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module-system.chapter.md: Elaborate on extendModules performance

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Robert Hensing 2023-04-16 19:44:18 +02:00
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doc/module-system/module-system.chapter.md
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@ -75,11 +75,20 @@ A function similar to `evalModules` but building on top of the already passed [`
If you're familiar with prototype inheritance, you can think of the current, actual `evalModules` invocation as the prototype, and the return value of `extendModules` as the instance.
This functionality is also available to modules as the `extendModules` module argument.
::: {.note}
**Evaluation Performance**
`extendModules` returns a configuration that shares very little with the original `evalModules` invocation, because the module arguments may be different.
So if you have a configuration that has been (or will be) largely evaluated, almost none of the computation is shared with the configuration returned by `extendModules`.
The real work of module evaluation happens while computing the values in `config` and `options`, so multiple invocations of `extendModules` have a particularly small cost, as long as only the final `config` and `options` are evaluated.
If you do reference multiple `config` (or `options`) from before and after `extendModules`, evaluation performance is the same as with multiple `evalModules` invocations, because the new modules' ability to override existing configuration fundamentally requires constructing a new `config` and `options` fixpoint.
This functionality is also available to modules as the `extendModules` module argument.
:::
#### `_module` {#module-system-lib-evalModules-return-value-_module}