simple_floyd.hs

import Data.Array.IO
import Control.Monad (forM_)
import System.Time
import System.IO

type IData = Int

-- slow but simple load (better avoiding intermediate list)
readWeights :: IO (IOUArray (Int, Int) IData)
readWeights = do
  c <- getContents
  let a :: [IData]
      a = read c
      s = (floor.sqrt.fromIntegral.length) a - 1
  newListArray ((0, 0), (s, s)) a

main = do

  w <- readWeights

  putStrLn "Computing distances..."  
  t0 <- getClockTime
  
  ((_, _), (b, _)) <- getBounds w
  let r = [0..b]
  -- can be parallelized!!!
  forM_ r $ \k -> do
    forM_ r $ \x -> do
      forM_ r $ \y -> do
        xk <- readArray w (x, k)
        ky <- readArray w (k, y)
        xy <- readArray w (x, y)
        writeArray w (x, y) $ min xy (xk + ky)
  
  t1 <- getClockTime

  if b < 40
  then  
    forM_ [0..b] $ \x -> do
      forM_ [0..b] $ \y -> do
        readArray w (x, y) >>= putStr . (++ ", ") . show
      putStrLn "|"
  else
    return ()

  putStrLn ("Time: " ++ show (diffClockTimes t1 t0))