RosettaCodeData/Task/Fibonacci-sequence/Haskell/fibonacci-sequence-8.hs

{-# Language LambdaCase #-}
import Data.MemoTrie
fib :: Integer -> Integer
fib = memo $ \case
   0 -> 0
   1 -> 1
   n | n>0 -> fib (n-1) + fib (n-2)
     | otherwise -> fib (n+2) - fib (n+1)