From the standpoint of Perl 6 (the language), we've thought about a lot of these things from the standpoint of fundamentals. What you find in the draft concurrency spec is more like a laundry list of things people would like. However, part of the reason we've not bothered much with that document is precisely because concurrency must be built into the design from the ground up, not because we're ignoring the topic. Hence, just off the top of my head, in the basic design of Perl 6 you will find various reassuring tendencies:
- All blocks are (potentially) closures
- Data tends to be well-typed even if the user doesn't realize it
- Emphasis is on reentrancy via use of lexicals rather than globals
- Most Perl 5 globals are gone or are no longer global
- The global footprint of a Perl 6 thread can be a lot lighter than a Perl 5 interpreter
- Globals and other strangely scoped variables must be syntactically distinguished with twigils
- Strictness is now default, so no accidental globals
- Mutable vs immutable data is semantically distinguished
- Default of formal parameters is readonly
- State variables are explicitly declared and clone as lexical variables do
- Tied variables must be explicitly declared
- Context lexicals encouraged over temporized globals
- Most objects stored in opaque storage
- Mutating methods are syntactically distinguished
- Symbolic vs hard reference syntactically distinguished
- Constants are known to be constant at compile time
- Hyperops, junctions, and feeds all make specific promises about lack of interactions
- Exceptions may easily be treated as undefined data
when doing parallel operations
- Lazy lists encourage reentrant programming
- Patterns are reentrant so they can be used in multiple threads on different strings simultaneously
- Grammars and other logic programs are designed to be allow multiple "in flight" hypotheses in parallel
These bits of design all contribute to easier concurrency, but are spread out over all the existing synopses, and most of them are not explicitly labeled as "concurrency support". That is not to say that there are not still many open issues, and as you say, we'll have ample opportunity to discuss them.
Specifically with regard to STM, while STM doesn't itself address the issue of non-reversible external operations, I think it does potentially address the issue of acquiring the locks in preparation for doing such an operation, since the locks themselves are presumably in memory. And I suspect that STM can be extended to manage anything that can be construed as "memory" as long as it has reversible characteristics.
Anyway, in many cases I think the compiler will have enough hints to determine whether the code is reentrant and whether the data has to be shared without forcing ordinary users into a monadic lifestyle. And where the compiler is not so sure, it can likely ask the programmer politely for a few more clues. In any case, the Perl 6 language is mutable and strongly versioned, so we can hopefully fix it where it goes wrong.
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