nixos/doc: add documentation for formats.hocon

nixos/doc/manual/development/settings-options.section.md

@@ -46,6 +46,159 @@ have a predefined type and string generator already declared under
     `generate` to build a Java `.properties` file, taking
     care of the correct escaping, etc.
 
+`pkgs.formats.hocon` { *`generator`* ? `<derivation>`, *`validator`* ? `<derivation>`, *`doCheck`* ? true }
+
+:  A function taking an attribute set with values
+
+    `generator`
+
+    :   A derivation used for converting the JSON output
+        from the nix settings into HOCON. This might be
+        useful if your HOCON variant is slightly different
+        from the java-based one, or for testing purposes.
+
+    `validator`
+
+    :   A derivation used for verifying that the HOCON
+        output is correct and parsable. This might be
+        useful if your HOCON variant is slightly different
+        from the java-based one, or for testing purposes.
+
+    `doCheck`
+
+    :   Whether to enable/disable the validator check.
+
+    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://github.com/lightbend/config/blob/main/HOCON.md>
+
+    Inside of `lib`, you will find these functions
+
+    `mkInclude`
+
+    :   This is used together with a specially named
+        attribute `includes`, to include other HOCON
+        sources into the document.
+
+        The function has a shorthand variant where it
+        is up to the HOCON parser to figure out what type
+        of include is being used. The include will default
+        to being non-required. If you want to be more
+        explicit about the details of the include, you can
+        provide an attrset with following arguments
+
+        `required`
+
+        :   Whether the parser should fail upon failure
+            to include the document
+
+        `type`
+
+        :   Type of the source of the included document.
+            Valid values are `file`, `url` and `classpath`.
+            See upstream documentation for the semantics
+            behind each value
+
+        `value`
+
+        :   The URI/path/classpath pointing to the source of
+            the document to be included.
+
+        `Example usage:`
+
+        ```nix
+          let
+            format = pkgs.formats.hocon { };
+            hocon_file = pkgs.writeText "to_include.hocon" ''
+              a = 1;
+            '';
+          in {
+            some.nested.hocon.attrset = {
+              _includes = [
+                (format.lib.mkInclude hocon_file)
+                (format.lib.mkInclude "https://example.com/to_include.hocon")
+                (format.lib.mkInclude {
+                  required = true;
+                  type = "file";
+                  value = include_file;
+                })
+              ];
+              ...
+            };
+          }
+        ```
+
+    `mkAppend`
+
+    :   This is used to invoke the `+=` operator.
+        This can be useful if you need to add something
+        to a list that is included from outside of nix.
+        See upstream documentation for the semantics
+        behind the `+=` operation.
+
+        `Example usage:`
+
+        ```nix
+          let
+            format = pkgs.formats.hocon { };
+            hocon_file = pkgs.writeText "to_include.hocon" ''
+              a = [ 1 ];
+              b = [ 2 ];
+            '';
+          in {
+            _includes = [
+              (format.lib.mkInclude hocon_file)
+            ];
+
+            c = 3;
+            a = format.lib.mkAppend 3;
+            b = format.lib.mkAppend (format.lib.mkSubstitution "c");
+          }
+        ```
+
+    `mkSubstitution`
+
+    :   This is used to make HOCON substitutions.
+        Similarly to `mkInclude`, this function has
+        a shorthand variant where you just give it
+        the string with the substitution value.
+        The substitution is not optional by default.
+        Alternatively, you can provide an attrset
+        with more options
+
+        `optional`
+
+        :   Whether the parser should fail upon
+            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.mkSubstition "SOME_ENVVAR";
+            d = format.lib.mkSubstition {
+              value = "SOME_OPTIONAL_ENVVAR";
+              optional = true;
+            };
+          }
+        ```
+
 `pkgs.formats.json` { }
 
 :   A function taking an empty attribute set (for future extensibility)