Re: [Haskelldb-users] Problems with insert

[<bri...@cs...>]

Immanuel Normann wrote:
> Hello,
> 
> I want to use the insert function and try to follow the guide from
> http://haskelldb.sourceforge.net/guide/insert.html
> 
> My database has one table t1 with the following layout:
> 
> +-------+----------+------+-----+---------+-------+
> | Field | Type     | Null | Key | Default | Extra |
> +-------+----------+------+-----+---------+-------+
> | c1    | int(11)  | YES  |     | NULL    |       | 
> | c2    | char(32) | YES  |     | NULL    |       | 
> +-------+----------+------+-----+---------+-------+
> 
> Right now I am able to set up all such that I can send successfully a
> select query with e.g.:
> 
> select = do withDB $ do t <- table t1
>                         project (c1 << t!c1)
> 
> But I fail with insert though I follow the instruction (of course
> adapted to my table layout) as presented on that website:
> 
> 
> testInsert =
>    do  withDB $ (\db -> insert db t1 ((c1 << constant 3) #
> 				      (c2 << constant "foo")))
> 
> This yields the error:
>     The lambda expression `\ db -> ...' has one argument,
>     but its type `Query (Rel er)' has none
>     In the second argument of `($)', namely
> 	`(\ db
> 	      -> insert db t1 ((c1 << (constant 3)) # (c2 << (constant "foo"))))'
>     In the expression:
> 	  withDB
> 	$ (\ db
> 	       -> insert db t1 ((c1 << (constant 3)) # (c2 << (constant "foo"))))
>     In the expression:
> 	do withDB
> 	 $ (\ db
> 		-> insert db t1 ((c1 << (constant 3)) # (c2 << (constant "foo"))))
> 
> In fact the types are:
> (\db -> insert db t1 ((c1 << constant 3) # (c2 << constant "foo"))) :: Database -> IO ()
> withDB :: (Database.HaskellDB.Database.GetRec er vr) => Query (Rel er) -> IO [Record vr]
> 
> So, how does insert work correctly?
> 
> Thanks,
> Immanuel

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)

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) #
                                            (c2 << constant "foo"))


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.

/Björn