about summary refs log tree commit diff
path: root/tvix/docs/src/eval/language-issues.md
diff options
context:
space:
mode:
Diffstat (limited to 'tvix/docs/src/eval/language-issues.md')
-rw-r--r--tvix/docs/src/eval/language-issues.md46
1 files changed, 46 insertions, 0 deletions
diff --git a/tvix/docs/src/eval/language-issues.md b/tvix/docs/src/eval/language-issues.md
new file mode 100644
index 000000000000..152e6594a1d0
--- /dev/null
+++ b/tvix/docs/src/eval/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