Flake `overlays` output cannot be a list

[lovesegfault]

Describe the bug

You cannot define the overlays output of a flake to be a list, it must be a set. This is despite the documentation stating that it can be a list or set.

Steps To Reproduce

1. Write a flake

   {
   inputs = {
   agenix.url = "github:ryantm/agenix";
   deploy-rs.url = "github:serokell/deploy-rs";
   };
   
   outputs = { self, agenix, deploy-rs }: {
   overlays = [ agenix.overlay deploy-rs.overlay ];
   };
   }

2. nix flake check

Expected behavior

For the check to pass

nix-env --version output

nix-env (Nix) 2.4

[nrdxp]

I was confused by this a number of times as well, but maybe all that's needed is a documentation update. After all, requiring overlays to be an attribute set is a bit more versatile. That way, if there are multiple overlays inside, the user can pick and choose, and if one does want all the overlays, it's just a builtins.attrValues away. The only issue I see with that is that maybe attrValues puts them in an unintended order though :thinking:

[lovesegfault]

I think overlays as attrs and as a list serve different purposes, and perhaps should be separate outputs altogether.

I see a list of overlays as just a handy way of exposing a "combined" set of overlays ready-to-use. In contrast, I see attrset of overlays as a way of exposing a set of overlays that may not even be meant to be used together.

You can work around this currently by processing your [(final: prev: { ... }) ... ] into a single overlay:

{
  overlay = final: prev:
    lib.composeManyExtensions (import ./my-list-of-overlays.nix);
}

But it still strikes me as odd that there's no ready way of exposing a list of overlays.

[thufschmitt]

I don’t think that allowing a list of overlays is a really good idea as it makes things more painful for the consumer of the flake for no real benefit − either the list is just one big overlay, and it might as well be exported as such, or it’s a set of independent overlays, and it’d better be exported as an attribute set.