nixpkgs/doc/functions/generators.section.md

Generators

Generators are functions that create file formats from nix data structures, e. g. for configuration files. There are generators available for: INI, JSON and YAML

All generators follow a similar call interface: generatorName configFunctions data, where configFunctions is an attrset of user-defined functions that format nested parts of the content. They each have common defaults, so often they do not need to be set manually. An example is mkSectionName ? (name: libStr.escape [ "[" "]" ] name) from the INI generator. It receives the name of a section and sanitizes it. The default mkSectionName escapes [ and ] with a backslash.

Generators can be fine-tuned to produce exactly the file format required by your application/service. One example is an INI-file format which uses : as separator, the strings "yes"/"no" as boolean values and requires all string values to be quoted:

let
  inherit (lib) generators isString;

  customToINI = generators.toINI {
    # specifies how to format a key/value pair
    mkKeyValue = generators.mkKeyValueDefault {
      # specifies the generated string for a subset of nix values
      mkValueString = v:
             if v == true then ''"yes"''
        else if v == false then ''"no"''
        else if isString v then ''"${v}"''
        # and delegates all other values to the default generator
        else generators.mkValueStringDefault {} v;
    } ":";
  };

# the INI file can now be given as plain old nix values
in customToINI {
  main = {
    pushinfo = true;
    autopush = false;
    host = "localhost";
    port = 42;
  };
  mergetool = {
    merge = "diff3";
  };
}

This will produce the following INI file as nix string:

[main]
autopush:"no"
host:"localhost"
port:42
pushinfo:"yes"
str\:ange:"very::strange"

[mergetool]
merge:"diff3"

::: {.note} Nix store paths can be converted to strings by enclosing a derivation attribute like so: "${drv}". :::

Detailed documentation for each generator can be found here