I'm pushing hard to get unfoldr fixed ASAP, and looking to take advantage of that. Specifically, I like the idea of using the fusion that (I dearly hope) will come with unfoldr to make a variety of other things fuse that currently need their own special rules. I also want to extend fusion by one or two general rules. One that looks very good is similar to the rejected foldr/cons rule, but seems somewhat less likely to cause problems while still enhancing fusion in general: {-# RULES "cons/build" forall x (g::forall b . (a->b->b)->b->b) . x : build g = build (\c n -> x `c` g c n) #-} This will allow foldr/build fusion to look past conses. So for example, foldr f q (x : build g) will be rewritten (cons/build) to foldr f q (build (\c n -> x `c` g c n)) and then (fold/build) to (\c n -> x `c` g c n) f q This reduces to f x (g f q) A discussion about foldr k z (x1:x2:x3:...:x2000:build g), a possible way to fix it, and why I don't think it's important anyway. This means the same thing as foldr k z ([x1,x2,x3,...,x2000] ++ build g) The current ++ rule rewrites this to foldr k z (augment (\c n -> foldr c n [x1,...,x2000]) (build g)) Then foldr/augment: (\c n -> foldr c n [x1,..,x2000]) k (foldr k z (build g)) Then foldr/build: (\c n -> foldr c n [x1,...,x2000]) k (g k z) Then beta reduction: foldr k (g k z) [x1,...,x2000] So to take the example from earlier, foldr f q (x : build g) is currently bad, but foldr f q ([x] ++ build g) will end up foldr f (g f q) [x] and then in this case the foldr/single rule will give f x (g f q) but if more than one element were in the list, we'd end up with a fold over the literal list (not bad in general). This suggests a fix, but one that may be difficult to implement within GHCs stage framework as is (I'm not quite sure; someone who knows more about the system may see a way to do this, but I think it wants to happen before the ++ rule, which I think happens at the beginning of time): Translate x1:x2:x3:...:build g to [x1,x2,x3,...] ++ build g Then let everything else happen as usual. I think, however, that this is probably overkill. Just implement cons/build and tell users that if they want x1:x2:x3:....:x2000:foo not to expand under foldr then they need to write [x1,x2,x3,...,x2000]++foo instead.