Nix could support evaluating attribute sets more lazily, such that e.g. the following works:

(throw "" // { x = 0; }).x
-> 0

Currently this doesn't work because attribute sets are evaluated strictly in all names, meaning all // operations and co. will have to be applied to figure out all names, making the above code throw in the current version.

Making attribute names lazy in that way has the potential to solve many existing problems:

• .extend attributes of package scopes won't necessarily have to evaluate all package attributes. Similarly .override, etc. of packages won't have to call the derivation primop multiple times anymore
• types.lazyAttrsOf might not be needed anymore
• In overlays you'll be able to use self.lib for the attribute definitions if lib is added in a later overlay
• lib.mkIf might not be necessary for the module system anymore
• Potentially more

However this also changes semantics, so this should be thought out well.

I have implemented an initial prototype that supports above evaluation in infinisil@afeda4f. I'll continue with this if I have time.

Ping @edolstra @Profpatsch

NOTE (@roberth): To manage expectations, querying a non-existing attribute fundamentally requires evaluation of all attribute names.

• e.g. if attrs?attr then e_lazy else e_strict has to strictly evaluate the attrnames for e_strict.
• { foo, ...}: -style functions have to strictly evaluate all the attrnames in order to preserve their existing semantics.