[encouraging everybody to reply on haskell-cafe] On Tuesday 23 November 2004 12:02, you wrote:
Thanks to the encouraging post
http://www.haskell.org//pipermail/haskell/2004-November/014748.html
from Benjamin Franksen, I have implemented my proposal which allows the user to define new global variables without unsafePerformIO, NOINLINE and other such horrors.
This is funny. When I got no immediate reaction from you, I started implementing it myself. I ended up with something similar. It has less features but is also a lot simpler. This is the interface: initGlobal :: Typeable a => a -> IO () getGlobal :: Typeable a => IO a Some remarks: o The separation into two modules is only historical. o I use an MVar internally, not an IORef; since it is not exposed, no indefinite blocking can occur. It's just a mutex around the dictionary. o Storing (TypeRep,Dynamic) pairs is redundant, since Dynamics already contain their own TypeRep (that is how they are made to work). o Both our implementations use unsafePerformIO in an unsafe manner, which is why the NOINLINE flag is used. o I also use a list for the dictionary; and I share your view about TypeRep badly needing an Ord instance (probably trivial to provide but I could be wrong). *************** On a related note, there was some discussion recently about which IO actions should be considered as 'harmless' so that they are allowed for the proposed top-level '<-' bindings, and how to characterize them in an elegant way. Here is yet another solution: The only things allowed at top-level (other than pure values) will be unique labels (such as provided by Data.Unique). Then we take all the newXXX actions out of whatever monad they live in and provide them as pure functions that take a Unique as additional argument: newXXX :: Unique -> a -> XXX a This might be a bit tricky to do efficiently. Anyway, a Haskell program could then create top-level Unique labels instead of top-level XXX vars: myGlobalVarLabel <- unique myGlobalMVar = newMVar myGlobalVarLabel "initial content" The concrete syntax could be made even simpler (and clearer), i.e. without the '<-' operator: unique myGlobalVarLabel Advantages: o No question of what is in SafeIO and what is not. o No question of when top-level IO actions are performed. *************** Now what about combining the two aproaches? The point here is that in Haskell we can easily create new unique labels at the top-level without resorting to any kind of unsafe operations: data Uniq1 = Uniq1 data Uniq2 = Uniq2 ... Only these have not the same but different types. So we need a way to map them to a single type in such a way that their uniqueness is preserved. We already have such a tool and it is called "deriving Typeable": unique = typeOf type Unique = TypeRep data Uniq1 = Uniq1 deriving Typeable data Uniq2 = Uniq2 deriving Typeable Our unique labels can now simply be defined as label1 = unique Uniq1 label2 = unique Uniq2 and our global variables as global1 = functionalNewMVar label1 True global2 = functionalNewMVar label1 (117::Int) I think this is most elegant, although there remains the questions of an efficient implementation of functionalNewXXX. Ben