[dmd-concurrency] draft 4

[Steve Schveighoffer]

This all looks very cool for message passing.

I'm still more interested in how shared turns out, message passing seems to be a very straightforward problem with a very straightforward solution.  Also, having never really used a language with builtin message passing or a MP library (I have implemented it several times unwittingly not knowing the pattern), I can't really add any more insightful comment except to say it does look exciting :)

A comment on the introduction, I know that the other chapters of the book don't have an initial section header, but you may want to break up this section into a brief introduction and then title this section header appropriately.  Although it is a good lesson and backs up the design choices of D, it doesn't have anything to do with D's API.  Having to read 6 pages of history before reading anything about D is puzzling.  The header "A brief history of data sharing"  before the whole thing would cue uninterested readers to jump to the meaty parts.

Pretend your a person learning D, and you already know that message passing is the best, having dealt with some message passing library (or language that supports it).  You don't want to read through a history lesson confirming what you already know, you just want to answer the question "how does D do concurrency?"  Basically, I think you should explicitly identify the "how" and "why" parts, preferably putting some of the "how" first.

-Steve