mtl-2.3 needs a migration guide for removal of re-exports

[Bodigrim]

#74 (comment) says

@Bodigrim we are planning on this months from now, with as much tooling guidance, announcing, and help as possible.

#103 (comment) says

@emilypi is going to revert this revert later, and write a migration guide. Tonight or tomorrow I'm going to create the writeup with our reasoning.

And yet neither published changelog nor announcements acknowledge the removal of re-exports from Control.Monad / Data.Monoid; and tooling / migration guide / public reasoning failed to materialise. Could this please be amended?

With all due respect to fantastic efforts of @kozross to revitalise the development of mtl, which I hugely appreciate, silently breaking hundreds of downstream packages is no way to maintain core libraries.

[kozross]

The changelog missing the relevant entry was indeed my bad, and I did fix it in #114. This does need to be updated on Hackage admittedly, but I believe that's a metadata change, so shouldn't require another release.

[andreasabel]

but I believe that's a metadata change, so shouldn't require another release.

If you mean "metadata" in the sense of hackage, I am afraid the CHANGELOG isn't part of the metadata that can be modified. Only some contents of the .cabal file can be modified.

@kozross Could you please comment on whether the removal of ExceptT (also not mentioned in the CHANGELOG) was intentional?

• mtl-2.3 dropped ExceptT #117

If not, maybe you would anyway need a re-release.

[kozross]

@andreasabel It was not - I've commented on the original issue, and plan to issue a fix today.