lib/attrsets: add genAttrs'

[dali99]

Description of changes

As wished by @infinisil in #197004 I'm splitting each function into other PRs to merge incrementally.

Main changes from #197004 is that I'm no longer including them in lib/default.nix ref: #266103
And I've added types and tests.

I've also added a test for genAttrs, which seemed missing.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

Priorities

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[dali99]

@ofborg eval

[roberth]

Don't overload a and b, e.g.:

Suggested change

	genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
	(i: i.b.c) (i: i.a)
	genAttrs' [ {p = 2; q.r = "a" } {p = 1; q.r = "b"} ]
	(i: i.q.r) (i: i.p)

[roberth]

A denotationally equivalent function would be

-genAttrs' :: [ a ] -> (a -> String) -> (a -> Any) -> AttrSet
+genAttrs' :: [ a ] -> (a -> { name :: String; value :: a}) -> AttrSet

However, operationally, the latter can use a let binding to share computation between the two attributes.
The prior can not share a variable that depends on the a.

[roberth]

Yet another alternative is

genAttrs' :: [ a ] -> (a -> Attrs) -> Attrs

This more compact, more powerful, and you can still read that a name and value are computed.

genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
  (i: { ${i.b.c} = i.a; })

while also allowing filtering behavior:

genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
  (i: optionalAttrs (i.a > 1) { ${i.b.c} = i.a; })

or multiple keys per item

genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
  (i: {
    ${i.b.c} = i.a;
    "${i.b.c}-squared" = i.a * i.a;
  })

[roberth]

Whatever genAttrs' ends up being, its documentation should refer to the other functions that are similar, and the other functions should refer back.
Some functions I'd think of

• genAttrs
• concatMapAttrs
• listToAttrs

There's probably a more related function I'm not thinking of.