nixpkgs/doc/functions/generators.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         xml:id="sec-generators">
 <title>Generators</title>

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

 <para>
  All generators follow a similar call interface: <code>generatorName
  configFunctions data</code>, where <literal>configFunctions</literal> 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 <code>mkSectionName ? (name: libStr.escape [ "[" "]" ]
  name)</code> from the <literal>INI</literal> generator. It receives the name
  of a section and sanitizes it. The default <literal>mkSectionName</literal>
  escapes <literal>[</literal> and <literal>]</literal> with a backslash.
 </para>

 <para>
  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
  <literal>: </literal> as separator, the strings
  <literal>"yes"</literal>/<literal>"no"</literal> as boolean values and
  requires all string values to be quoted:
 </para>

<programlisting>
with lib;
let
  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 delegats 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";
  };
}
</programlisting>

 <para>
  This will produce the following INI file as nix string:
 </para>

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

[mergetool]
merge:"diff3"
</programlisting>

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

 <para>
  Detailed documentation for each generator can be found in
  <literal>lib/generators.nix</literal>.
 </para>
</section>