Making sense of nix-shell

Help
1

I’m trying to understand some apparently magic behavior of nix-shell. I have following nix expression which relates to a gem file, here with fake name gem_name:

  { lib, bundlerEnv, ruby }:                                                                                                     
                                                                                                                                 
  bundlerEnv rec {                                                                                                               
    name = "gem_name-${version}";                                                                                                
                                                                                                                                 
    version = (import gemset).gem_name.version;                                                                                  
    inherit ruby;                                                                                                                
    # expects Gemfile, Gemfile.lock and gemset.nix in the same directory                                                         
    gemdir = ./.;                                                                                                                                                                                                                                          
  }

gemset.nix has been generated through bundix in the recommended way.

  1. nix-shell man page says it accepts a file with a nix expression defining a derivation. In fact, this file is not a derivation, but a function which if called with proper arguments defines a derivation. Reading at --arg argument documentation I have deduced that nix-shell has some kind of autocall behavior when the expression defines a function. How does it exactly work in this case, where --arg is not even provided? I haven’t found any documentation about it.

  2. If I run nix-shell I get an error:

error: undefined variable 'gemset' at /my/path/default.nix:6:21

However, if I run nix-shell -p <any_package>, for example, nix-shell -p hello it works. Reading at the docs, -p just makes a package available in the shell. However, it seems that it also makes the import to work.

  1. Which is the difference between nix-shell -p package and nix-shell -A attribute? Isn’t -A also used to load a package via its attribute?

Thanks for any insight

2

Edit: sorry, cutting out my use of numbers; I was numbering for my own benefit and not trying to map them to your questions; this is only a partial answer.

I’m not familiar with any of the nix+ruby ecosystem stuff, so I’m shooting from the hip a bit…

  • When you run nix-shell -p hello, it isn’t building your default.nix at all.
  • I think the reason you get the variable error is that nix is interpreting gemset as a variable and not a path type or string type. I think you want something like import ./gemset.nix
1 Like
3

generally import works on a path, you can think of it as copying and pasting all the text from that file into the expression. In general, the nix expression that’s imported is either a dictionary of configuration, or a function which given something like a version of nodejs is able to produce a package set.

4

Thanks for your answers @abathur & @jonringer.

About the import, it makes total sense. In fact, I found it weird having import gemset as I hadn’t seen anything like that in nix expression language documentation. However, I was just copying from nixpkgs documentation for ruby packages. Is it a typo there?

Changing gemset for the actual path makes nix-shell complain with:

error: cannot auto-call a function that has an argument without a default value ('lib')

But now I realize that, of course, that documentation expects the use of callPackage or something similar to build the derivation (so there is no autocall magic as I told), as it is documentation intended to build a package. I’ve updated it and now it works.

When you run nix-shell -p hello , it isn’t building your default.nix at all.

Oh!!! Thanks! I misunderstood the behavior of -p. I also realized that -A is completely different, as it selects an attribute from the current expression and creates the environment from it instead of from the whole expression.

5

Yes. In the example as written, import gemset doesn’t work. If you modify the example to include the attribute gemset = ./gemset.nix; (next to the definition of gemdir) then it would work, but you normally omit that definition because it has a default value based on gemdir already.