Am I missing something about unfoldr?

[David Feuer]

Foldr/build fusion can't remove bottoms; it *can* add them. That's why we
have to be very careful. I *think* we're safe in this case, though.
On Jul 26, 2014 4:28 PM, "Carter Schonwald" <carter.schonwald at gmail.com>
wrote:

> its worth point out that many uses of fusion in haskell libs punt on
> preserving bottoms, that is, fusion can make *more* programs terminate. I
> don't have a good example off  hand  mind you
>
>
> On Sat, Jul 26, 2014 at 4:02 PM, David Feuer <david.feuer at gmail.com>
> wrote:
>
>> Hmm... I found something I missed. I need to prove that this doesn't
>> break when the arguments to unfold use seq (if in fact it doesn't). I will
>> have to look into it later.
>> On Jul 26, 2014 3:34 PM, "David Feuer" <david.feuer at gmail.com> wrote:
>>
>>> 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
>>>   where
>>>     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 place.
>>>
>>> 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,
>>> unfortunately.
>>>
>>
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>
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