I think something like this needs some good use cases to be included in full. Right now a lot of them seem like they have a pretty limited use case. I'd definitely be more comfortable with some getting included if it leads to less total code in Nixpkgs.

I had a similar try at something like writers.write in #43074. buildTree does I think the same thing so that these two are equivalent:

write "name" { "/etc/test" = { text = "hello world"; }; }

buildTree "name" {
  "/etc/test" = writeText "test" "hello world";
} {}

So I think something like that is definitely useful.

I think most of the use-cases would be outside of nixpkgs or in nixos modules.
It's very cool to have inline packages in your shell.nix or ExecStart scripts in other languages than bash.

What would be the recommended way to test these? use nixos-tests?

We have pkgs/test, but maybe something in the writers directory would make more sense.

Soo I guess I'm done. I'm not sure about the tests though

I think we need better criteria of where this can be used. Is this intended for internal use within Nixpkgs? If so, we need some usages inside the tree. If not, why not just release this as a separate repo? I just don't think we have a good enough use for this right now.

Also, it critically doesn't support external files, which seems pretty important. You can kind of "fake" it with something like:

writeBash "examples" (builtins.readFile ./example.sh)

But this just reads the file and writes it back to the store twice! Anyway, this seems like something that is an interesting use of Nix, but should be left out of Nixpkgs.

There are usecases inside of nixpkgs. Having something like services.xserver.windowManager.xmonad.config comes to my mind. I can implement this next. Also a lot of suckless software rely upon recompliation and there is no abstraction inside configuration.nix

About the files ending up twice in the store, does writeScript(Bin) handle this?
I guess, I could extend the writers to take strings or files.