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docs(tvix/eval): start doc about problematic/weird lang behavior

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The idea is that we can keep track of the more unexpected behavior,
behavior that maybe should not be a thing at all and behavior we are not
sure about yet.

Change-Id: I70933f00af1230a7ab9d30e917b61199fe571caf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6803
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
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sterni 2022-09-29 16:29:40 +02:00
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tvix/eval/docs/language-issues.md Normal file
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# Nix language issues
In the absence of a language standard, what Nix (the language) is, is prescribed
by the behavior of the C++ Nix implementation. Still, there are reasons not to
accept some behavior:
* Tvix aims for nixpkgs compatibility only. This means we can ignore behavior in
edge cases nixpkgs doesn't trigger as well as obscure features it doesn't use
(e.g. `__overrides`).
* Some behavior of the Nix evaluator seems to be unintentional or an
implementation detail leaking out into language behavior.
Especially in the latter case, it makes sense to raise the respective issue and
maybe to get rid of the behavior in all implementations for good. Below is an
(incomplete) list of such issues:
* [Behaviour of nested attribute sets depends on definition order][i7111]
* [Partially constructed attribute sets are observable during dynamic attr names construction][i7012]
* [Nix parsers merges multiple attribute set literals for the same key incorrectly depending on definition order](i7115)
On the other hand, there is behavior that seems to violate one's expectation
about the language at first, but has good enough reasons from an implementor's
perspective to keep them:
* Dynamic keys are forbidden in `let` and `inherit`. This makes sure that we
only need to do runtime identifier lookups for `with`. More dynamic (i.e.
runtime) lookups would make the scoping system even more complicated as well
as hurt performance.
* Dynamic attributes of `rec` sets are not added to its scope. This makes sense
for the same reason.
* Dynamic and nested attributes in attribute sets don't get merged. This is a
tricky one, but avoids doing runtime (recursive) merges of attribute sets.
Instead all necessary merging can be inferred statically, i.e. the C++ Nix
implementation already merges at parse time, making nested attribute keys
syntactic sugar effectively.
[i7111]: https://github.com/NixOS/nix/issues/7111
[i7012]: https://github.com/NixOS/nix/issues/7012
[i7115]: https://github.com/NixOS/nix/issues/7115