[Haskell-cafe] Generating random graph

[Bertram Felgenhauer]

Hi Mitar,

> I have made this function to generate a random graph for
> Data.Graph.Inductive library:
> 
> generateGraph :: Int -> IO (Gr String Double)
> generateGraph graphSize = do
>   when (graphSize < 1) $ throwIO $ AssertionFailed $ "Graph size out
> of bounds " ++ show graphSize
>   let ns = map (\n -> (n, show n)) [1..graphSize]
>   es <- fmap concat $ forM [1..graphSize] $ \node -> do
>     nedges <- randomRIO (0, graphSize)
>     others <- fmap (filter (node /=) . nub) $ forM [1..nedges] $ \_ ->
> randomRIO (1, graphSize)
>     gen <- getStdGen

Others have already remarked that you could implement this as a pure
function. However, the mistake is the use of  getStdGen  here, which
is (almost?) never what you need: two consecutive valls of getStdGen
will return the same generator. You should call newStdGen  instead.

Best regards,

Bertram