lib/attrsets: add concatMapAttrs

[figsoda]

Description of changes

concatMapAttrs: Map each attribute in the given set and merge them into a new attribute set.

concatMapAttrs ::
  (String -> a -> AttrSet)
  -> AttrSet
  -> AttrSet

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.11 Release Notes (or backporting 22.05 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[roberth]

This function is related to concatMap.

Let's first generalize concatMap. I'll use List a instead of [a].

concatMap :: (a -> List b) -> List a -> List b

-- generalize List -->

bind :: (a -> f b) -> f a -> f b

-- specialize f to Attrset -->

concatMapAttrs :: (a -> Attrset b) -> Attrset a -> Attrset b

This is a slightly different, but perhaps nicer interface.

# Example:
       concatMapAttrs
         (name: value: {
           ${name} = value; 
           "${name + value}" = value;
         })
         { x = "a"; y = "b"; }
       => { x = "a"; xa = "a"; y = "b"; yb = "b"; }

It'd be something like this, I think.

flip pipe [ (mapAttrs f) attrValues (foldl' merge {}) ]

I would think that the last function was already available somewhere.

I also think joinAttrs :: (String -> String -> String) -> Attrset (Attrset a) -> Attrset a would be a good function to have.
It could be generalized to joinAttrsRecursiveUntil as well, with (List String -> String).

Well, the join ones are just a thought, but concatMapAttrs will make this PR a bit cleaner.

[figsoda]

I'm not sure if bind is possible with nix's type system, maybe we just need something like foldMap

[roberth]

bind was just to illustrate the transformation. Might have been a bit too elaborate.

[roberth]

Please also add one or two test cases to make sure we won't accidentally invert the priority of attributes in the case of colliding names.

[figsoda]

I changed mapAttrsMulti to concatMapAttrs and changed filterMapAttrs to a similar definition, also added a test for each

[roberth]

EDIT: Comment turned into issue

• Decide on a recursive attribute set tree flattening function #237776