Nix eval to read a value or provide a default

Help
1

The simple explanation

I have a program that’s going to read my configuration from flake.nix and do something with the remote host based on some information in there.

I can read a value from the Nix flake like this:

nix eval --read-only --quiet .#nixosConfigurations.my-host.config.my-feature.enabled

This works great. Nice and fast too. Returns true or false, based on what that value is set to.

The problem: I want it to default to false if the value is not defined at all (it’s defined in a module, so I want it to assume false if the module is not present). Currently nix eval just returns a non zero error code, so I can’t tell if it wasn’t defined or if there was a syntax error.

The icky details

I’m working on an experimental system for managing robots.
I’ve created this module to automatically revert to a previous generation if the robot doesn’t get confirmation that ssh still works. I want to make my deployment system get grumpy at the user if this module is disabled or not present.

2

You might be able to use --apply with something involving tryEval?

3

Can’t tell what you want, seems like there’s conflicting requirements here.

Use options.<whatever>.isDefined to check if it’s set.
Or provide default to mkOption.

4

@waffle8946 I’m confused by that answer…
It is defined as an option, with a default here.

The issue is that if the user doesn’t include the module, then that option, with its default, will not be present and the evaluation fails.

@tejing Is this what you had in mind? Looking at the documentation for tryEval, I’m guessing this doesn’t count as a “thrown error”.

[thecarl@the-carl-tower:~/Projects/ros_assistant/packages/rass-cli/example_projects/nvidia-xavier-agx]$ nix eval --read-only --apply builtins.tryEval .#nixosConfigurations.generic-x86.config.auto-revert.enable
error: flake 'git+file:///home/thecarl/Projects/ros_assistant?dir=packages/rass-cli/example_projects/nvidia-xavier-agx' does not provide attribute 'packages.x86_64-linux.nixosConfigurations.generic-x86.config.auto-revert.enable', 'legacyPackages.x86_64-linux.nixosConfigurations.generic-x86.config.auto-revert.enable' or 'nixosConfigurations.generic-x86.config.auto-revert.enable'
5

Guess that idea won’t work.

You could still use --apply, though. Just call it on nixosConfigurations.generic-x86.config and index into the attrset using the --apply expression, with defaults, and probably still tryEval, for the case where the option is created but never set, since the module system should throw in that case.

6

I don’t think I follow.

Is this in the ballpark of what you’re thinking?

nix eval --read-only --apply builtins.tryEval .#nixosConfigurations.generic-x86.config

I’m starting to suspect what I’m trying to do may just be too far outside of the scope of Nix and I should rethink the whole idea. The ultimate goal is to detect if the auto-revert.nix module has been included into the configuration.

7

More like

nix eval --read-only --apply 'config: (config.my-feature or false).enabled or false' .#nixosConfigurations.generic-x86.config

It sounds like you don’t need to catch a “not-set” error, since you provide a default value so long as the module is loaded at all. If that’s correct, then you shouldn’t need the tryEval after all.

8

Thank you so much for your time and patience! I had to massage it a little but that seems to do exactly what I need.

I had tried to do something similar to this but hadn’t figured out that making it a function would let me use it under apply (I still consider myself new to Nix). I’m just trying to make sure I grasp exactly how this works.

#                                      |- If this fails to be resolved, false
#                                      |                           |- Just kicks back the actual value of "enabled"
#                                      |                           |          |- If enabled failed to resolve, false
#                                      |                           |          |      | - Path to our system's config. Is used as the input to the function.
#                                      V                           V          V      V
nix eval --read-only --apply 'config: (config.my-feature or false).enabled or false' .#nixosConfigurations.generic-x86.config
9

Yup, you’ve got that about right. Technically the first false is a bit misleading, since it isn’t actually the return value in the case where my-feature doesn’t exist. It just ensures the second lookup fails because false isn’t an attrset and thus doesn’t have any keys. You could replace the first false with {} and it might be a little clearer how that part works.

1 Like