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diff --git a/tvix/eval/docs/language-issues.md b/tvix/eval/docs/language-issues.md new file mode 100644 index 0000000000..152e6594a1 --- /dev/null +++ b/tvix/eval/docs/language-issues.md @@ -0,0 +1,46 @@ +# 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. + +Other behavior is just odd, surprising or underdocumented: + +* `builtins.foldl'` doesn't force the initial accumulator (but all other + intermediate accumulator values), differing from e.g. Haskell, see + the [relevant PR discussion][p7158]. + +[i7111]: https://github.com/NixOS/nix/issues/7111 +[i7012]: https://github.com/NixOS/nix/issues/7012 +[i7115]: https://github.com/NixOS/nix/issues/7115 +[p7158]: https://github.com/NixOS/nix/pull/7158 |