Haskell STM : How to store ThreadID as per their execution sequence - haskell

Question

In the following program Fibonacci number is generated from a given integer (generated randomly) and that value is stored into a TVar. As the execution time for generating the Fibonacci is different for different number, thus threads will not run sequentially. I want to store theadID, may be in a list, to check their execution pattern.
Please help me. Thanks in advance.
module Main
where
import Control.Parallel
import Control.Concurrent.STM
import Control.Concurrent
import System.Random
import Control.Monad
import Data.IORef
import System.IO
nfib :: Int -> Int
nfib n | n <= 2 = 1
| otherwise = par n1 (pseq n2 (n1 + n2 ))
where n1 = nfib (n-1)
n2 = nfib (n-2)
type TInt = TVar Int
updateNum :: TInt -> Int -> STM()
updateNum n v = do x1 <- readTVar n
let y = nfib v
x2 <- readTVar n
if x1 == x2
then writeTVar n y
else retry
updateTransaction :: TInt -> Int -> IO ()
updateTransaction n v = do atomically $ updateNum n v
incR :: IORef Int -> Int -> IO ()
incR r x = do { v <- readIORef r;
writeIORef r (v - x) }
main :: IO ()
main = do
n <- newTVarIO 10
r <- newIORef 40;
forM_ [1..10] (\i -> do
incR r i
;v <- readIORef r
;forkIO (updateTransaction n v)
)
I want to store [TreadID,FibNo] into a List for all the threads as per their execution. Suppose T1 has executed Fib30, T2 Fib35, T3->32 and T4->40. And if the commit sequence of threads like T1,T3, T2 and T4 then I want to store T1-35,T3-32,t2-35,t4-40 in a list.
Edit:
As suggested by #MathematicalOrchid, I have modified updateTrasaction as follows:-
updateTransaction :: MVar [(ThreadId, Int)] -> TInt -> Int -> IO ()
updateTransaction mvar n v = do
tid <- myThreadId
atomically $ updateNum n v
list <- takeMVar mvar
putMVar mvar $ list ++ [(tid, v)]
Now I am trying to print the values from that list in main
main :: IO ()
main = do
...
...
m <- newEmptyMVar
...
...
mv <- readMVar m
putStrLn ("ThreadId, FibVal : " ++ " = " ++ (show mv))
At the time of execution. MVar values couldn't be read and generates error
Exception: thread blocked indefinitely in an MVar operation
What to do? Thank in advance.

Answer 1

Did you want something like
updateTransaction :: TInt -> Int -> IO ()
updateTransaction n v = do
tid <- myThreadId
putStrLn $ "Start " ++ show tid
atomically $ updateNum n v
putStrLn $ "End " ++ show tid
Or perhaps something like
updateTransaction :: TInt -> Int -> IO ThreadId
updateTransaction n v = do
atomically $ updateNum n v
myThreadId
and change forM_ to forM?
Also, this part:
do
x1 <- readTVar n
...
x2 <- readTVar n
if x1 == x2 ...
If x1 /= x2 then GHC will automatically abort and restart your transaction. You do not need to manually check this yourself. Indeed, the else-branch can never execute. That's kind of the point of STM; it will appear to your transaction that nobody else changes the data you're looking at, so you don't ever have to worry about concurrent writes.
Edit: If you want to record the actual order in which the transactions committed, you're going to need some more inter-thread communication. Obviously you could do that with STM, but just for a list of stuff, maybe this could work?
updateTransaction :: MVar [(ThreadId, Int)] -> TInt -> Int -> IO ()
updateTransaction mvar n v = do
tid <- myThreadId
fib <- atomically $ updateNum n v
list <- takeMVar mvar
putMVar mvar $ list ++ [(tid, fib)]
(Obviously you have to make updateNum return the number it calculated.)