[Haskell] Re: ANN: HDBC (Haskell Database Connectivity)

oleg at pobox.com oleg at pobox.com
Tue Jan 10 00:25:16 EST 2006

Axel Simon wrote, in response to Keean Schupke
> > 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.

It seems using types to enforce the policy that a data base handle
cannot ever `leak' out might be a better idea, in Haskell. It is quite
straight-forward to do so: cf. the design of the ST monad guarantees
that an STRef cannot `leak' out in the result of runST. Monadic
Regions (implemented in Haskell) generalize the idea to multiple
nested regions: a pointer (or any other resource, e.g., a database
cursor) allocated in one region can be used in nested regions but
cannot leak outside of the region that created it (Or, to be more
precise, cannot be used outside -- and so, can be safely deallocated
when the region is exited).

Incidentally, the difficulty with finalizers was precisely the
argument for using enumerators rather than cursors in database
APIs. Takusen has implemented that idea; takusen currently supports
Sqlite, PostgreSQL and Oracle, has a test suite. Its performance test
shows that takusen can retrieve 2 million rows from a table without
running out of memory.

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