Key arguments

[Bill Baxter]

baleog wrote:
> How can i implement key arguments in D? I want something like this:
> --
> void f(int x, double y = 2.0, int z = 3);
> ...
> f(1, z:10, y:5.1); // x=1, y=5.1, z=10
> f(1, z:10); // x=1, y=2.0, z=10
> --
> I tried to do this with Variant type and explicit type checking. But it takes a lot of routine actions. Maybe exists a better way?
> 
> Best regards

If you have a lot of arguments with various optional arguments that each 
have their own defaults, I think a struct is the most straightforward 
solution right now.

struct FOptions {
    double y = 2.0;
    int z = 3;
}

void f(int x, FOptions opt) {...}

{FOptions opt;
  opt.z = 10;
  opt.y = 5.1;
  f(1, opt);
}

{FOptions opt;
  opt.z = 10;
  f(1, opt);
}

It would be a lot nicer if static struct initializers worked for 
non-static structs too.  Then you could do:

Foptions opt = {z:10};
f(1,opt);

Or even f(1,{z:10});

There have been proposals to make struct initializers more useful along 
these lines.  The next step logical step in that evolution would be to 
recognize a trailing struct argument as a keyword set, and let you call

  f(1,z:10)
as a shortcut for
  f(1,{z:10})
which would be a shortcut for something like
  f(1,cast(FOptions){z:10})
which would work if struct initializers didn't have to be static.

Walter doesn't seem particularly interested, though.

--bb