suggestion: default values

[David B. Held]

bobef wrote:
> It would be nice if there was some character like $ or something, if used as function argument, the default value for this argument to be used. What I mean is this:
> 
> void myfun(int a,char[] b,bool d=false,int e=55){}
> 
> void main()
> {
>     myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
> }

While some languages provide named parameters, they tend to be langs 
which encourage functions with many arguments.  If you find you are 
creating functions like this, you should first ask yourself if this is 
the best way.  If you decide that it is, one pattern to use is analogous 
to the Builder pattern:

struct myFunArgs
{
     int a;
     char[] b;
     bool d = false;
     int e = 55;
}

void myfun(myFunArgs args) { ... }

myFunArgs args;
args.a = 5; args.b = "5";
myfun(args);

This is the *very* Poor Man's named parameters mechanism, but if you 
have enough parameters to need named ones, then this probably isn't at 
all burdensome (and may increase clarity).

Another alternative, if you are creating a library function that you 
want to be *very* friendly, you can create a variadic type-safe template 
function and parse the args inside the function (though this doesn't 
work very well with args of the same type).  Of course, you can use the 
Perl convention of name, value pairs to make it completely general, but 
that's not very elegant or idiomatic D style.

Dave