add Cthulhu workshop

[aviatesk]

Okay I've done with the Cthulhu part. I'd appreciate if you could have a look on it.

[timholy]

It looks good, I am looking forward to it!

Small comments:

• some will notice it's easy to circumvent @code_warntype's problem by using it on inner(Int, somes), but I think it's fine as-is (my examples have similar issues)
• for inner, would it be better to do val = s.value; if isa(val, T) ...? Then you don't have to annotate.
• "Cthulhu lets us to traverse all these forms quite easily" not Expr as far as I know
• "Reasoning about multithreading code is hard even with Cthulhu" I guess that's true, but is that example really about multithreaded code or about Core.Box (15276)?

[aviatesk]

Thanks for your feedback ! Some responses:

• for inner, would it be better to do val = s.value; if isa(val, T) ...? Then you don't have to annotate.

Yeah, I will refactor the code.

• "Cthulhu lets us to traverse all these forms quite easily" not Expr as far as I know

True, I will add a caveat on it.

• "Reasoning about multithreading code is hard even with Cthulhu" I guess that's true, but is that example really about multithreaded code or about Core.Box (15276)?

Yes, the actual problem is from Core.Box. But here I want to highlight how difficult it is to inspect the multithreading code where a closure is wrapped in Task, and so I believe this will make sense.