Bulat Ziganshin
It makes sense to me that the above behaviour is seen: length is now a good consumer, but it generates 1+(1+(1+(1+... as it is consuming, and this causes a stack overflow. I don't think we can fix this while staying with fold/build fusion, so it looks to me like the patch should be reverted and the whole problem looked at post-6.6.
in general, for any function we can either implement 1) good consumer based on using foldr 2) tail-recursion
are you agree?
I'd like to point out that, if you are prepared to abandon foldr/build fusion in favour of hylo fusion, you can code good consumers using either foldr or foldl. (Length is a foldl, because it uses an accumulator.) Foldl is tail-recursive, but this does not prevent it being a good consumer, provided your framework of fusion laws is sufficiently flexible. Regards, Malcolm