• timotheecour (36/36) Feb 01 2013 currently there's a limitation with logging using variadic

"timotheecour" <thelastmammoth gmail.com> writes:

currently there's a limitation with logging using variadic 
templates:

eg:
the following is not allowed because it's ambiguous:
void DEBUG(T...)(T a, string file=__FILE__) {...}

proposal:
create a new kind of optional argument that's uninstantiable by 
user code, only by the compiler. We could use the following new 
syntax "==" instead of "=":
void DEBUG(T...)(T a, string file==__FILE__) {write(file,":"); 
foreach(ai ;a) write(ai," "); }

user code:
DEBUG(1+1, 2.2); //at file=__FILE__="myfun.d"
//this will print:"myfun.d:1+1 2.2"

the compiler would apply the same rules as for deducing the 
variadic T in "void DEBUG(T...)(T a)" and append one more arg to 
it (file). Note that file can't directly be instantiated by user 
code, it has to be implicit, to avoid the ambiguity.

This would be also very helpful in conjunction with my previous 
proposal earlier today ("feature request: __ARGS__ for logging 
(cf __FILE__, __LINE__, __FUNC___)". One could simply write:
---
void DEBUG(T...)(T a, string[]args==__ARGS__){
foreach(ai,vai;lockstep(a,args) writeln(vai,"=",ai,"; ");
}
int a=3,b=4;
DEBUG(1+2,3*3,a,b);//prints 1+2=3; 3*3=9;a=3;b=4;
---
Plain dead simple logging that just works with any number of args 
and avoids verbosity of tuple syntax; very useful for quick 
debugging.

Note: we can have more such special optional args at the end:
void DEBUG(T...)(T a, string[]args==__ARGS__, string 
file==__FILE__)
the limitation: they have to be implicitly convertible as above 
by such macros.