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module-system.chapter.md: Add mental model to type and extendModules

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Robert Hensing 2023-04-16 19:25:09 +02:00
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doc/module-system/module-system.chapter.md
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@ -64,12 +64,17 @@ The option definitions that are typed with this type will extend the current set
However, the value returned from the type is just the [`config`](#module-system-lib-evalModules-return-value-config), like any submodule.
If you're familiar with prototype inheritance, you can think of this `evalModules` invocation as the prototype, and usages of this type as the instances.
This type is also available to the [`modules`](#module-system-lib-evalModules-param-modules) as the module argument `moduleType`.
<!-- TODO: document the module arguments. Using moduleType is like saying: suppose this configuration was extended. -->
#### `extendModules` {#module-system-lib-evalModules-return-value-extendModules}
A function similar to `evalModules` but building on top of the already passed [`modules`](#module-system-lib-evalModules-param-modules). Its arguments, `modules` and `specialArgs` are added to the existing values.
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.
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.