= Solution
The <list unfold> operation has the following <type signature> and definition:
``
unfoldr :: (s -> Maybe (a,s)) -> s -> [a]
unfoldr step s = case step s of
Nothing -> []
Just (x,s') -> x : unfoldr step s'
groupUnfold :: Eq a => [a] -> [[a]]
groupUnfold = unfoldr next
where
next [] = Nothing
next (x:xs) = Just (x:ys,zs)
where (ys,zs) = span (== x) xs
``
The seed is the unprocessed <suffix of a list>. An empty seed terminates the unfold; otherwise `next` returns the first maximal run and precisely the seed for the next call. Substituting the definition of `unfoldr` recovers part (c)'s equations, proving correctness. The `Just` pair and first group constructor can be produced before `span` finishes, retaining the same <lazy evaluation> behaviour.
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