Are you sure about this point? I'd like to see a purely functional implementation of this, e.g. in Haskell without monoids. I believe it's significantly harder than a C implementation (which is indeed a few lines)
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)
]