[Haskell-cafe] Other transactional arrays?

9 Jul 2010


      Currently, Haskell have transactional arrays in
"Control.Concurrent.STM.TArray" implemented as "Array i (TVar e)"
which is array of transactional variables.  But what if I need to
place an array into TVar itself?  This is something like "TVar
(IOArray i e)", but with ability to read/write array elements inside
an STM transaction, and then commit changes with "atomically".

I've tried to make up this arrays in the following way:
...
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# OPTIONS_GHC -fno-cse #-}
...
module Data.Vector.STM
  ( TVector
  , newTVector
  , newTVectorIO
  , readTVector
  , readTVectorIO
  , writeTVector
  ) where
...
import GHC.Conc
import GHC.Prim
import GHC.Types (Int(..), IO(..))
...
data MutableArray a = MutableArray (MutableArray# RealWorld a)
...
data TVector a = TVector (TVar# RealWorld (MutableArray a))
...
stm = STM
{-# NOINLINE stm #-}
...
newTVector :: Int -> a -> STM (TVector a)
newTVector (I# i#) x = stm $ \s1# ->
    case newArray# i# x s1#             of { (# s2#, a# #) ->
    case newTVar# (MutableArray a#) s2# of { (# s3#, t# #) ->
    (# s3#, (TVector t#) #) }}
...
newTVectorIO :: Int -> a -> IO (TVector a)
newTVectorIO (I# i#) x = IO $ \s1# ->
    case newArray# i# x s1#             of { (# s2#, a# #) ->
    case newTVar# (MutableArray a#) s2# of { (# s3#, t# #) ->
    (# s3#, (TVector t#) #) }}
...
readTVector :: TVector a -> Int -> STM a
readTVector (TVector t#) (I# i#) = stm $ \s1# ->
    case readTVar# t# s1#     of { (# s2#, (MutableArray a#) #) ->
    case readArray# a# i# s2# of { (# s3#, a #) ->
    (# s3#, a #) }}
...
readTVectorIO :: TVector a -> Int -> IO a
readTVectorIO (TVector t#) (I# i#) = IO $ \s1# ->
    case readTVarIO# t# s1#   of { (# s2#, (MutableArray a#) #) ->
    case readArray# a# i# s2# of { (# s3#, a #) ->
    (# s3#, a #) }}
...
writeTVector :: TVector a -> Int -> a -> STM ()
writeTVector (TVector t#) (I# i#) x = stm $ \s1# ->
    case readTVar# t# s1#                    of { (# s2#, (MutableArray a#) #) ->
    case writeArray# a# i# x s2#             of { s3# ->
    case writeTVar# t# (MutableArray a#) s3# of { s4# ->
    (# s4#, () #) }}}
It seems like it works, but I'm in doubt about it's correctness.
Unfortunately, I don't know much about STM mechanics, so I'm asking
Cafe users (you :-) for help.

[Haskell-cafe] Other transactional arrays?

Emil Melnicov