[Nix-dev] Question about "package sets"

[Karn Kallio]

Nixpkgs has several "package sets" such as emacsPackages, haskellPackages, 
pythonPackages that generally group together expressions depending on a 
particular interpreter or compiler.  Sometimes they use recurseIntoAttrs  ( 
python, perl ) and sometimes not ( emacs, haskell ) .  There is no package set 
like cPackages or gccPackages that groups expressions depending on a C 
compiler nor a javaPackages.

Is there a policy or convention as to when to form package sets and if so 
whether or not to use recurseIntoAttrs?