Haskell STM : Main thread will not exit until child thread completes execution - multithreading

Question

In the following program, I want that main thread will not exit until all its child threads complete execution. Please note that I have used bang patterns to evaluate the Fibonacci call so that it returns an evaluated thunk to main thread.
{-# LANGUAGE BangPatterns #-}
module Main
where
import Control.Concurrent.STM
import Control.Concurrent
import System.IO
nfib :: Int -> Int
nfib n | n <= 2 = 1
| otherwise = (n1 + n2 )
where n1 = nfib (n-1)
n2 = nfib (n-2)
type TInt = TVar Int
updateNum :: TInt -> Int -> STM()
updateNum n v = do writeTVar n v
updateTransaction :: TInt -> Int -> IO ()
updateTransaction n v = do
atomically $ do
updateNum n v
main :: IO ()
main = do
n <- newTVarIO 10
forkIO $ do
let v = 30
let !x = nfib v
updateTransaction n x
forkIO $ do
let v = 15
let !x = nfib v
updateTransaction n x
forkIO $ do
let v = 25
let !x = nfib v
updateTransaction n x
nv <- readTVarIO n
putStrLn ("Fib number of " ++ " = " ++ (show nv))
nv <- readTVarIO n
putStrLn ("Fib number of " ++ " = " ++ (show nv))
nv <- readTVarIO n
putStrLn ("Fib number of " ++ " = " ++ (show nv))
I have tired to solve this problem as per [link] (Haskell MVar : How to execute shortest job first?). I don't know whether approach is correct not not and also getting an error when try to print the value of TMVar.
Here is the code :- (nfib is same as above)
type TMInt = TMVar Int
updateNum1 :: TMInt -> Int -> STM()
updateNum1 n v = do putTMVar n v
updateTransaction1 :: TMInt -> Int -> IO ()
updateTransaction1 n v = do
atomically $ do
updateNum1 n v
main1 :: IO ()
main1 = do
n <- newTMVarIO 0
forkIO $ do
let v = 30
let !x = nfib v
updateTransaction1 n x
forkIO $ do
let v = 15
let !x = nfib v
updateTransaction1 n x
forkIO $ do
let v = 25
let !x = nfib v
updateTransaction1 n x
-- t <- takeTMVar n
-- putStrLn( "result: " ++ (show t))
** Error is as follows:-
Couldn't match type `STM' with `IO'
Expected type: IO Int
Actual type: STM Int
In the return type of a call of `takeTMVar'
In a stmt of a 'do' block: t <- takeTMVar n
Please help. Thanks.

Answer 1

main1 is in IO but takeTMVar returns an STM Int. You need to run the transaction:
t <- atomically $ takeTMVar n
putStrLn( "result: " ++ (show t))