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lib/generators: add an example of overriding defaults

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An example of overriding the `toINI` generator is added,
hopefully clarifying the expressiveness of generators.
This commit is contained in:
Profpatsch 2018-03-26 17:31:51 +02:00
parent fa71407f36
commit e01d485ce4
1 changed files with 57 additions and 4 deletions
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61
doc/functions.xml
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@ -221,16 +221,69 @@
<para>
All generators follow a similar call interface: <code>generatorName
configFunctions data</code>, where <literal>configFunctions</literal> is a
set of user-defined functions that format variable parts of the content.
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 gets the name
of a section and returns a sanitized name. The default
] 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>