What is __filter in flake.nix?

[Septias]

The top-level flake.nix uses a lot of dunder methods like __filter which look like the library functions, but aren't. I also don't find any definition for them in the file so I wonder how the whole flake even works. Can someone explain this shortly?

[deemp]

@Septias, I believe you've discovered them in nix repl. Try running repl inside this repository without :lf. You'll see these functions. They're builtins. They aren't a part of the flake.