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docs(tvix/eval): optimization potential for inherit (from) exprs

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Change-Id: Ibddaa111a5b7a86c42dbe153ae8e53f9a5601a54
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10112
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>
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sterni 2023-08-23 15:06:05 +02:00
parent 6b685ec4a5
commit c5961e7774
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19
tvix/eval/docs/known-optimisation-potential.md
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@ -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.