f3c27f1717
Working on https://github.com/NixOS/nix/pull/7158, I discovered that C++ Nix actually is strict in the accumulator, just not in the first value. This seems due to the fact that in the C++ evaluator, function calls don't seem to be thunked unconditionally and foldl' just elects not to wrap it in a thunk (don't quote me on this summary, even though it seems to line up with the code for primop_foldlStrict and testable behavior). It doesn't seem worth it to risk breaking the odd Nix expression just to be strict in one more value per invocation of foldl' (i.e. the initial accumulator value `nul`), so let's match the existing C++ Nix behavior here. Change-Id: If59e62271a90d97cb440f0ca72a58ec7840d1690 Reviewed-on: https://cl.tvl.fyi/c/depot/+/7022 Autosubmit: sterni <sternenseemann@systemli.org> Tested-by: BuildkiteCI Reviewed-by: tazjin <tazjin@tvl.su>
46 lines
2.3 KiB
Markdown
46 lines
2.3 KiB
Markdown
# Nix language issues
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In the absence of a language standard, what Nix (the language) is, is prescribed
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by the behavior of the C++ Nix implementation. Still, there are reasons not to
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accept some behavior:
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* Tvix aims for nixpkgs compatibility only. This means we can ignore behavior in
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edge cases nixpkgs doesn't trigger as well as obscure features it doesn't use
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(e.g. `__overrides`).
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* Some behavior of the Nix evaluator seems to be unintentional or an
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implementation detail leaking out into language behavior.
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Especially in the latter case, it makes sense to raise the respective issue and
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maybe to get rid of the behavior in all implementations for good. Below is an
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(incomplete) list of such issues:
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* [Behaviour of nested attribute sets depends on definition order][i7111]
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* [Partially constructed attribute sets are observable during dynamic attr names construction][i7012]
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* [Nix parsers merges multiple attribute set literals for the same key incorrectly depending on definition order](i7115)
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On the other hand, there is behavior that seems to violate one's expectation
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about the language at first, but has good enough reasons from an implementor's
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perspective to keep them:
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* Dynamic keys are forbidden in `let` and `inherit`. This makes sure that we
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only need to do runtime identifier lookups for `with`. More dynamic (i.e.
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runtime) lookups would make the scoping system even more complicated as well
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as hurt performance.
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* Dynamic attributes of `rec` sets are not added to its scope. This makes sense
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for the same reason.
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* Dynamic and nested attributes in attribute sets don't get merged. This is a
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tricky one, but avoids doing runtime (recursive) merges of attribute sets.
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Instead all necessary merging can be inferred statically, i.e. the C++ Nix
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implementation already merges at parse time, making nested attribute keys
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syntactic sugar effectively.
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Other behavior is just odd, surprising or underdocumented:
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* `builtins.foldl'` doesn't force the initial accumulator (but all other
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intermediate accumulator values), differing from e.g. Haskell, see
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the [relevant PR discussion](p7158).
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[i7111]: https://github.com/NixOS/nix/issues/7111
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[i7012]: https://github.com/NixOS/nix/issues/7012
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[i7115]: https://github.com/NixOS/nix/issues/7115
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[p7158]: https://github.com/NixOS/nix/pull/7158
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