builtins.attrNames
#builtins
#AttrSets
AttrSets -> String []
AttrSetsがもつkeyの一覧を返す
https://nixos.org/manual/nix/stable/language/builtins.html#builtins-attrNames
code: exmaple.nix
builtins.attrNames { x=1; } /* =>
"x"
*/