writeSTRef twice for each iteration

fib3 :: Int -> Integer
fib3 n = runST $ do
    a <- newSTRef 1
    b <- newSTRef 1
    replicateM_ (n-1) $ do
        !a' <- readSTRef a
        !b' <- readSTRef b
        writeSTRef a b'
        writeSTRef b $! a'+b'
    readSTRef b

writeSTRef once for each iteration

fib4 :: Int -> Integer
fib4 n = runST $ do
    a <- newSTRef 1
    b <- newSTRef 1
    replicateM_ (n-1) $ do
        !a' <- readSTRef a
        !b' <- readSTRef b
        if a' > b'
          then writeSTRef b $! a'+b'
          else writeSTRef a $! a'+b'
    a'' <- readSTRef a
    b'' <- readSTRef b
    if a'' > b''
      then return a''
      else return b''

Benchmark, given n = 20000:

benchmarking 20000/fib3 mean: 5.073608 ms, lb 5.071842 ms, ub 5.075466 ms, ci 0.950 std dev: 9.284321 us, lb 8.119454 us, ub 10.78107 us, ci 0.950

benchmarking 20000/fib4 mean: 5.384010 ms, lb 5.381876 ms, ub 5.386099 ms, ci 0.950 std dev: 10.85245 us, lb 9.510215 us, ub 12.65554 us, ci 0.950

fib3 is a bit faster than fib4.