Javacc error reporting results in “Expansion can be matched by empty string.” - javacc

Question

I am trying to add some custom error messages to my javacc parser to hopefully make the error messages more specific and the language problems easier to find and correct.
The first error that I am trying to focus in on is how to detect that the correct number of arguments have been provided to a 'function' call. Rather than the default message, I would like to print out something like "missing argument to function".
My simplified language and my attempt to catch a missing argument error looks something like:
double arg(boolean allowMissing):
{ double v; Token t; }
{
t = <INT> { return Double.parseDouble(t.image); }
| t = <DOUBLE> { return Double.parseDouble(t.image); }
| v = functions() { return v; }
| { if (!allowMissing) throw new ParseException("Missing argument");} // #1 Throw error if missing argument
}
double functions() :
{ double v1, v2, result;
double[] array;
}
{
(<MIN> "(" v1=arg(false) "," v2=arg(false) ")") { return (v1<v2)?v1:v2; }
| (<MAX> "(" v1=arg(false) "," v2=arg(false) ")") { return (v1>v2)?v1:v2; }
| (<POW> "(" v1=arg(false) "," v2=arg(false) ")") { return Math.pow(v1, v2); }
| (<SUM> "(" array=argList() ")") { result=0; for (double v:array) result+=v; return result;}
}
double[] argList() :
{
ArrayList<Double> list = new ArrayList<>();
double v;
}
{
( (v=arg(true) { list.add(v);} ( "," v=arg(false) {list.add(v);} )*)?) { // #2 Expansion can be matched by empty string here
double[] arr = new double[list.size()];
for (int i=0; i<list.size(); i++)
arr[i] = list.get(i);
return arr;
}
}
As you can see functions will recursively resolve their arguments, and this allows function call to be nested.
Here are a few valid expressions that can be parsed in this language:
"min(1,2)",
"max(1,2)",
"max(pow(2,2),2)",
"sum(1,2,3,4,5)",
"sum()"
Here is an invalid expression:
"min()"
This all works well until I tried to check for missing arguments (code location #1). This works fine for the functions that have a fixed number of arguments. The problem is that the sum function (code location #2) is allowed to have zero arguments. I even passed in a flag to not throw an error if missing arguments are allowed. however, javacc gives me an error at location #2 that "Expansion within "(...)?" can be matched by empty string". I understand why I get this error. I have also read the answer for JavaCC custom errors cause "Expansion can be matched by empty string." but it did not help me.
My problem is that I just cannot see how I can have this both ways. I want to throw an error for missing arguments in the functions that have a fixed number of arguments, but I don't want an error in the function that allows no arguments. Is there a way to refactor my parser so that I still use the recursive style, catch missing arguments from the functions that take a fixed arguments, yet allow some functions to have zero arguments?
Or is there a better way to add in custom error messages? I am not really seeing much in the documentation.
Also, any pointers to examples that use more sophisticated error reporting would be greatly appreciated. I am actually using jjtree, but I simplified it down for this example.

Answer 1

Here's what I would do.
Instead of using a boolean argument in function arg, I would use the ? operator:
double arg():
{ double v; Token t; }
{
t = <INT> { return Double.parseDouble(t.image); }
| t = <DOUBLE> { return Double.parseDouble(t.image); }
| v = functions() { return v; }
}
double functions() :
{ double v1=0, v2=0, result;
double[] array;
}
{
(<MIN> "(" (v1=arg())? "," (v2=arg())? ")") { return (v1<v2)?v1:v2; }
| (<MAX> "(" (v1=arg())? "," (v2=arg())? ")") { return (v1>v2)?v1:v2; }
| (<POW> "(" (v1=arg())? "," (v2=arg())? ")") { return Math.pow(v1, v2); }
| (<SUM> "(" array=argList() ")") { result=0; for (double v:array) result+=v; return result;}
}
double[] argList() :
{
List<Double> list = new ArrayList<Double>();
double v;
}
{
( (v=arg() { list.add(v); } | { list.add(0.); } )
( "," (v=arg() { list.add(v); } | { list.add(0.); } ) )*) {
double[] arr = new double[list.size()];
for (int i=0; i<list.size(); i++)
arr[i] = list.get(i);
return arr;
}
}

Answer 2

You could do this
double arg():
{ double v; Token t; }
{
t = <INT> { return Double.parseDouble(t.image); }
| t = <DOUBLE> { return Double.parseDouble(t.image); }
| v = functions() { return v; }
}
double argRequired():
{ double v; }
{
v = arg() { return v ; }
| { if (!allowMissing) throw new ParseException("Missing argument");} // #1 Throw error if missing argument
}
double argOptional( double defaultValue ): // Not needed for this example, but might be useful.
{ double v; }
{
v = arg() { return v ; }
| { return defaultValue ; }
}
double functions() :
{ double v1, v2, result;
double[] array;
}
{
(<MIN> "(" v1=argRequired() "," v2=argRequired() ")") { return (v1<v2)?v1:v2; }
| (<MAX> "(" v1=argRequired() "," v2=argRequired() ")") { return (v1>v2)?v1:v2; }
| (<POW> "(" v1=argRequired() "," v2=argRequired() ")") { return Math.pow(v1, v2); }
| (<SUM> "(" array=argList() ")") { result=0; for (double v:array) result+=v; return result;}
}
double[] argList( ) :
{
ArrayList<Double> list = new ArrayList<>();
double v;
}
{
( v=arg() { list.add(v);}
( "," v=argRequired() {list.add(v);}
)*
)?
{
double[] arr = new double[list.size()];
for (int i=0; i<list.size(); i++)
arr[i] = list.get(i);
return arr;
}
}