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add zipAttrsWith primop

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nixpkgs can save a good bit of eval memory with this primop. zipAttrsWith is
used quite a bit around nixpkgs (eg in the form of recursiveUpdate), but the
most costly application for this primop is in the module system. it improves
the implementation of zipAttrsWith from nixpkgs by not checking an attribute
multiple times if it occurs more than once in the input list, allocates less
values and set elements, and just avoids many a temporary object in general.

nixpkgs has a more generic version of this operation, zipAttrsWithNames, but
this version is only used once so isn't suitable for being the base of a new
primop. if it were to be used more we should add a second primop instead.
This commit is contained in:
pennae 2021-12-25 15:29:49 +01:00
parent 96d08fcd66
commit 00c993f48b
4 changed files with 96 additions and 0 deletions
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2
doc/manual/src/release-notes/rl-next.md
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@ -4,3 +4,5 @@
more compliant one](https://github.com/ToruNiina/toml11).
* Added `:st`/`:show-trace` commands to nix repl, which are used to
set or toggle display of error traces.
* New builtin function `builtins.zipAttrsWith` with same functionality
as `lib.zipAttrsWith` from nixpkgs, but much more efficient.