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doc: refactor documentation for lib.{composeExtensions,composeManyExtensions}

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Johannes Kirschbauer 2024-08-15 11:11:07 +02:00
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95
lib/fixed-points.nix
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@ -311,28 +311,8 @@ rec {
);
/**
Compose two extending functions of the type expected by 'extends'
into one where changes made in the first are available in the
'super' of the second
# Inputs
`f`
: 1\. Function argument
`g`
: 2\. Function argument
`final`
: 3\. Function argument
`prev`
: 4\. Function argument
Compose two overlay functions and return a single overlay function that combines them.
For more details see: [composeManyExtensions](#function-library-lib.fixedPoints.composeManyExtensions).
*/
composeExtensions =
f: g: final: prev:
@ -341,14 +321,69 @@ rec {
in fApplied // g final prev';
/**
Compose several extending functions of the type expected by 'extends' into
one where changes made in preceding functions are made available to
subsequent ones.
Composes a list of [`overlays`](#chap-overlays) and returns a single overlay function that combines them.
:::{.note}
The result is produced by using the update operator `//`.
This means nested values of previous overlays are not merged recursively.
In other words, previously defined attributes are replaced, ignoring the previous value, unless referenced by the overlay; for example `final: prev: { foo = final.foo + 1; }`.
:::
# Inputs
`extensions`
: A list of overlay functions
:::{.note}
The order of the overlays in the list is important.
:::
: Each overlay function takes two arguments, by convention `final` and `prev`, and returns an attribute set.
- `final` is the result of the fixed-point function, with all overlays applied.
- `prev` is the result of the previous overlay function.
# Type
```
composeManyExtensions : [packageSet -> packageSet -> packageSet] -> packageSet -> packageSet -> packageSet
^final ^prev ^overrides ^final ^prev ^overrides
# Pseudo code
let
# final prev
# ↓ ↓
OverlayFn :: ( { ... } -> { ... } -> { ... } );
in
composeManyExtensions :: [ OverlayFn ] -> OverlayFn
```
# Examples
:::{.example}
## `lib.fixedPoints.composeManyExtensions` usage example
```nix
let
# The "original function" that is extended by the overlays.
# Note that it doesn't have prev: as argument since it can be thought of as the first function.
original = final: { a = 1; };
# Each overlay function has 'final' and 'prev' as arguments.
overlayA = final: prev: { b = final.c; c = 3; };
overlayB = final: prev: { c = 10; x = prev.c or 5; };
extensions = composeManyExtensions [ overlayA overlayB ];
# Caluculate the fixed point of all composed overlays.
fixedpoint = lib.fix (lib.extends extensions original );
in fixedpoint
=>
{
a = 1;
b = 10;
c = 10;
x = 3;
}
```
:::
*/
composeManyExtensions =
lib.foldr (x: y: composeExtensions x y) (final: prev: {});
@ -357,17 +392,17 @@ rec {
Create an overridable, recursive attribute set. For example:
```
nix-repl> obj = makeExtensible (self: { })
nix-repl> obj = makeExtensible (final: { })
nix-repl> obj
{ __unfix__ = «lambda»; extend = «lambda»; }
nix-repl> obj = obj.extend (self: super: { foo = "foo"; })
nix-repl> obj = obj.extend (final: prev: { foo = "foo"; })
nix-repl> obj
{ __unfix__ = «lambda»; extend = «lambda»; foo = "foo"; }
nix-repl> obj = obj.extend (self: super: { foo = super.foo + " + "; bar = "bar"; foobar = self.foo + self.bar; })
nix-repl> obj = obj.extend (final: prev: { foo = prev.foo + " + "; bar = "bar"; foobar = final.foo + final.bar; })
nix-repl> obj
{ __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }