[Haskell-cafe] Re: [Haskell] Top Level <-

[Ashley Yakeley]

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