I have added make-thread-parameter
to SRFI 226 (see my personal repo). Am Mi., 8. Sept. 2021 um 10:19 Uhr schrieb Shiro Kawai <shiro.ka...@gmail.com>: > > Regarding the semantics of parameter mutation. The draft says: > > Moreover, this specification makes a decision on the behavior of mutable > parameter objects with respect to multiple threads. If a parameter that is > inherited by a newly created thread and has not been reparameterized is > mutated by one of the threads, the mutation will also (eventually) be > observed by the other thread. This is a clear semantics because this is the > same behavior as implemented by lexical variables. > > Although I agree that the proposed semantics can be considered parallel to > the lexical variable semantics, I argue against it: Shared mutable parameters > are of little use. > > It is unreliable to use it for inter-thread communication, for the parameter > can be reparameterized without your knowledge, because of its "dynamic > scoping" nature. That means you can't locally determine the effect of > mutating a parameter (whether it's thread-local or shared), so you have to > always reparametize when you need to mutate a parameter with a consistent > behavior. > > Besides, there's no guarantee on atomicity of mutating parameters. It's > always better to use dedicated synchronization primitives for inter-thread > communication. > > On the other hand, thread-local parameter mutation comes handy. For example, > suppose you want a library that collects per-thread event logging. > > (define log-sink (make-parameter #f)) ;; (<thread> . <sink>) > > (define (get-sink) > (let ((p (log-sink))) > (if (and p (eq? (car p) (current-thread))) > (cdr p) > (let ((sink (make-sink))) > (log-sink (cons (current-thread) sink)) > sink)))) > > (deifne (log-push! item) > (write-log-to-sink item (get-sink))) > > We want log-push! to work just by importing to the code that uses it. We > don't want to (or even can't) go to where the thread is created and wrap the > thread thunk with (parameterize ((log-sink #f)) ...). > > Gauche has been using this thread-local parameters, so I don't know if > there's a useful case for shared-mutable-state parameters. I'd like to know > if there are some. > > > > > >
