Re: [Haskelldb-users] Problems with insert

[Immanuel N. <i.n...@iu...>]

Am Dienstag, den 23.10.2007, 13:54 +0200 schrieb Björn Bringert:

> 
> Hi Immanuel,
> 
> your withDB type looks strange. You say that it is:
> 
> withDB :: GetRec er vr => Query (Rel er) -> IO [Record vr]
> 
> This looks like it takes a query, runs it, and returns the results. That 
> is, I expect that your withDB implementation is something like this:
> 
> withDB :: GetRec er vr => Query (Rel er) -> IO [Record vr]
> withDB q = connect (... db connection arguments ...) (\db -> query db q)
Actually I copied this withDB from a HaskellDB Wiki
http://www.haskell.org/hawiki/HaskellDbTutorial
Which appearently does not exist anymore. 
> 
> But you are then trying to use the same withDB function to do an insert, 
> even though the withDB in itself does a query. What I would do instead 
> is to have withDB be something like (not tested):
> 
> withDB :: (Database -> IO a) -> IO a
> withDB f = connect (... db connection arguments ...) f
> 
> Then your query example would be:
> 
> select = do withDB $ \db -> query db $ do t <- table t1
>                                         project (c1 << t!c1)
> 
> And the insert would be as you wrote it:
> 
> testInsert = withDB $ \db -> insert db t1 ((c1 << constant 3) #
>                                  &#-1;&#-1;          (c2 << constant "foo"))
Thank you! I will try this. 
> 
> 
> Note that every use of withDB opens and closes a database connection,so 
> in real programs, you want to make sure not to do this over and over for 
> performance reasons.
In fact I haven't considered this. Since I am not that familliar with
HaskellDB. Could you sketch how to insert efficiently without redundant
connection open/close? say given the list of type [(Int,String)] that
should be inserted into table with two columns of appropriate types.

Thanks,
Immanuel