@@ -112,6 +112,7 @@ module Streamly.Internal.Data.Fold
112112 , lmap
113113 --, lsequence
114114 , lmapM
115+ , scan
115116 , indexed
116117
117118 -- ** Filtering
@@ -375,6 +376,16 @@ transform (Pipe pstep1 pstep2 pinitial) (Fold fstep finitial fextract) =
375376
376377 extract (Tuple' _ fs) = fextract fs
377378
379+-- See StreamD/Transform.scanOnce for implementing this.
380+--
381+-- | Scan the input of a 'Fold' to change it in a stateful manner using another
382+-- 'Fold'.
383+--
384+-- /Unimplemented/
385+{-# INLINE scan #-}
386+scan :: Fold m a b -> Fold m b c -> Fold m a c
387+scan = undefined
388+
378389------------------------------------------------------------------------------
379390-- Left folds
380391------------------------------------------------------------------------------