[Haskell-cafe] Unfolding alongside folding

David Feuer david.feuer at gmail.com
Mon Aug 19 10:14:20 UTC 2019

Thinking about fusing indexed maps for lists led me to consider the
following function, which generalizes mapWithIndex to allow arbitrary
stateful computation

  :: (a -> s -> Maybe (b, s))
  -> [a]
  -> s
  -> [b]
foldWithUnfold f = go
    go (a : as) s
      | Just (b, s') <- f a s
      = b : go as s'
    go _ _ = []

which we can fit into fold/build fusion like so:

foldWithUnfold f as s0 =
  build $ \c n ->
      go a k s
        | Just (b, s') <- f a s
        = b `c` k s'
        | otherwise
        = n
    in foldr go (const n) as s0

Does a function of this general nature exist in some package already?

Also, I see that the type can be expressed

  :: (a -> StateT s Maybe b)
  -> [a]
  -> s
  -> [b]

Is that a better or worse way to say it?
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