extensibility)
    and returning a set with JSON-specific attributes `type` and
    `generate` as specified [below](#pkgs-formats-result).

`pkgs.formats.yaml` { }

:   A function taking an empty attribute set (for future extensibility)
    and returning a set with YAML-specific attributes `type` and
    `generate` as specified [below](#pkgs-formats-result).

`pkgs.formats.ini` { *`listsAsDuplicateKeys`* ? false, *`listToValue`* ? null, \.\.\. }

:   A function taking an attribute set with values

    `listsAsDuplicateKeys`

    :   A boolean for controlling whether list values can be used to
        represent duplicate INI keys

    `listToValue`

    :   A function for turning a list of values into a single value.