This is a good opportunity to use the ol' sliding windows trick in Haskell list processing!
Basically if you want sliding windows over a list "abcdefg" to get, ie,
["abc","bcd","cde","def","efg"], one thing you can do is iteratively drop items:
map (`drop` "abcdefg") [0,1,2]
-- ["abcdefg","bcdefg","cdefg","defg"]and then take the transpose:
transpose $ map (`drop` "abcdefg") [0,1,2]
-- ["abc","bcd","cde","def","efg","fg","g"]
You get the sliding windows, plus a few trailing elements as the window slides off the edge of the list.
Now at this point we just need to find the first unique sliding window:
windows :: Int -> [a] -> [[a]]
windows n xs = transpose $ map (`drop` xs) [0..n-1]
solve :: Int -> String -> Int
solve n xs = find isUnique ws + n
where
ws = windows n xs
isUnique w = S.size (S.fromList w) == n
part1, part2 :: String -> Int
part1 = solve 4
part2 = solve 14