Adrian Hey wrote:
Brian Hulley wrote:
The use of a plain value to initialize the ref rather than a monadic
computation would ensure that there would be no problems with trying
to work out which order to initialize top level refs that are
dependent on values of refs in other modules since there could be no
dependencies.
I'm not sure what problem you see with the ACIO monad proposal. It
was designed to prevent the kind of ordering dependency problems
which (I think) you're refering to. You can't read or write
IORefs/MVars or do any other "real IO" operation from ACIO.
All you can do is create them (and more complex data structures
based on them). So they could be evaluated at compile time, in
principle (AFAICS etc..). I think it should also be possible to
fork threads from ACIO, provided you arrange that they're initially
blocked on an empty MVar or something.
If it turns out that that isn't enough to properly initialise them
(like you need to do real IO), then you just can't have them as
top level identifiers. But you can still use something like the
"oneShot" function to implement "get" actions at the top level (in
the IO monad).
I was thinking that there would be a problem with polymorphic refs but now I
think I understand that there would not be a problem, seeing the description
of ACIO in
http://www.haskell.org//pipermail/haskell-cafe/2004-November/007664.html
where it is clear that the ACIO actions are just prepended to main.
So I'll retract my proposal in favour of ACIO :-)
Regards, Brian.
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