foldM.idr

fold : (Monad m) => (a -> b -> b) -> b -> Sink a m b
fold f = recur where
  recur acc = do
    ma <- await           -- await for more inputs
    case ma of                   
      Just x => do        -- in case there is a value to process
        acc' <- f x acc   -- * combine it the current accumulator
        recur acc'        -- * and recurse with the new value
      Nothing => pure acc -- otherwise, return the result