[Haskell-cafe] Operator #

[c8h10n4o2]

What does # mean in this code ? (from Data.List)

findIndices p ls = loop 0# ls
                 where
                   loop _ [] = []
                   loop n (x:xs) | p x       = I# n : loop (n +# 1#) xs
                                      | otherwise = loop (n +# 1#) xs

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