When given two lists each of which are conditional on basis of mkIf, how to merge those two lists (akin to mkMerge for sets)?
I have run into the following pattern several times now and I cannot seem to find documentation explaining how to idiomatically solve this.

Say you’re starting out with this snippet:

someConfigValue = mkIf (cfg.condition1) [ "some value 1" ];

Then you add another conditional value:

someConfigValue = [
  (mkIf (cfg.condition1) [ "some value 1" ])
  (mkIf (cfg.condition2) [ "some value 2" ])
];

In the specific case at hand (trying to pass this to systemd.services.<name>.LoadCredential) I get the following error since the internal representation of the two mkIf values are sets:

error: cannot coerce a set to a string

Using mkMerge does not seem to work either (since it takes a list of sets and merges those sets).
I could replace the mkIfs with optionals and then concatenate the lists using ++, but that’s dumping all the module logic, so I’m not sure whether that’s the right choice here.

>Solution :

Ordinarily, I would use optionals for something like this.

If you really want to make it mergable, I think you can instead put whateverService.serviceConfig.LoadCredentials = [ "cred 1" ]; on one side of a merge and whateverService.serviceConfig.LoadCredentials = [ "cred 2" ]; on the other side, and the standard merging logic will put them in the same list.

This more or less results in just pushing someConfigValue in one level:

config = mkMerge [
  (mkIf (cfg.condition1) { someConfigValue = [ "some value 1" ]; })
  (mkIf (cfg.condition2) { someConfigValue = [ "some value 2" ]; })
];

Which is more or less how I tend to see mkIf used: It predicates whole sections of config at the top level, rather than individual values inside of config. Individual values tend to use things like optionals or optionalString.