diff options
author | sterni <sternenseemann@systemli.org> | 2023-08-23T13·06+0200 |
---|---|---|
committer | sterni <sternenseemann@systemli.org> | 2023-11-24T23·36+0000 |
commit | c5961e77749d1d002a5e747468b87c34a32821f3 (patch) | |
tree | be912a20603afa28a731a3e2506d3d74c0596350 /tvix/eval/docs/known-optimisation-potential.md | |
parent | 6b685ec4a562693f56dee27624074b7cc38e70c8 (diff) |
docs(tvix/eval): optimization potential for inherit (from) exprs r/7052
Change-Id: Ibddaa111a5b7a86c42dbe153ae8e53f9a5601a54 Reviewed-on: https://cl.tvl.fyi/c/depot/+/10112 Tested-by: BuildkiteCI Reviewed-by: Adam Joseph <adam@westernsemico.com>
Diffstat (limited to 'tvix/eval/docs/known-optimisation-potential.md')
-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. |