Streamline keys and elems

[treeowl]

Make sure keys and elems won't build silly pairs and
thunks to deconstruct them.

[phadej]

Isn't a guarantee that map . build should fuse? (toList is defined in terms of build)?

The current implementation is way simpler to understand,

[sjakobi]

It would be nice to get some numbers that demonstrate the usefulness of this change.

[treeowl]

Ah, good questions. We need to look at the Core and see if the pairs and selector thunks are eliminated by fusion. @sjakobi, I've never had a clue of the correct way to ask Cabal to ask GHC to give me -ddump-simpl for a module. I've always just hacked around by running bare GHC. There must be a right way to do this. Do you know it?

[sjakobi]

I've never had a clue of the correct way to ask Cabal to ask GHC to give me -ddump-simpl for a module. I've always just hacked around by running bare GHC. There must be a right way to do this. Do you know it?

What I usually do is insert a OPTIONS_GHC pragma in the relevant module and just cabal build. I usually copy the one from this blog post: https://www.joachim-breitner.de/blog/763-Faster_Winter_5__Eta-Expanding_ReaderT 😄

[sjakobi]

I'll mark this as a draft to clarify that we need benchmark results and/or insights regarding the Core before we can properly consider a merge.