fcd5e53703
When investigating discrepancies between foldl' in tvix and C++ Nix, I discovered that C++ Nix's foldl' doesn't seem to be strict at all. Since this seemed wrong, I looked into Haskell's foldl' implementation which doesn't force the list elements (`val` in our code), but the accumulation value (`res` in our code). You can look at the code here: https://hackage.haskell.org/package/base-4.17.0.0/docs/src/GHC.List.html#foldl%27 This actually makes a lot of sense: If `res` is not forced after each application of `op`, we'll end up thunks nested as deeply as the list is long, potentially taking up a lot of space. This can be limited by forcing the `res` thunk before applying `op` again (and creating a new thunk). I've also PR-ed an equivalent change for C++ Nix at https://github.com/NixOS/nix/pull/7158. Since this is not merged nor backported to our Nix 2.3 fork, I've not copied the eval fail test yet, since it wouldn't when checking our tests against C++ Nix in depot. Change-Id: I34edf6fc3031fc1485c3e714f2280b4fba8f004b Reviewed-on: https://cl.tvl.fyi/c/depot/+/6947 Autosubmit: sterni <sternenseemann@systemli.org> Reviewed-by: grfn <grfn@gws.fyi> Tested-by: BuildkiteCI
41 lines
2.1 KiB
Markdown
41 lines
2.1 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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* [builtins.foldl' doesn't seem to be strict](p7158)
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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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[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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