On Monday 22 Nov 2004 11:26 am, Keean Schupke wrote: > Adrian Hey wrote: > >Just repeating this again and again doesn't make it any more true. > > Ditto... I for one think the best solution is to use the language as > intended and pass the values as function arguments.
I guess you mean the usual handle based approach, but this makes no sense at all for a Haskell interface to some *unique* stateful resource (eg. a piece of raw hardware or "badly designed" C library). The handle is a completely redundant argument to all interface functions (there's no need identify which thing is being referenced because there is only one). Furthermore it still leaves you with the problem ensuring that users don't use whatever "openUniqueThing" routine that creates and initialises the state multiple times to end up with two or more different TWIs which are all trying to reflect state changes in the same unique resource. AFAICS the only way of preventing this requires the use of top level mutable state, so this solution is a non-solution (the only safe way of using this "solution" still leaves you with the original problem). Of course this problem could be solved quite simply with a top level.. userOpenUniqueThing <- oneShot openUniqueThing Unfortunately this is not an option because it creates a top level MVar (and so it must be evil). The only thing to be said in favour of the handle based approach is that forcing users to get the state handle does ensure that any necessary external initialisation has been performed prior to using the resource. But there are other ways to do this too (like prefixing every exported interface function with "userInit" instead of exporting userInit itself). > As pointed out > on this list - the only possible situation where you cannot do this is > when interfacing to a badly written C library. This is one situation, but certainly not the only possible one. You have the same problem with interfacing to any unique stateful resource (or even if you have a multiple but finite supply of these resources). > This is true - but only because unsafePerformIO exists. Without it > World is simply a value passed via the IO Monad. Huh? Top level TWIs are just part of the initial world state (as seen by main). We can argue about whether or not they are needed, but their existence surely doesn't make the situation any worse than it already is. > I would ask an alternative question - is it possible to live without > unsafePerformIO? Not at present. > I have never needed to use it! I have a feeling that those folk who think they don't need it are those who enjoy the luxury of doing all their IO via pre-supplied "Haskell user friendly" libraries and haven't given much thought to how these libraries actually work or how they could be implemented in Haskell if they didn't already exist (without using the unsafePerformIO hack of course). Regards -- Adrian Hey _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
