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doc: lib.composeExtensions reference to overlays (#325479)
This commit is contained in:
Robert Hensing
GitHub
commit
b4292c4370
@ -63,7 +63,6 @@ rec {
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See [`extends`](#function-library-lib.fixedPoints.extends) for an example use case.
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There `self` is also often called `final`.
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# Inputs
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`f`
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@ -90,7 +89,12 @@ rec {
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:::
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*/
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fix = f: let x = f x; in x;
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fix =
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f:
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let
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x = f x;
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in
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x;
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/**
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A variant of `fix` that records the original recursive attribute set in the
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@ -99,14 +103,20 @@ rec {
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This is useful in combination with the `extends` function to
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implement deep overriding.
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# Inputs
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`f`
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: 1\. Function argument
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*/
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fix' = f: let x = f x // { __unfix__ = f; }; in x;
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fix' =
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f:
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let
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x = f x // {
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__unfix__ = f;
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};
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in
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x;
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/**
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Return the fixpoint that `f` converges to when called iteratively, starting
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@ -117,7 +127,6 @@ rec {
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0
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```
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# Inputs
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`f`
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@ -134,13 +143,12 @@ rec {
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(a -> a) -> a -> a
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```
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*/
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converge = f: x:
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converge =
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f: x:
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let
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x' = f x;
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in
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if x' == x
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then x
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else converge f x';
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if x' == x then x else converge f x';
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/**
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Extend a function using an overlay.
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@ -149,7 +157,6 @@ rec {
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A fixed-point function is a function which is intended to be evaluated by passing the result of itself as the argument.
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This is possible due to Nix's lazy evaluation.
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A fixed-point function returning an attribute set has the form
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```nix
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@ -257,7 +264,6 @@ rec {
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```
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:::
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# Inputs
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`overlay`
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@ -299,8 +305,7 @@ rec {
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:::
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*/
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extends =
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overlay:
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f:
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overlay: f:
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# The result should be thought of as a function, the argument of that function is not an argument to `extends` itself
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(
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final:
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@ -311,63 +316,98 @@ rec {
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);
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/**
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Compose two extending functions of the type expected by 'extends'
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into one where changes made in the first are available in the
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'super' of the second
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# Inputs
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`f`
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: 1\. Function argument
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`g`
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: 2\. Function argument
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`final`
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: 3\. Function argument
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`prev`
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: 4\. Function argument
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Compose two overlay functions and return a single overlay function that combines them.
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For more details see: [composeManyExtensions](#function-library-lib.fixedPoints.composeManyExtensions).
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*/
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composeExtensions =
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f: g: final: prev:
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let fApplied = f final prev;
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prev' = prev // fApplied;
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in fApplied // g final prev';
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let
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fApplied = f final prev;
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prev' = prev // fApplied;
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in
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fApplied // g final prev';
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/**
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Compose several extending functions of the type expected by 'extends' into
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one where changes made in preceding functions are made available to
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subsequent ones.
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Composes a list of [`overlays`](#chap-overlays) and returns a single overlay function that combines them.
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:::{.note}
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The result is produced by using the update operator `//`.
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This means nested values of previous overlays are not merged recursively.
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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; }`.
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:::
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# Inputs
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`extensions`
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: A list of overlay functions
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:::{.note}
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The order of the overlays in the list is important.
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:::
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: Each overlay function takes two arguments, by convention `final` and `prev`, and returns an attribute set.
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- `final` is the result of the fixed-point function, with all overlays applied.
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- `prev` is the result of the previous overlay function(s).
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# Type
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```
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composeManyExtensions : [packageSet -> packageSet -> packageSet] -> packageSet -> packageSet -> packageSet
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^final ^prev ^overrides ^final ^prev ^overrides
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# Pseudo code
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let
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# final prev
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# ↓ ↓
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OverlayFn = { ... } -> { ... } -> { ... };
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in
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composeManyExtensions :: ListOf OverlayFn -> OverlayFn
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```
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# Examples
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:::{.example}
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## `lib.fixedPoints.composeManyExtensions` usage example
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```nix
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let
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# The "original function" that is extended by the overlays.
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# Note that it doesn't have prev: as argument since no overlay function precedes it.
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original = final: { a = 1; };
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# Each overlay function has 'final' and 'prev' as arguments.
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overlayA = final: prev: { b = final.c; c = 3; };
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overlayB = final: prev: { c = 10; x = prev.c or 5; };
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extensions = composeManyExtensions [ overlayA overlayB ];
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# Caluculate the fixed point of all composed overlays.
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fixedpoint = lib.fix (lib.extends extensions original );
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in fixedpoint
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=>
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{
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a = 1;
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b = 10;
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c = 10;
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x = 3;
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}
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```
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:::
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*/
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composeManyExtensions =
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lib.foldr (x: y: composeExtensions x y) (final: prev: {});
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composeManyExtensions = lib.foldr (x: y: composeExtensions x y) (final: prev: { });
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/**
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Create an overridable, recursive attribute set. For example:
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```
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nix-repl> obj = makeExtensible (self: { })
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nix-repl> obj = makeExtensible (final: { })
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nix-repl> obj
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{ __unfix__ = «lambda»; extend = «lambda»; }
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nix-repl> obj = obj.extend (self: super: { foo = "foo"; })
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nix-repl> obj = obj.extend (final: prev: { foo = "foo"; })
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nix-repl> obj
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{ __unfix__ = «lambda»; extend = «lambda»; foo = "foo"; }
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nix-repl> obj = obj.extend (self: super: { foo = super.foo + " + "; bar = "bar"; foobar = self.foo + self.bar; })
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nix-repl> obj = obj.extend (final: prev: { foo = prev.foo + " + "; bar = "bar"; foobar = final.foo + final.bar; })
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nix-repl> obj
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{ __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }
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@ -379,7 +419,6 @@ rec {
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Same as `makeExtensible` but the name of the extending attribute is
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customized.
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# Inputs
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`extenderName`
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@ -390,8 +429,13 @@ rec {
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: 2\. Function argument
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*/
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makeExtensibleWithCustomName = extenderName: rattrs:
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fix' (self: (rattrs self) // {
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${extenderName} = f: makeExtensibleWithCustomName extenderName (extends f rattrs);
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});
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makeExtensibleWithCustomName =
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extenderName: rattrs:
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fix' (
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self:
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(rattrs self)
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// {
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${extenderName} = f: makeExtensibleWithCustomName extenderName (extends f rattrs);
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}
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);
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}
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