Non-standard overlays

[qrilka]

Ordinary nixpkgs overlays are functions of 2 parameters as it's described in the manual and https://github.com/input-output-hk/haskell.nix/blob/master/docs/user-guide.md also talks about overlays passed as pkg-def-overlays. Only after reading haskell.nix sources it became clear that I should put either packages or function over hackage.
It would be good to either rename this attribute or at least update documentation about what's expected as its contents.

[angerman]

@qrilka excellent point! Where would you expect such documentation? Also do you have a better naming suggestion?

[qrilka]

With Nix being dynamically typed it's not that easy to have a good place for a documentation. As currently this attribute seem to appear only in the User Guide probably it's the best place right now. But if you're OK to rename it then I don't have a good alternative, the best name I could find now is something like pkg-def-extra

[angerman]

@ElvishJerricco, @rvl: pkg-def-extra instead of pig-def-overlay? Yay or Nay?

[rvl]

It's neither an overlay nor underlay. "Extra" seems like a workable name. Or "extensions". "Customizations" is a bit too long.

Since our API is in flux at the moment, there's no great problem with renaming.

I would also like to change what arguments get passed to the package set extras. Instead of just hackage, I would like { hackage, ghc, ...}.

[ElvishJerricco]

I have a rough design for how to fix this stuff much better, but I've yet to figure out how to make it ergonomic enough for it not to confuse people. Basically, I want an attrsOf packagSetType option, where packageSetType lets you declare your ordering with respect to other entries. Then the package set, the extra-deps, and the local packages can just be different package sets overlaid using this ordering, and arbitrary other sets can be interleaved as desired.