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digitalmars.D.learn - Trouble with receiveOnly and immutable(JSONValue)

reply rx <mr b5.re> writes:
alias SyncData = immutable(JSONValue);

void worker(string filename) {
   SyncData data = filename.readText.parseJSON;
   send(ownerTid, data);
}

void main(string[] args) {
   spawn(&worker, args[1]);
   writeln(receiveOnly!SyncData);
}

I'm trying to send this immutable(JSONValue) back to the main 
thread but when trying to compile, I get the following error:


 /usr/include/dmd/phobos/std/concurrency.d(764): Error: cannot 
 modify immutable expression ret.__expand_field_0
 cc.d(15): Error: template instance 
 `std.concurrency.receiveOnly!(immutable(JSONValue))` error 
 instantiating


Can anyone help out?
Jan 10 2019
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 1/10/19 12:15 PM, rx wrote:

 alias SyncData = immutable(JSONValue);
 
 void worker(string filename) {
    SyncData data = filename.readText.parseJSON;
    send(ownerTid, data);
 }
 
 void main(string[] args) {
    spawn(&worker, args[1]);
    writeln(receiveOnly!SyncData);
 }
 
 I'm trying to send this immutable(JSONValue) back to the main thread but 
 when trying to compile, I get the following error:
 

 /usr/include/dmd/phobos/std/concurrency.d(764): Error: cannot modify 
 immutable expression ret.__expand_field_0
 cc.d(15): Error: template instance 
 `std.concurrency.receiveOnly!(immutable(JSONValue))` error instantiating


 
 Can anyone help out?


Looks like a long-standing bug (there are several related issues about 
sending/receiving immutable data). The way std.concurrency.receiveOnly 
works is it creates a temporary result, then puts the received item into 
the result, and then returns it. But you can't copy an immutable into a 
temporary that's already immutable.

e.g. it looks like this:

immutable int x;
x = 5; // error

I don't know if there's a specific "can't receive immutable data" issue 
report, but certainly, you can add your issue to the list.

-Steve
Jan 10 2019
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 1/10/19 1:20 PM, Steven Schveighoffer wrote:


 I don't know if there's a specific "can't receive immutable data" issue 
 report, but certainly, you can add your issue to the list.


Actually, this one is nearly identical and quite new, you can just add 
to that one:

https://issues.dlang.org/show_bug.cgi?id=19345

-Steve
Jan 10 2019
parent reply rx <mr b5.re> writes:
On Thursday, 10 January 2019 at 18:25:44 UTC, Steven 
Schveighoffer wrote:

 On 1/10/19 1:20 PM, Steven Schveighoffer wrote:


 I don't know if there's a specific "can't receive immutable 
 data" issue report, but certainly, you can add your issue to 
 the list.


 Actually, this one is nearly identical and quite new, you can 
 just add to that one:

 https://issues.dlang.org/show_bug.cgi?id=19345

 -Steve


Thanks Steve. I suppose when the documentation talks about 
preferring message passing immutable data it means just use plain 
old receive. I can just use a template I anyway.

Would you estimate this Phobos bug to be particularly hard to 
solve?
Jan 10 2019
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 1/10/19 2:36 PM, rx wrote:

 On Thursday, 10 January 2019 at 18:25:44 UTC, Steven Schveighoffer wrote:

 On 1/10/19 1:20 PM, Steven Schveighoffer wrote:


 I don't know if there's a specific "can't receive immutable data" 
 issue report, but certainly, you can add your issue to the list.


 Actually, this one is nearly identical and quite new, you can just add 
 to that one:

 https://issues.dlang.org/show_bug.cgi?id=19345


 
 Thanks Steve. I suppose when the documentation talks about preferring 
 message passing immutable data it means just use plain old receive. I 
 can just use a template I anyway.
 
 Would you estimate this Phobos bug to be particularly hard to solve?


Not sure. It would involve casting away immutable, which has to be done 
carefully. Probably nobody has had motivation enough to fix it yet.

-Steve
Jan 10 2019
parent rx <mr b5.re> writes:
On Thursday, 10 January 2019 at 20:21:04 UTC, Steven 
Schveighoffer wrote:

 Thanks Steve. I suppose when the documentation talks about 
 preferring message passing immutable data it means just use 
 plain old receive. I can just use a template I anyway.
 
 Would you estimate this Phobos bug to be particularly hard to 
 solve?


 Not sure. It would involve casting away immutable, which has to 
 be done carefully. Probably nobody has had motivation enough to 
 fix it yet.

 -Steve


I might take a look at it but obviously I don’t want to screw it 
up if it’s complex.
Jan 10 2019