StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

POreturn TVar after initialisation
primarykey
Id
14096954
data
AcceptedAnswerId
0
AnswerCount
1
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2012-12-31T02:59:57.037
FavoriteCount
0
LastActivityDate
2012-12-31T03:28:05.403
LastEditDate
2012-12-31T03:24:46.690
LastEditorUserId
1080126
OwnerUserId
1080126
ParentId
0
PostTypeId
1
Score
1
ViewCount
163
LastEditorDisplayName
text
Body
Is it possible to return the newly created TVar in a do block? I tried to implement this using this code: <pre><code>type Buffer a = TVar [[(a,a)]] newBuffer :: STM (Buffer a) newBuffer = newTVar [[]] launchGhosts :: [[(String,String)]] -> Buffer String launchGhosts unblocked = do buff <- atomically newBuffer atomically $ put buff unblocked return buff computeBlock :: Buffer String -> IO() computeBlock buff = do i <- atomically $ get buff putStrLn $ show i put :: Buffer a -> [[(a,a)]] -> STM () put buff x = do writeTVar buff x get :: Buffer a -> STM [[(a,a)]] get buff = do x <- readTVar buff return x </code></pre> And this should allow me to initialize the shared memory and use it at another place in the program. The main reason that I want to separate the memory initialization is to call the concurrent functions multiple times, without initialising the memory again and again. The type checker trows these two errors: <pre><code>pacman.hs:65:29: No instance for (Monad TVar) arising from a do statement Possible fix: add an instance declaration for (Monad TVar) In a stmt of a 'do' block: buff <- atomically newBuffer In the expression: do { buff <- atomically newBuffer; atomically $ put buff unblocked; computeBlock buff; return buff } In an equation for `launchGhosts': launchGhosts unblocked = do { buff <- atomically newBuffer; atomically $ put buff unblocked; computeBlock buff; .... } pacman.hs:65:37: Couldn't match expected type `TVar t0' with actual type `IO a0' In the return type of a call of `atomically' In a stmt of a 'do' block: buff <- atomically newBuffer In the expression: do { buff <- atomically newBuffer; atomically $ put buff unblocked; computeBlock buff; return buff } </code></pre> Does anybody know what the problem is, or maybe another way to implement the idea behind this code? update: <pre><code>launchGhosts :: [[(String,String)]] -> IO(Buffer String) launchGhosts unblocked = do buff <- atomically newBuffer atomically $ put buff unblocked return buff computeBlock :: IO(Buffer String) -> IO() computeBlock buff = do i <- atomically $ get buff putStrLn $ show i </code></pre> update: <pre><code>pacman.hs:71:46: Couldn't match expected type `Buffer a0' with actual type `IO (Buffer String)' In the first argument of `get', namely `buff' In the second argument of `($)', namely `get buff' In a stmt of a 'do' block: i <- atomically $ get buff </code></pre>
Tags
<haskell><concurrency>
Title
return TVar after initialisation
singulars
PostAcceptedAnswerId
1. This table or related slice is empty.
PostParentId
1. This table or related slice is empty.
PostTypePostTypeId
1. PTQuestion
UserLastEditorUserId
1. USConsec
UserOwnerUserId
1. USConsec
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
PostLinksRelatedPostIdPostId
1. This table or related slice is empty.
PostsAcceptedAnswerId
1. This table or related slice is empty.
PostsParentIdCreationDate
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
VotesPostIdCreationDate
1. VO
 singulars
 PostPostId
 POreturn TVar after initialisation
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTDownMod
2. VO
 singulars
 PostPostId
 POreturn TVar after initialisation
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
3. VO
 singulars
 PostPostId
 POreturn TVar after initialisation
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
CommentsPostId
1. This table or related slice is empty.

Querying!

Guidance

A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.