Error: "sync variables cannot currently be written" - chapel

Question

One of my students is having a severe problem with their project (assignment is: http://turing.plymouth.edu/~kgb1013/?course=4310&project=0). They (and I) are getting a weird error when compiling with my test file, testSemaphore.chpl, available at: http://turing.plymouth.edu/~kgb1013/4310/semaphoreProject/testSemaphore.chpl . The error is this:
$CHPL_HOME/modules/standard/IO.chpl:3038: error: sync variables cannot currently be written - apply readFE/readFF() to those variables first
I've had them remove the body of all of their methods, but they're still getting the error. Here is the body of their code:
use Time;
class Semaphore {
var gate1$ : sync int;
//Constructor
proc Semaphore(){
}
//secondary Constructor
proc Semaphore(givenTokens : int){
}
//returns the number of tokens available
proc getNumTokens() : int {
return 1;
}
//gives out a token
proc p() {
}
//returns a token
proc v() {
}
}
Why are they getting this error?

Answer 1

Sorry you're having trouble! It turns out that some of this would be better as a GitHub issue on the Chapel project, but let me first explain what's happening.
The class Semaphore includes a sync variable field. Meanwhile, the default implementation for writing a class is to write each field. So the error message in IO.chpl is trying to say that it doesn't have a reasonable way of outputting a sync variable. E.g.
var x: sync int;
writeln(x);
gives the same error. I think it would be reasonable to open up a GitHub issue on the Chapel project about how inscrutable this error is (and how it doesn't report a line number that is useful to you). Note that I personally used chpl testSemaphore.chpl --print-callstack-on-error to better understand the error - it often helps to add --print-callstack-on-error when you get an error referring to an internal/standard module.
Now, as for the assignment, there are two ways to resolve it:
Adjust testSemaphore.chpl to not print out the semaphore representation in 'writeln' calls. I commented out both writelns starting with "Testing that" and got it to compile.
Adjust the class Semaphore to include a writeThis to replace the compiler-generated write-each-field default, as below:
Here is an example of such a writeThis (see also The readThis, writeThis, and readWriteThis methods )
class Semaphore {
var gate1$ : sync int;
// Other methods as before
proc writeThis(f) {
f <~> "sync semaphore";
}
}