[Haskell-cafe] Newbie vs. laziness

[Jeff Polakow]

Hello,

> dbCreateIndices con dtn dof = do
>    let dummy = newDbItem
>    let query = "SELECT id FROM " ++ (dtn dummy) ++ " ORDER BY " ++ (dof 
dummy)
>    ids <- liftM (map fromSql . concat ) $! quickQuery con query []
>    return $ IntMap.fromList $ zip ids [0..]
> 
> quickQuery returns a lazy list of results, and I expected ($!) to make
> it strict. But my program crashes if I use it this way. But, if I add
> a print to the function:
> 
When using lazy DB connections in HDBC (I'm assuming you're using HDBC 
since quickQuery is the pre-packaged lazy query call in HDBC), you have to 
be careful about forcing all the data your'e expecting from a query on a 
connection before using the same connection for something else. As the 
previous poster mentioned, $! will only evaluate the list to weak head 
normal form. Printing out the list will indeed force everything in the 
list to be evaluated. 

There is an extended discussion of this same issue (in the context of lazy 
file io) in this thread from haskell-cafe: 
    http://www.haskell.org/pipermail/haskell-cafe/2007-March/023498.html

-Jeff


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