Ganesh Sittampalam wrote:
On Sat, 30 Aug 2008, Ashley Yakeley wrote:
OK. Let's call it "top-level scope". Haskell naturally defines such a
thing, regardless of processes and processors. Each top-level <- would
run at most once in top-level scope.
If you had two Haskell runtimes call by C code, each would have its
own memory allocator and GC; IORefs, Uniques and thunks cannot be
shared between them; and each would have its own top-level scope, even
though they're in the same process.
That sounds more feasible - though it does constrain a plugin
architecture (in which Haskell code can dynamically load other Haskell
code) to cooperate with the loading RTS and not load multiple copies of
modules; this might make linking tricky.
This is a good idea anyway. It's up to the dynamic loading architecture
to get this right.
There's also the problem Duncan
Coutts mentioned about loading multiple versions of the same module -
what are the semantics of <- in relation to that?
If they are different versions, they ought to be considered different
modules with different names. Thus, Unique in base-3.0.2.0 ought to be a
different type than Unique in base-4.0. Thus any top-level initialisers
ought to be considered different and be run separately.
What's the current static behaviour? What happens if I link with
packages B & C, which link with different versions of A?
Also, it's no use for mediating access to a resource or library that can
only be accessed once, right? In fact, even without the problem of two
Haskell runtimes in one process this can't work, since some library in
another language might also choose to access that resource or library.
What applications does this leave beyond Data.Unique and Random?
So far we've just looked at declaring top-level IORefs and MVars.
By declaring top-level values of type IOWitness, you can generate open
witnesses to any type, and thus solve the expression problem. See my
open witness library and paper:
<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/open-witness>
<http://semantic.org/stuff/Open-Witnesses.pdf>
--
Ashley Yakeley
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