How do specialArgs work?

[zogstrip]

@zogstrip

I need to do more testing to better understand this.

I may have found a hint in @djacu’s excellent Function Arguments - NixOS Modules Lessons

Function Arguments

When you define a module as a function like we did previously, there are certain arguments that are automatically provided.

You do not have to explicitely put them in the function signature as we have the ellipsis ... to manage any arguments we do not use.

All modules are passed the following arguments:

• lib: The nixpkgs library.
• config: The results of all options after merging the values from all modules together.
• options: The options declared in all modules.
• specialArgs: An attribute set of extra arguments to be passed to the module functions.
• All attributes of specialArgs.

NOTE

The fact that all the attributes of specialArgs are automatically provided means you don’t need to add specialArgs to the module function signature if we want access to specialArgs.thing.
We can just add thing to the function signature and use it directly.

It looks like specialArgs is always passed as an argument as well as all its attributes!

Which seems to be what the line @zimward pointed at do

({ inherit lib options config specialArgs; } // specialArgs)

Which means, pass lib, options, config, specialArgs and “merge” with all the attributes of specialArgs (using the // operator).