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| -rw-r--r-- | tvix/eval/docs/known-optimisation-potential.md | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/tvix/eval/docs/known-optimisation-potential.md b/tvix/eval/docs/known-optimisation-potential.md index e2e0497aeea1..951771e739a7 100644 --- a/tvix/eval/docs/known-optimisation-potential.md +++ b/tvix/eval/docs/known-optimisation-potential.md @@ -138,3 +138,22 @@ optimisations, but note the most important ones here. determining if finalisation is necessary or not. This wouldn't be necessary if `OpFinalise` would just noop on any values that don't need to be finalised (anymore). + +* Phantom binding for from expression of inherits [easy] + + The from expression of an inherit is reevaluated for each inherit. This can + be demonstrated using the following Nix expression which, counter-intuitively, + will print “plonk” twice. + + ```nix + let + inherit (builtins.trace "plonk" { a = null; b = null; }) a b; + in + builtins.seq a (builtins.seq b null) + ``` + + In most Nix code, the from expression is just an identifier, so it is not + terribly inefficient, but in some cases a more expensive expression may + be used. We should create a phantom binding for the from expression that + is reused in the inherits, so only a single thunk is created for the from + expression. |