Original proposal by Andrew Martin:
I'm pretty sure you don't need a bang pattern on acc when you use foldl'. Also, <> is preferable to mappend here, since SMP has happened.
I was confused initially by the fact that the revision title talks about adding foldMap' while in fact this changes an existing implementation. LGTM.
Oops. Looks like I lost my first commit that in fact added the foldMap' method.
I'm afraid this has missed the window for 8.6. Can you bump this to 4.13?
Can you add an entry to libraries/base/Changelog.md?
Rebase & changelog entry
The test failures look unrelated to me.
Whoops, looks like I dropped the ball on this one. Looks good to me. I'll amend the changelog appropriately.