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Merge pull request #242312 from NixOS/doc-lib-render-fixedPoints

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doc: Render lib.fixedPoints
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Robert Hensing 2023-07-08 19:57:44 +02:00 committed by GitHub
parent 1cfa64e7bd aa1beb0ab5
commit e809af74eb
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GPG key ID: 4AEE18F83AFDEB23
3 changed files with 126 additions and 88 deletions
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1
doc/default.nix
View file

@ -11,6 +11,7 @@ let
{ name = "strings"; description = "string manipulation functions"; }
{ name = "versions"; description = "version string functions"; }
{ name = "trivial"; description = "miscellaneous functions"; }
{ name = "fixedPoints"; baseName = "fixed-points"; description = "explicit recursion functions"; }
{ name = "lists"; description = "list manipulation functions"; }
{ name = "debug"; description = "debugging functions"; }
{ name = "options"; description = "NixOS / nixpkgs option handling"; }

17
doc/doc-support/lib-function-docs.nix
View file

@ -14,13 +14,16 @@ stdenv.mkDerivation {
buildInputs = [ nixdoc ];
installPhase = ''
function docgen {
# TODO: wrap lib.$1 in <literal>, make nixdoc not escape it
if [[ -e "../lib/$1.nix" ]]; then
nixdoc -c "$1" -d "lib.$1: $2" -l ${locationsJSON} -f "$1.nix" > "$out/$1.md"
name=$1
baseName=$2
description=$3
# TODO: wrap lib.$name in <literal>, make nixdoc not escape it
if [[ -e "../lib/$baseName.nix" ]]; then
nixdoc -c "$name" -d "lib.$name: $description" -l ${locationsJSON} -f "$baseName.nix" > "$out/$name.md"
else
nixdoc -c "$1" -d "lib.$1: $2" -l ${locationsJSON} -f "$1/default.nix" > "$out/$1.md"
nixdoc -c "$name" -d "lib.$name: $description" -l ${locationsJSON} -f "$baseName/default.nix" > "$out/$name.md"
fi
echo "$out/$1.md" >> "$out/index.md"
echo "$out/$name.md" >> "$out/index.md"
}
mkdir -p "$out"
@ -29,8 +32,8 @@ stdenv.mkDerivation {
```{=include=} sections
EOF
${lib.concatMapStrings ({ name, description }: ''
docgen ${name} ${lib.escapeShellArg description}
${lib.concatMapStrings ({ name, baseName ? name, description }: ''
docgen ${name} ${baseName} ${lib.escapeShellArg description}
'') libsets}
echo '```' >> "$out/index.md"

196
lib/fixed-points.nix
View file

@ -1,34 +1,49 @@
{ lib, ... }:
rec {
# Compute the fixed point of the given function `f`, which is usually an
# attribute set that expects its final, non-recursive representation as an
# argument:
#
# f = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
#
# Nix evaluates this recursion until all references to `self` have been
# resolved. At that point, the final result is returned and `f x = x` holds:
#
# nix-repl> fix f
# { bar = "bar"; foo = "foo"; foobar = "foobar"; }
#
# Type: fix :: (a -> a) -> a
#
# See https://en.wikipedia.org/wiki/Fixed-point_combinator for further
# details.
/*
Compute the fixed point of the given function `f`, which is usually an
attribute set that expects its final, non-recursive representation as an
argument:
```
f = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
```
Nix evaluates this recursion until all references to `self` have been
resolved. At that point, the final result is returned and `f x = x` holds:
```
nix-repl> fix f
{ bar = "bar"; foo = "foo"; foobar = "foobar"; }
```
Type: fix :: (a -> a) -> a
See https://en.wikipedia.org/wiki/Fixed-point_combinator for further
details.
*/
fix = f: let x = f x; in x;
# A variant of `fix` that records the original recursive attribute set in the
# result. This is useful in combination with the `extends` function to
# implement deep overriding. See pkgs/development/haskell-modules/default.nix
# for a concrete example.
/*
A variant of `fix` that records the original recursive attribute set in the
result, in an attribute named `__unfix__`.
This is useful in combination with the `extends` function to
implement deep overriding.
*/
fix' = f: let x = f x // { __unfix__ = f; }; in x;
# Return the fixpoint that `f` converges to when called recursively, starting
# with the input `x`.
#
# nix-repl> converge (x: x / 2) 16
# 0
/*
Return the fixpoint that `f` converges to when called iteratively, starting
with the input `x`.
```
nix-repl> converge (x: x / 2) 16
0
```
Type: (a -> a) -> a -> a
*/
converge = f: x:
let
x' = f x;
@ -37,75 +52,94 @@ rec {
then x
else converge f x';
