[Nix-dev] nix os modules: how to filter out unset values

[phreedom at yandex.ru]

On Monday, June 30, 2014 22:56:27 Max Ivanov wrote:
> That would work if not type of default should match type of option. So
> to make it work, I'd need to invent magic values for every type I use,
> and then filter them all out, which doesn't seem to be very elegant
> solution. 

Unless the magic value is null :) Try this:
opt1 = mkOption { type = types.nullOr types.str; default=null; };

> The problem is that access to any attribute in
> config.services.xyz triggers evaluation of the value and if default is
> missing the error is thrown.
> 
> So in normal case it enforces values to be set, unless mkOption is
> used with default attribute. My minimal goal here is at least to keep
> enforcing as it is, and mark "optional" values so that they don't
> appear (or can be removed) from config.services.xyz
> 
> On Mon, Jun 30, 2014 at 8:48 AM, Mateusz Kowalczyk
> 
> <fuuzetsu at fuuzetsu.co.uk> wrote:
> > 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}'";
> >>           
> >>           };
> >>       
> >>       };
> >>   
> >>   };
> >> 
> >> }
> >> _______________________________________________
> >> nix-dev mailing list
> >> nix-dev at lists.science.uu.nl
> >> http://lists.science.uu.nl/mailman/listinfo/nix-dev
> > 
> > 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.
> > _______________________________________________
> > nix-dev mailing list
> > nix-dev at lists.science.uu.nl
> > http://lists.science.uu.nl/mailman/listinfo/nix-dev
> 
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