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Turns out, my lib value was actually a function; unfortunately, since nix flakes is still unstable, it didn't quite show where this was happening.

nixos.wiki has an example of overriding the source of a package.

You do need to provide a reproducible source. A github repo url is mutable, so you need to specify the revision.

Ok I finally managed to get it working. After more searching and getting lost it turns out nix has pkgs.vimUtils.buildVimPlugin This solved my problem. My files now read:

Since your using the --pure option, nothing from your previous environment will be imported into the shell. This includes the swift binary itself. So first I would add swift to the buildInputs.

The error your getting indicates that gcc doesn't even understand the command line options being passed. I don't know anything about Swift, but after seeing that I assumed that it needs a different compiler. Sure enough, after a quick search it looks like it is llvm based. You will thus need to also add clang to your build inputs. In addition, you'll need to use mkShell instead of mkDerivation to setup the proper environment.

The former is a wrapper to the latter that is specifically designed to set up a nix-shell, and ensures the environment is handled correctly. You'll need to pass in the shellHook attribute to override the default c compiler to clang using the CC envvar. The shellHook is a way of running arbitrary commands at the start of the shell.

As a final note, the nix-shell command searches for shell.nix by default so you don't have to pass it as an argument. If shell.nix does not exist, then it will try default.nix. If neither exist, then you'll have to explicitly specify the nix expressions location.

TLDR

to put it all together:

You should return false from your interceptor if you are done with execution.

Returns: true if the execution chain should proceed with the next interceptor or the handler itself. Else, DispatcherServlet assumes that this interceptor has already dealt with the response itself.

http://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/servlet/HandlerInterceptor.html

Change

It can be done. The lib attribute in Nixpkgs has a host of functions that come in handy from time to time.

I am a little bit concerned by your example, because rewriting this as an algorithm doesn't provide any value in my perspective. I will assume that such a solution does provide value in your actual problem.

Here's a little nix repl session with a solution:

I want to declare variable dotfiles_dir so all other files can see and use it.

Sorry, but that's not possible. In Nix, there's no such thing as global variables. If there were, it would ruin it's ability to provide reproducible builds because then Nix expressions would have access to undeclared inputs.

/etc/nixos/configuration.nix is not place store global information, it's technically a NixOS module. But more importantly, it's a function.

However... there's a way to define a value in one place and use it where ever you need it. Something like this:

/etc/nixos/dotfiles-dir.nix

Just two things:

1 - I think you have to much code for a simple thing like this, we can reduce it.

2 - I highly adivise you saving your user in it's uuid created when the user creates an account, it's safier. So you'll have and structure like