Yes, I see... I edited it from the source code, which actually has the type:
dbConnectWith :: VerifyTables t => (SqlHandle () -> IO ()) -> t -> Query
() -> IO ()
dbConnectWith confn tabs query = confn (do { _ <- runST (do verifyTables
tabs ; query) 0; return () })
I obviously did not think hard enough about the changes I was making for
readability...
Keean
Benjamin Franksen wrote:
On Monday 09 January 2006 10:03, Axel Simon wrote:
On Sun, 2006-01-08 at 14:51 +0000, Keean Schupke wrote:
My solution to this when developing a database library for my own
use was to define the API
in a bracket notation style, and only provide safe functions. The
idea is that the function obtains the resource, calls a function
passed as an argument, then frees the resource, so all resouces are
guaranteed to be freed in the correct order... for example:
dbConnectWith :: DbName -> (DbHandle -> IO Result) -> Result
dbConnectWith name workFn = do
handle <- dbConnectTo name
workFn handle `finally` dbDisconnect handle
In this way you avoid finalizers... and everthing is safe providing
you only export the "with" style functions from the library...
Here's an example from the library, the connect function:
I suppose you meant to write "result" rather than "Result". This
style of functions is only safe if the user ensures that DbHandle is
never returned as part of the result. You should have that in your
documentation.
I wanted to mention this too, but you were quicker ;)
As far as I can tell, the only general solution is to use finalizers
and, if you really need to enforce a sequence of finialization,
touchForeignPtr.
Repeat: touchForeignPtr can NOT be used to enforce finalization order.
A practical issue with touchForeignPtr is that it
cannot be conveniently called from another finalizer, since the
latter live in C.
What do you mean "live in C"? Can't or shouldn't finalizers be written
in Haskell, too?
Ben
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