On 06/30/2014 09:42 AM, Max Ivanov wrote:
> Hi Nixers,
>
> I am building a nix os module and stumbled upon a problem which I
> might need your advice on how to approach it.
>
> What I can't figure out, is how to filter out unset values even if
> they don't have a default value set in mkOption. The idea is that
> application itself handles missing values using defaults and I don't
> want to duplicate them in a config.
>
> On the other hand, there are some values which are mandatory, so that
> if they are missing from configuration.nix config build should fail.
> Worth mentioning is that number of mandatory attrs is << number of
> handled by default attrs, so I'd prefer to whitelist mandatories if it
> is possible. Also the solution whatever it is should process cfg
> recursively as cfg is actually nested set of types.submodules.
>
> Here is a minimal example, which demonstrates the problem I am trying
> to solve. It doesn't make sense overall, so don't judge me for missing
> mkIf or silly system services ;) it just shows what I am trying to
> achieve.
>
> { config, lib, pkgs, ... }:
> with lib;
> let
> # question is how to write this function
> filter_cfg = cfg: cfg;
>
> cfg = filter_cfg config.services.xyz;
> confFile = pkgs.writeText "xyz.json" (builtins.toJSON cfg);
> in
> {
> options.services.xyz = {
> opt1 = mkOption { type = types.str; default="default_opt1"; };
>
> # should be excluded from cfg if missing in configuration
> opt_devs_defaults = mkOption { type = types.str; };
>
> # error should be raised if missing in conf
> opt_mandatory = mkOption { type = types.str; };
> };
>
> config = {
> systemd.services.xyz = {
> serviceConfig = {
> ExecStart = "${pkgs.bash} -c 'touch ${confFile}'";
> };
> };
> };
> }
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>
For optional values, you could set default to null and then checking for
null later. If it's null, don't include it in the command and if it
isn't then do include it. Effectively ‘not set’ becomes default.
I'm unsure how you can enforce a presence of an option. Things like
‘assert’ &c could be used to catch such things but I don't know whether
this is recommended course of action.
--
Mateusz K.
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