test = gather (segmentedIndices info) vec

segs :: Segments Int
segs = fromList (Z:.4) [1,7,0,2]

starts :: Vector Int
starts = fromList (Z:.4) [0,10,5,5]

info :: Acc (Vector (Int,Int))
info = A.zip (use starts) (use segs)

vec :: Acc (Vector Float)
vec = use $ fromList (Z:.20) [0..]

--

segmentedIndices
    :: Acc (Vector (Int,Int))
    -> Acc (Vector Int)
segmentedIndices info
  = A.map A.snd
  $ A.scanl1 (segmented (+)) (A.zip idx ones)
  where
    idx   = mkHeadIndices info
    ones  = fill (shape idx) 1

mkHeadIndices
    :: Acc (Vector (Int,Int))
    -> Acc (Vector Int)
mkHeadIndices info
  = A.init
  $ A.permute const zeros (\ix -> seg!ix A.== 0 ? ( ignore, index1 (offset ! ix) )) start
  where
    (start, seg)  = A.unzip info
    (offset, len) = unlift (scanl' (+) 0 seg)
    zeros         = fill (index1 $ the len + 1) 0

segmented
    :: (Exp Int -> Exp Int -> Exp Int)
    -> Exp (Int, Int)
    -> Exp (Int, Int)
    -> Exp (Int, Int)
segmented f a b =
  let (aF, aV) = unlift a
      (bF, bV) = unlift b
  in
  lift ( aF A..|. bF
       , bF A./= 0 ? (bF, f aV bV))