On Saturday 13 Nov 2004 10:39 am, Keean Schupke wrote:
Actually, I Think I'm wrong - I think its not even safe if you cannot export the '<-' def. If any functions which use it are exported you are in the same situation. I cannot say the kind of code in the example I gave is good, can you? Infact the availability of these top level IO actions seems to completely change the feel of the language...
I've looked at your example, and the behaviour you describe is exactly what would be expected and what it is intended. That's the whole point of things having identity. The reason they have identity is because they are mutable and all users of a particular TWI are indeed mutating the same thing, just as all users of stdout are writing to the same file. The point is that if the shared TWI is something like an IORef this is (of course) extremely dangerous because anybody can write anything they like to it at any time. But that is not how this should be used. The module exporting one or more TWIs typically will not be exporting raw IORefs. It will be exporting a well designed stateful api which access IORefs etc via closures. It's the responsibility of the author of the exporting module to organise the code so that it delevers (on whatever promisies it's making) to all clients, and clients should not rely on anything that isn't being promised. So it seems to me that the only thing that's wrong here is your expectations (I.E. that a module should assume it has exclusive access to whatever state the TWI's it imports mutate). This is not so. If it wants it's own private TWI (a mutable queue say) it should not be importing another modules queue (not that any good design should be exporting such a thing anyway), it should be importing a newQueue constructor and making it's own queue (either at the top level or via normal IO monadic operations).. myQueue <- newQueue But there's no magic here. All IO actions have potentially unknown state dependencies and mutating effects, that's why they're in the IO monad. All the top level <- extension does is enable the user to extend the initial world state (as seen by main) with user defined state, but it doesn't fundamentally change nature or hazards of programming via the IO monad, for better or worse. Regards -- Adrian Hey