[Haskell-cafe] STM problems

[Michael Speer]

Thank you for pointing me back on track.

I got caught in a sleepy mindset of thinking of (STM a) as a datatype
only reachable while in an STM context rather than an action returning
an a.

I wanted a fixed-sized array with mutable variables ( no need to
mutate the array ) so here is the working next iteration.

import Control.Concurrent.STM
import Control.Concurrent

import Array

data HashTable a b = HashTable (a -> Int) (Array Int (TVar (Maybe b)))

mkHashTable :: ( a -> Int ) -> Int -> STM (HashTable a b)
mkHashTable hashfn numBuckets | numBuckets > 0 = do
  vs <- mapM (\v -> newTVar Nothing >>= \nt -> return (v,nt)) [1..numBuckets]
  return $ HashTable hashfn $ array (1,numBuckets) vs

readHashTable :: HashTable a b -> a -> STM (Maybe b)
readHashTable hashtable key =
  let (HashTable hashfn table) = hashtable
  in readTVar (table ! hashfn key)

writeHashTable :: HashTable a b -> a -> (Maybe b) -> STM ()
writeHashTable hashtable key value =
    let (HashTable hashfn table) = hashtable
    in writeTVar (table ! hashfn key) value >> return ()

main = do
  v <- atomically $ do
         ht <- mkHashTable id 10 :: STM (HashTable Int String)
         mapM_ (\n -> writeHashTable ht n (Just $ "hello " ++ show n)) [1..10]
         mapM (\n -> readHashTable ht n) [1..10]
  print v


On Sun, Aug 23, 2009 at 5:12 AM, Luke Palmer<lrpalmer at gmail.com> wrote:
> On Sun, Aug 23, 2009 at 1:33 AM, Michael Speer<knomenet at gmail.com> wrote:
>> data HashTable a b = HashTable (a -> Int) (STM (Array Int (TVar (Maybe b))))
>
> So wait... a HashTable is a hash function together with an *action
> returning an array of TVars?*    I don't think that's right.  It looks
> like that action is recreating an empty table each time it is
> accessed.  You probably want:
>
> data HashTable a b = HashTable (a -> Int) (TVar (Array Int (TVar (Maybe b))))
>
> instead.  The rest of the implementation will follow the type.
>
>>
>> mkHashTable :: ( a -> Int ) -> Int -> HashTable a b
>> mkHashTable hashfn numBuckets | numBuckets > 0 = do
>>  HashTable hashfn $ do
>>    vs <- mapM (\v -> newTVar Nothing >>= \tv -> return (v,tv) ) [1..numBuckets]
>>    return $ array (1,numBuckets) vs
>>
>> readHashTable :: HashTable a b -> a -> STM (Maybe b)
>> readHashTable hashtable key =
>>  let (HashTable hashfn table) = hashtable
>>  in table >>= \tb -> readTVar (tb ! hashfn key)
>>
>> writeHashTable :: HashTable a b -> a -> (Maybe b) -> STM ()
>> writeHashTable hashtable key value =
>>    let (HashTable hashfn table) = hashtable
>>    in table >>= \tb -> writeTVar (tb ! hashfn key) value >> return ()
>>
>> main =  let ht = mkHashTable id 10 :: HashTable Int String
>>        in do
>>          v <- atomically $ do
>>             mapM_ (\n -> writeHashTable ht n (Just $ "hello " ++ show
>> n)) [1..10]
>>             mapM (\n -> readHashTable ht n) [1..10]
>>          print v
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