nix-repl> lib.mkIf true "test"  
{ _type = "if"; condition = true; content = "test"; }

In reply to @throwachimera:matrix.org
Hi, I'm trying to learn Nix for NixOS and sometimes I want to evaluate expressions to understand them better. However, when I try using libraries from nixpkgs, they don't fully evaluate. For example:

nix-repl> lib.mkIf true "test"  
{ _type = "if"; condition = true; content = "test"; }

I'm not sure if this is due to lazy evaluation or if there’s some other reason. How can I work around this? In general, I'd be interested in hearing about the ways people here debug and learn the language, as I guess I'm not usually seeing the resulting attribute sets directly when using it to configure NixOS.

This isn't actually something directly related to the language or evaluation, I think. The result in your example actually is fully evaluated! The mkIf funtion in nixpkgs is just defined in a way that evaluates to that attribute set.

The NixOS module system would tzen take that result if you put that in your config and use it to calculate yout final config. That is implemented in Nix, but not a feature of the Nix language itself. The Nix language has no concept of modules and so on, so NixOS uses things like Nix attribute sets with _type attribute as in your example to implement its features.

In reply to@eisfunke:eisfunke.com

This isn't actually something directly related to the language or evaluation, I think. The result in your example actually is fully evaluated! The mkIf funtion in nixpkgs is just defined in a way that evaluates to that attribute set.

The NixOS module system would tzen take that result if you put that in your config and use it to calculate yout final config. That is implemented in Nix, but not a feature of the Nix language itself. The Nix language has no concept of modules and so on, so NixOS uses things like Nix attribute sets with _type attribute as in your example to implement its features.

throwachimera: Here's something that works:

nix-repl> moduleEval = type: config: lib.mergeDefinitions [ "root" ] type [ { file = "<expr>"; value = config; } ]

nix-repl> :p moduleEval lib.types.str (lib.mkIf true "test")                                                       
{ defsFinal = [ { file = "<expr>"; value = "test"; } ]; defsFinal' = { highestPrio = 100; values = [ «repeated» ]; }; isDefined = true; mergedValue = "test"; optionalValue = { value = "test"; }; }

Or since .mergedValue is usually most interesting:

nix-repl> moduleEval = type: config: (lib.mergeDefinitions [ "root" ] type [ { file = "<expr>"; value = config; } ]).mergedValue

nix-repl> :p moduleEval lib.types.str (lib.mkIf true "test")                                                                     
"test"

In reply to@infinisil:matrix.org

Or since .mergedValue is usually most interesting:

nix-repl> moduleEval = type: config: (lib.mergeDefinitions [ "root" ] type [ { file = "<expr>"; value = config; } ]).mergedValue

nix-repl> :p moduleEval lib.types.str (lib.mkIf true "test")                                                                     
"test"