Dream Feature Regarding Default Arguments

[Mason McGill]

On Sunday, 6 April 2014 at 01:33:36 UTC, Nick Sabalausky wrote:
> On 4/5/2014 9:26 PM, Nick Sabalausky wrote:
>>
>> It would be *fantastic* if D recognized the disambiguation of 
>> using
>> incompatible types and permitted this:
>>
>>     interface Foo {}
>>     interface Bar {}
>>     class FooBar : Foo, Bar {}
>>
>>     void func(Foo foo=someFoo, Bar bar=someBar) {...}
>>
>>     func(myFoo); // Unambiguous, OK
>>     func(myBar); // Unambiguous, OK
>>     func(myFooBar); // Ambiguous, ERROR
>
> Actually, that last line should be:
>
>      func(myFooBar); // Unambiguous, this is still interpreted 
> as the first parameter (with the second parameter left as 
> default) just as it is right now.

I think D is actually one of the better languages around in terms 
of factoring this sort of thing out.  Here's an example argument 
parser that lets you define pairs of optional arguments: 
http://pastebin.com/RaNfwH6X.  It can be extended to allow 
n-tuples of optional arguments.

Argument parsing is encapsulated so writers only have to declare 
one function template, with a constraint that clearly expresses 
intent:

void func(Args...)(Args args)
     if (Pattern!(bool, AnyOf!(int, string)).matches!Args)
{
     alias Parser = Pattern!(bool, AnyOf!(int, string));
     auto parsedArgs = Parser.parse(tuple(args), tuple(false, 0, 
"0"));
     writeln(parsedArgs);
}

void main()
{
     func(true);         // true, 0, "0"
     func(true, 1);      // true, 1, "0"
     func(true, "1");    // true, 0, "1"
     func(true, 1, "1"); // true, 1, "1"
}