Re: [Haskell-cafe] global variables

Aaron Denney wrote:

On 2007-05-24, David House wrote:

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On 24/05/07, Adrian Hey wrote:

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Or even at compile time (which is why I think it's reasonable to regard operations like newIORef etc.. as not really being "IO" operations at all). You can allocate heap space at compile time? (Well, I guess you could, but that wouldn't still be usable at run time...) I imagine newIORef as mallocing() some room, then returning a pointer to that memory. That doesn't seem like something that could be done at compile time.

You can allocate bss or data space at compile time for the executable you are compiling. (Well, if you read compile as compile and link. It's a bit fuzzy.)

Well we don't need to get too bogged down with the details of how any particular compiler/linker/rts might work. The point being that with any.. myIORef <- newIORef initialExpression whether or not it's at the top level, the only information needed to create the IORef is the initialExpression, and if it's at the top level then this is available at compile time (it doesn't even have to be evaluated at compile time in order to create the IORef). But it doesn't require any information from, nor should it have any effect on, the outside world that an executing program is interacting with. It is conceivable that for some newIORef implementations this would not be true, but in that case it's difficult to see how such implementations could safely put their newIORef in the ACIO monad anyway. Regards -- Adrian Hey