Parallelism tutorial

[lukemaurer]

Basic introduction to data-race-free parallelism, focusing on fork/join parallelism and data parallelism provided by the parallel library as a way to introduce the portable and contended modes.

Currently pretty drafty:

• Needs an introduction
• Should probably be reorganized a bit—the section on portable and contended was supposed to be sort of an intermission but it's the biggest part
• Possibly a bit too formal in places? I think having numbered rules for things is good but then I've written conference papers
• Some of the example code could be more motivated: a Thing.t is just something with an immutable price and a mutable mood. (If nothing else, if my time at a trading firm has taught me anything, it's that prices are in fact mutable.)
• Other not-quite-finished bits

[goldfirere]

Nice tutorial. I enjoyed reading it.

I wondered in one of my inline comments about whether we want all the general portability/contention stuff in this tutorial. But after reflecting about this with @ccasin, I think it's good. We'll have general documentation for this stuff. But pointing readers to it in the middle of the tutorial is annoying, and the general documentation will inevitably have more detail than is needed at this point. So the stuff here might be duplicative, but I think on balance it's good duplication.

Happy to chat about anything here if you'd like.

[goldfirere]

Really happy with where this is going!

[jonludlam]

Generally really nice, I feel I've got a much better understanding of this now. I think mainly my comments are to ensure that anyone trying to follow along with a code editor doesn't get tripped up. In fact, if you want to encourage someone doing that, then a bit of detail at the start of how to set it up would be useful.