There is also https://github.com/purenix-org/purenix
That's interesting, looks like it is transpiler that could be used immediately if someone doesn't mind the extra step. Thank you for sharing it.
BTW: the Nickel has chance to end up replace Nix language as I saw author of Nix (Eelco) is also contributing to it. I don't think it can transpile to a nix code.
1. https://github.com/tweag/nickel - it is a typed language, that might in the future replace nix language
2. looks like likely is addressed by flakes