Attach zero-based indices before filtering:
findStates :: [State] -> String -> [(State, Integer)]
findStates states str =
  [(state, i) |
     (state, i) <- zip states [0..],
     isPrefix str state]
The zip function pairs each state with its position in the original sequence. The list comprehension retains exactly those pairs satisfying the prefix test, preserving their order and their original indices. The declared result makes [0..] a stream of Integer indices. An empty search string matches every state, including an empty state, because the empty list is a prefix of every list.

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