# Modify the contents of an explicitly recursive attribute set in a way that
# honors `self`-references. This is accomplished with a function
#
# g = self: super: { foo = super.foo + " + "; }
#
# that has access to the unmodified input (`super`) as well as the final
# non-recursive representation of the attribute set (`self`). `extends`
# differs from the native `//` operator insofar as that it's applied *before*
# references to `self` are resolved:
#
# nix-repl> fix (extends g f)
# { bar = "bar"; foo = "foo + "; foobar = "foo + bar"; }
#
# The name of the function is inspired by object-oriented inheritance, i.e.
# think of it as an infix operator `g extends f` that mimics the syntax from
# Java. It may seem counter-intuitive to have the "base class" as the second
# argument, but it's nice this way if several uses of `extends` are cascaded.
#
# To get a better understanding how `extends` turns a function with a fix
# point (the package set we start with) into a new function with a different fix
# point (the desired packages set) lets just see, how `extends g f`
# unfolds with `g` and `f` defined above:
#
# extends g f = self: let super = f self; in super // g self super;
# = self: let super = { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }; in super // g self super
# = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; } // g self { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
# = self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; } // { foo = "foo" + " + "; }
# = self: { foo = "foo + "; bar = "bar"; foobar = self.foo + self.bar; }
#
/*
Modify the contents of an explicitly recursive attribute set in a way that
honors `self`-references. This is accomplished with a function
```nix
g = self: super: { foo = super.foo + " + "; }
```
that has access to the unmodified input (`super`) as well as the final
non-recursive representation of the attribute set (`self`). `extends`
differs from the native `//` operator insofar as that it's applied *before*
references to `self` are resolved:
```
nix-repl> fix (extends g f)
{ bar = "bar"; foo = "foo + "; foobar = "foo + bar"; }
```
The name of the function is inspired by object-oriented inheritance, i.e.
think of it as an infix operator `g extends f` that mimics the syntax from
Java. It may seem counter-intuitive to have the "base class" as the second
argument, but it's nice this way if several uses of `extends` are cascaded.
To get a better understanding how `extends` turns a function with a fix
point (the package set we start with) into a new function with a different fix
point (the desired packages set) lets just see, how `extends g f`
unfolds with `g` and `f` defined above:
```
extends g f = self: let super = f self; in super // g self super;
= self: let super = { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }; in super // g self super
= self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; } // g self { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; }
= self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; } // { foo = "foo" + " + "; }
= self: { foo = "foo + "; bar = "bar"; foobar = self.foo + self.bar; }
```
*/
extends = f: rattrs: self: let super = rattrs self; in super // f self super;
# 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
/*
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
*/
composeExtensions =
f: g: final: prev:
let fApplied = f final prev;
prev' = prev // fApplied;
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.
#
# composeManyExtensions : [packageSet -> packageSet -> packageSet] -> packageSet -> packageSet -> packageSet
# ^final ^prev ^overrides ^final ^prev ^overrides
/*
Compose several extending functions of the type expected by 'extends' into
one where changes made in preceding functions are made available to
subsequent ones.
```
composeManyExtensions : [packageSet -> packageSet -> packageSet] -> packageSet -> packageSet -> packageSet
^final ^prev ^overrides ^final ^prev ^overrides
```
*/
composeManyExtensions =
lib.foldr (x: y: composeExtensions x y) (final: prev: {});
# Create an overridable, recursive attribute set. For example:
#
# nix-repl> obj = makeExtensible (self: { })
#
# nix-repl> obj
# { __unfix__ = «lambda»; extend = «lambda»; }
#
# nix-repl> obj = obj.extend (self: super: { 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
# { __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }
/*
Create an overridable, recursive attribute set. For example:
```
nix-repl> obj = makeExtensible (self: { })
nix-repl> obj
{ __unfix__ = «lambda»; extend = «lambda»; }
nix-repl> obj = obj.extend (self: super: { 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
{ __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }
```
*/
makeExtensible = makeExtensibleWithCustomName "extend";
# Same as `makeExtensible` but the name of the extending attribute is
# customized.
/*
Same as `makeExtensible` but the name of the extending attribute is
customized.
*/
makeExtensibleWithCustomName = extenderName: rattrs:
fix' (self: (rattrs self) // {
${extenderName} = f: makeExtensibleWithCustomName extenderName (extends f rattrs);