I've worked out the basics of how to make more functions from GHC.Base, GHC.List, and Data.List participate in foldr/build fusion, but I could really use some help figuring out how to write the RULES to accompany them. I have too little experience with GHC's simplification process to manage this on my own, it seems. One complication I recognize already: REVERSE reverse xs = build $ \c n -> foldl (\a x -> x `c` a) n xs works as well as we can probably expect for fusion (it fuses nicely with map and a modified unfoldr), but when it doesn't fuse, it ends up duplicating its "worker" at the top level, potentially multiple times. I tried to write a rule to rewrite that to a simpler version, but that's complicated by the fact that foldl is INLINEd unconditionally. I'm thinking maybe the "right" thing is to allow all this duplication to happen, and then clean it up at the end ( see https://ghc.haskell.org/trac/ghc/ticket/9441 ), but that will not happen soon if it does at all.