I think I may have figured out at least part of the reason that cons/build gives bad results. I actually ran into a clue when working on scanl. It seems at least part of the problem is that a rule like x : build g = build (\c n -> c x (g c n)) makes (:) look "interesting" to the inliner. Unfortunately, as I discovered after much extreme puzzlement about why rules relating to scanl were affecting things that had nothing to do with scanl, it turns out that making (:) look interesting is really quite bad, and something that we probably never want to happen. As a result, the only ways I see to try to make rules like that work properly are 1. If constructors are *always* best treated as boring, and the inliner knows when's a constructor, make it treat them all as boring. 2. Offer a BORINGRULE annotation to indicate that the rule should not make its LHS "interesting", or 3. (I don't like this option much) Make a special case forcing (:) in particular to be boring. David