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evaluate inherit (from) exprs only once per directive

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desugaring inherit-from to syntactic duplication of the source expr also
duplicates side effects of the source expr (such as trace calls) and
expensive computations (such as derivationStrict).
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pennae 2024-01-27 16:33:34 +01:00
parent ecf8b12d60
commit cefd0302b5
9 changed files with 109 additions and 15 deletions
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doc/manual/rl-next/inherit-from-by-need.md Normal file
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---
synopsis: "`inherit (x) ...` evaluates `x` only once"
prs: 9847
---
`inherit (x) a b ...` now evaluates the expression `x` only once for all inherited attributes rather than once for each inherited attribute.
This does not usually have a measurable impact, but side-effects (such as `builtins.trace`) would be duplicated and expensive expressions (such as derivations) could cause a measurable slowdown.