[Haskell-cafe] Db and STRef

Benjamin Franksen benjamin.franksen at bessy.de
Mon Jan 16 07:45:14 EST 2006

On Sunday 15 January 2006 19:50, Brandon Moore wrote:
> John Goerzen wrote:
> ...
> > I didn't honestly follow the STRef discussion, or how something so
> > I/O-based could work there.
> >
> > -- John
> The only message I've found was Oleg mentioning the typing that keeps
> an STRef limited to the scope of the runST it came from.
> I don't think database access could live in ST either.
> I think the proposal was to build something like Oleg's Monadic
> Regions http://okmij.org/ftp/Haskell/types.html#monadic-regions
> on top of IO
> I'm assuming you understand how the type on runST and the STRef
> operations ensure that, even though you *can* smuggle out an STRef in
> the result from runST, you will never be able to use it again.
> The idea was to do the equivalent thing with databases: use fancy
> types to ensure that handle can only be used inside to origination
> withDB or withCursor or whatever, and the bracketing function can
> release the resource on the way out, without worrying about it being
> used again.

I think this is an extremely good idea. I have been very frustrated with 
finalizers because of their limitations (can't rely on them being 
called at all), so have (reluctantly) been using the unsafe bracket 
version. Making it safe via a type system trick is really the way to 
go. I'll go read Oleg's paper and see if it's a workable solution.


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