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2 points by shader 4519 days ago | link | parent

Maybe you could get versioning to work by making a real alias as opposed to just a copied reference. If somehow (i'm getting really far fetched here) expansion of macros, etc, could happen in any position instead of just application, you could have the alias expand to the other variable name. Then, you wouldn't have to worry about messing with your aliases ;)

So instead of merely 'evaluating' identifiers, you 'expanded' them, and variables expanded to their values, etc. you could have better aliases, and neater macros. Of course, I've probably misunderstood something horribly, but that's why you guys are here.



1 point by absz 4519 days ago | link

The problem is that macro expansion happens at "compile" time, so local variables don't have their values yet. But you reminded me of something in arc2c: to quote the "NOTES" file, "...if a local variable is ever mutated, a 'sharedvar' C-struct is used to represent that variable. These are constructed by %sharedvar primitives, and are accessed by %sharedvar-write and %sharedvar-read primitives."

So, since I believe almkglor said he was planning to use arc2c for this VM, only changing the code generation step, then the solution seems to be to just use the shared flag on values (probably the reference-counting version, as shader suggested). It seems like since we have sharedvars, the problem solves itself: when you send an object to another thread, you don't send the sharedvar, you send the value the sharedvar is wrapping. Then, when you modify something, then if it's a sent copy, that copy is modified, not affecting the original (because of the shared flag); if it's a local with aliases, then it's modified through the sharedvar, and since the sharedvar is still aliased and only its internals are modified, there's no problem!

Well, maybe---that assumes that I actually understand sharedvars well enough. Almkglor? Stefano (since you suggested/invented them)?

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1 point by almkglor 4519 days ago | link

> It seems like since we have sharedvars, the problem solves itself: when you send an object to another thread, you don't send the sharedvar, you send the value the sharedvar is wrapping.

You never actually send sharedvars - they are not accessible at the Arc level. However, they are necessary in order to send functions across processes; the generated code expects to see sharedvars, so it has to have (a copy of) sharedvars.

Edit:

> ...if a local variable is ever mutated, a 'sharedvar' C-struct is used to represent that variable. These are constructed by %sharedvar primitives, and are accessed by %sharedvar-write and %sharedvar-read primitives.

This means this case:

  (let foo 1
    (set foo 42))
Importantly, this does not use sharedvars!

  (let foo "hello"
    (sref foo #\H 0))

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