sng2c's problem

FindPattern.hs

-- | Predicates strings of startWith
-- Just for clarify, the same of `isPrefixOf`
startWith :: (Eq a) => [a] -> [a] -> Bool
startWith []       _        = True
startWith _        []       = False
startWith (x : xs) (y : ys) = x == y && startWith xs ys


main :: String -> [(Int, String)]
main input =
  [ (offset, sub)
  | sub    <- drop 1 . inits $ input
  , offset <- [0 .. length input `div` 2 + 1] 
  , startWith sub (drop (offset + length sub) input)
  ]

-- tests
main "ABCABC"
main "A"
main "AA"
main "AAAA"
main "ABABABAB"
main "ABCABCAC"
main "AABAAC"

https://gist.github.com/sng2c/6077247