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