RosettaCodeData/Task/Stern-Brocot-sequence/Haskell/stern-brocot-sequence-1.hs

import Data.List (elemIndex)

sb :: [Int]
sb = 1 : 1 : f (tail sb) sb
  where
    f (a:aa) (b:bb) = a + b : a : f aa bb

main :: IO ()
main = do
  print $ take 15 sb
  print
    [ (i, 1 + (\(Just i) -> i) (elemIndex i sb))
    | i <- [1 .. 10] ++ [100] ]
  print $ all (\(a, b) -> 1 == gcd a b) $ take 1000 $ zip sb (tail sb)