[Haskell-beginners] very impure [global] counter

[David McBride]

This is what I'd do:

{-# LANGUAGE NoMonomorphismRestriction #-}
module Counter where

import Control.Monad.State

main = runStateT procedure (0 :: Integer) >> return ()

incCounter = do
  n <- get
  modify (+1)
  return n

execFile = do
  n <- incCounter
  liftIO $ putStrLn $ ("command --createfile=tempfile" ++ show n ++ ".tmp")

procedure = do
  execFile
  execFile
  liftIO $ putStrLn "do something"
  execFile

On Fri, Jul 22, 2011 at 3:10 AM, Davi Santos <dps.abc at gmail.com> wrote:
> Hello all,
> I have massive (parallel if possible) system calls to an external
> non-deterministic program.
> Each time it is executed, it creates a file depending on a command line
> option 'opt' (input files path, for example).
> How can I ensure the file name will be unique? maybe with a global counter?
> My temporary solution have been to use a large random number:
> -----------
> mysteriousExecution :: String -> IO ()
> mysteriousExecution opt = do
>    number <- rand
>    run $ "mysterious-command " ⊕ opt ⊕ " --create-file=" ⊕ number
> rand = do
>    a ←  getStdRandom (randomR (1,999999999999999999999999999999999)) ∷  IO
> Int
>    let r = take 20 $ randomRs ('a','z') (mkStdGen a) ∷  String
>    return r
> ========
> I'm trying to avoid additional parameters to 'mysteriousExecution'.
> I tried a counter also (to replace rand), but I don't know how could I start
> it inside  'mysteriousExecution'.
> c ∷  IO Counter
> c = do
>     r ←  newIORef 0            -- start
>     return (do
>         modifyIORef r (+1)
>         readIORef r)
> If somebody says everything is wrong, ok.
> I understand. 18 years of imperative programming world can damage the brain.
> Thanks
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