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nix/doc/manual/rl-next/inherit-from-by-need.md
pennae cefd0302b5 evaluate inherit (from) exprs only once per directive 
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).
2024-02-26 19:07:08 +01:00

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synopsis prs
`inherit (x) ...` evaluates `x` only once 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.