[jaspervdj]

Not really, if you wanted to do a version using only immutable arrays:

  import Data.Array
  
  fw :: Int -> Array (Int, Int) Int -> Array (Int, Int) Int
  fw 0 graph0 = graph0
  fw k graph0 =
    let fw' = fw (k - 1) graph0 in array (bounds graph0)
    [ ((i, j), min (fw' ! (i, j)) ((fw' ! (i, k)) + (fw' ! (k, j))))
    | (i, j) <- range (bounds graph0)
    ]

The only difference with a mutable implementation is that you use a new array for every k, whereas in a mutable version you would reuse these.