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Shared Transactional Memory
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Shared Transactional Memory

Haskell allows you to do imperative programming through its shared memory transactional system. Shared memory variables are created with newTVar(), read with readTVar(), and written with writeTVar(). Threads can be created with the forkIO() call.

You can wrap any combination of read/write/new inside an atomically() call, and it will safely execute concurrently with other threads.

The atomically() call has another nice benefit, it converts an STM to an IO. This allows us to print values obtained from the STM Monad.

Here's a small example of how to do some basic imperative programming with Haskell.

stmtest.hs
1import Control.Concurrent.STM
2 
3pr x = do
4    y <- atomically(readTVar x)
5    putStrLn(show(y))
6 
7main = do
8    x <- atomically(newTVar 3)
9    pr x
10    atomically(writeTVar x 4)
11    pr x
12    atomically(writeTVar x 2)
13    pr x
$ ghc --make stmtest.hs
[1 of 1] Compiling Main             ( stmtest.hs, stmtest.o )
Linking stmtest ...

$ ./stmtest
3
4
2

Building a Mutex with STM

The "retry" statement is key to making this code work. When it is encountered, the transaction unrolls and waits for one of the variables to change. When this happens it tries again.

mutex.hs
1import Control.Concurrent
2import Control.Concurrent.STM
3 
4data Mutex = Locked | Unlocked
5 
6newMutex = atomically(newTVar Unlocked)
7 
8acquireMutex mut = atomically(do
9    val <- readTVar mut
10    case val of
11        Unlocked -> writeTVar mut Locked
12        Locked -> retry)
13 
14releaseMutex mut = atomically(writeTVar mut Unlocked)
15 
16threads mut counter n = doall [
17    forkIO(do
18        acquireMutex mut
19        id <- myThreadId
20        atomically(do
21            val <- readTVar counter
22            writeTVar counter (val-1))
23        putStrLn("I am thread "++show(id))
24        releaseMutex mut) | i <- [1..n]]
25 
26doall [] = putStr ""
27doall (a:b) = do
28    a
29    doall b
30 
31main = let
32        n = 10
33    in
34        do
35            mut <- newMutex
36            counter <- atomically(newTVar n)
37 
38            -- start a group of n threads
39            threads  mut counter n
40 
41            -- block until all threads finish
42            atomically(do
43                val <- readTVar counter
44                case val of
45                    0 -> return val
46                    otherwise -> retry)
47            putStrLn("done")
$ ghc --make mutex.hs
[1 of 1] Compiling Main             ( mutex.hs, mutex.o )
Linking mutex ...

$ ./mutex
I am thread ThreadId 2
I am thread ThreadId 3
I am thread ThreadId 4
I am thread ThreadId 5
I am thread ThreadId 6
I am thread ThreadId 7
I am thread ThreadId 8
I am thread ThreadId 9
I am thread ThreadId 10
done
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