Joachim Breitner wrote:

Did you profile first, and did it show up there? You know, premature optimization... so it might be that your fix is a nice improvement and useful exercise (and very welcome as such), but without much real-world effect.

You're right, of course. I read the comment and figured it made sense, but it may not really make much. In particular, the lists involved will usually be short, so O(n^2) may not actually be bad. It would probably be more valuable for me to start by trying to make it easier to follow what's going on (and I've submitted one patch to do so for one of the functions) and think about efficiency later or not at all.

BTW, code review is easier on Phabricator. Maybe you want to get started using that? See https://ghc.haskell.org/trac/ghc/wiki/Phabricator for instructions.

I'll take a look. There are a million tools to learn.