Am I missing something about unfoldr?

David Feuer david.feuer at
Sat Jul 26 19:34:19 UTC 2014

The current definition:

unfoldr :: (b -> Maybe (a, b)) -> b -> [a]
unfoldr f b = case f b of
  Just (a,new_b) -> a : unfoldr f new_b
  Nothing -> []

I haven't yet looked over the Core to see if GHC is able to do a
worker/wrapper transformation to inline it, but we definitely want to
inline it (in some appropriate phase), because doing so has the potential
to erase the Maybes. It also isn't a good producer for foldr/build.
However, if we (re)write it like this:

unfoldrB f b c n = go b
    go b = case f b of
             Just (a,new_b) -> a `c` go new_b
             Nothing -> n

unfoldr f b = build (unfoldrB f b)

then we get some really nice fusion:

foldr c n (unfoldr f b)
= foldr c n (build (unfoldrB f b))
= unfoldrB f b c n

The list is gone, and there are no new closures or data structures in its

As a side benefit, we could write groupBy as an unfoldr, and make it a
somewhat better producer, especially when the groups are small. I don't
*think* there's any way to make groupBy a good consumer for foldr/build,
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