John Meacham wrote: On Fri, Aug 29, 2008 at 04:33:50PM -0700, Dan Weston wrote:
C++ faced this very issue by saying that with global data, uniqueness of initialization is guaranteed but order of evaluation is not. Assuming that the global data are merely thunk wrappers over some common data source, this means that at minimum, there can be no data dependencies between plugins where the order of evaluation matters.
Fortunately, we can do a whole lot better with haskell, the type system guarentees that order of evaluation is irrelevant :) no need to specify anything about implementations.
Can't you write two recursive modules with <- that depend on each other, so that there's no valid initialisation order? Contrived example follows: module Module1 where glob1 :: IORef Int glob1 <- mod2 >>= newIORef mod1 :: IO Int mod1 = readIORef glob1 module Module2 where glob2 :: IORef Int glob2 <- mod1 >>= newIORef mod2 :: IO Int mod2 = readIORef glob2 It might need some strictness annotations to actually cause non-termination at initialisation rather than just make the results of mod1 and mod2 be _|_. I think those initialisers do satisfy ACIO, though I'm not certain - from the point of view of dataflow, you can certainly remove them both together if the rest of the code doesn't use mod1 or mod2, and likewise they commute with any other IO operations. But on the other hand there's no way to actually put them in an order that doesn't cause non-termination. Cheers, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================