Support for Indexed Folds? #484

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1. Lysxia commented on Feb 4, 2023

   

   Lysxia

   on Feb 4, 2023

   Contributor

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   I don't know if we want this, but note that you can define ifoldl' using Text.foldr or Text.foldl. Or if you have it for lists, you can compose it with Text.unpack.

   ifoldlText :: (Int -> r -> Char -> r) -> r -> Text -> r
   ifoldlText f r t = Text.foldr (\c k i x -> k (i+1) (f i x c)) (\_ x -> x) t 0 r
   

   Test example

   main :: IO ()
   main = do
     let f i x c = x ++ [(i, c)]
     print (ifoldlText f [] "hello")
     -- Output: [(0,'h'),(1,'e'),(2,'l'),(3,'l'),(4,'o')]
   
2. Lysxia

   added

   feature request

   questionRequires more investigationRequires more investigation

   on Feb 4, 2023
3. HuwCampbell commented on Feb 5, 2023

   

   HuwCampbell

   on Feb 5, 2023

   ContributorAuthor

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   Thanks. I ended up rolling this:

   ifoldl' :: (Int -> b -> Char -> b) -> b -> Text -> b
   ifoldl' f z0 =
     go . stream
       where
     go (Stream next s0 _len) =
       loop 0 z0 s0
         where
       loop !ix !z !s =
         case next s of
           Done -> z
           Skip s' -> loop ix z s'
           Yield x s' -> loop (ix + 1) (f ix z x) s'

   I'm not sure if it actually fuses any better.