A Nix query language?

[roberth]

the main reason we wrote https://github.com/clan-lol/nix-select was that the nix language is quite limited (but it doesn't have to be). We would be very happy if the builtin selection language inside nix could take some (or all?) of the features from nix-select :)

Originally posted by @Lassulus in #13140

[roberth]

This would ideally fit into the existing language and be concise enough for command line use.
Ideally we coordinate this to be a superset and generalization of #13140 that returns multiple, structured results instead of identifying a single value.

Related ideas

• [RFC 0181] List index syntax rfcs#181
• [RFC 0110] Add "inherit-as-list" syntax construct to the Nix language rfcs#110
• Haskell monad comprehensions
  • Generalized, query-like
• Python list comprehensions
• lib.concatMapAttrs (also, attrsets aren't monads, but that may be ok)
• jq, XPath