| 17 Jan 2026 |
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| 18 Jan 2026 |
 roberth | I would much appreciate your feedback on https://github.com/NixOS/rfcs/pull/196: Self-describing store
This RFC proposes that stores provide a nix-store.json file declaring their properties, enabling:
- Discovery of store settings (storeDir, encoding, etc.)
- Forward compatibility through explicit feature requirements
- A unified replacement for the ad hoc nix-cache-info format
| 16:55:25 |
| roberth | (best discussed over there) | 16:56:38 |
|  losvedar joined the room. | 17:37:54 |
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| 19 Jan 2026 |
 niksnut | looks good to me in principle | 10:01:39 |
 raboof | is it expected that // is not lazy? i.e. it evaluates the LHS even if the RHS holds the thing I'm taking from it? | 14:26:57 |
| raboof | * is it expected that // is not lazy? i.e. it evaluates the left parameter even if the right parameter holds the thing I'm taking from it? | 14:27:44 |
 Alyssa Ross | nix-repl> ({ foo = throw "oh no"; } // { foo = 1; }).foo
1
| 14:28:09 |
| Alyssa Ross | looks lazy to me? | 14:28:13 |
| raboof | nix-repl> ((throw "oh no") // { foo = "foo"; }).foo
error: oh no
| 14:31:30 |
 ma27 | attr-names are generally not lazy in Nix, hence the throw is being evaluated. | 14:32:02 |
| raboof | I read // as a function (in infix notation) with two parameters, perhaps that's not how this works? | 14:32:41 |
| raboof | right, so // creates its set of attr-names at first access. that makes some sense I guess. | 14:40:42 |
 Sergei Zimmerman (xokdvium) | In reply to @raboof:matrix.org
is it expected that // is not lazy? i.e. it evaluates the left parameter even if the right parameter holds the thing I'm taking from it? Yes, attribute names are evaluated eagerly in part due to how attrsets are stored internally | 14:42:23 |
 Mic92 | Also because they have to be lexical sorted, no? | 14:56:23 |
| roberth | It's the simple thing to do, and simple is fast, usually. Not being simple is what stalled https://github.com/NixOS/nix/pull/4154 | 14:57:32 |
| roberth | Could be revisited, but it does change the language semantics so that's tricky, and hard to backtrack away from in the future | 14:58:10 |
| Mic92 | Yeah, feels hard to make this bug-compatible with the current behaviour. | 14:58:51 |
| Mic92 | One could have an infinite attrset that would work with new nix, but not with old nix. | 14:59:24 |
| Mic92 | Also that would be less likely the biggest issue | 14:59:51 |
| roberth | It'd be subtler current errors that would be the problem, mostly infinite recursions that would be lifted in the new Nix language | 15:01:10 |
| roberth | (or at least those would be the ones people would complain to me about :) ) | 15:01:50 |
 Taeer Bar-Yam | Generally turning errors into correct behaviour is the least bad way to break compatibility, right? Though it causes more problems now that we have other forks of nix | 15:07:26 |
| raboof | yeah I wasn't saying we should change this, just calibrating my expectations ;) - but nice to see more people have been pondering (and even hacking on) this ;) | 15:07:34 |
| roberth | no, raboof, you are in the rabbit hole with us now! | 15:08:04 |
| roberth | I agree it's a good kind of change, but the second order effects are real. E.g. you upgrade, you use it, I don't, it fails for me, I complain, we lose | 15:09:02 |
| roberth | not to say we can't get over that. It's still an improvement, but is it the right thing to do now? | 15:10:05 |
| roberth | I'd want to know the long term impact on evaluator performance including opportunities missed. tricky. | 15:10:47 |
