A critic of Guido's blog on Python's lambda

[Ken Tilton]

sross wrote:
>>I do wonder what would happen to Cells if I ever want to support
>>multiple threads. Or in a parallel processing environment.
> 
> 
> AFAIK It should be fine.
> In LW, SBCL and ACL all bindings of dynamic variables are thread-local.
> 

Ah, I was guilty of making an unspoken segue: the problem is not with 
the *dependent* special variable, but with the sequentially growing 
numeric *datapulse-id* ("the ID") that tells a cell if it needs to 
recompute its value. The ID is not dynamically bound. If threads T1 and 
T2 each execute a toplevel, imperative assignment, two threads will 
start propagating change up the same dependency graph... <shudder>

Might need to specify a "main" thread that gets to play with Cells and 
restrict other threads to intense computations but no Cells?

Actually, I got along quite a while without an ID, I just propagated to 
dependents and ran rules. This led sometimes to a rule running twice for 
one change and transiently taking on a garbage value, when the 
dependency graph of a Cell had two paths back to some changed Cell.

Well, Cells have always been reengineered in the face of actual use 
cases, because I am not really smart enough to work these things out in 
the abstract. Or too lazy or something. Probably all three.

kenny

-- 
Cells: http://common-lisp.net/project/cells/

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