Tuple enhancement

[Steven Schveighoffer via Digitalmars-d]

On 10/16/16 9:58 AM, Andrei Alexandrescu wrote:
> I was thinking it would be handy if tuples had a way to access a field
> by name at runtime. E.g.:
>
> Tuple!(int, "a", double, "b") t;
> string x = condition ? "a" : "b";
> double v = t.get!string(x, 3.14);
>
> The get method takes the field name and the type of the presumed field,
> and it returns the value of the field in the tuple. If no field by that
> name and type, return the second argument.
>
> Rquirements:
>
> * Do not throw - allow the second argument to be a throwing delegate
>
> * Do not add overhead if the method is never used
>
> * Figure out a reasonable (but not all too complicated) way to deal with
> implicit conversions, e.g. if x == "a" in the the code above, should it
> return 3.14 or convert the int to double?
>
> * Handle qualifiers appropriately

Why not something like this:

Val get(T, Val)(auto ref T item, string memberName, Val defaultValue)
{
    switch(memberName)
    {
    foreach(n; __traits(allMembers, T))
    {
       static if(is(typeof(__traits(getMember, item, n)) : Val))
         case n: mixin("return item." ~ n ~ ";");
    }
    default:
      return defaultValue;
    }
}

1. It's opt-in, no code is added to the binary if you don't use it.
2. works for any type, not just Tuple.

Note: no testing, this needs probably some work to get it reasonable, 
e.g. need to filter out private members. Maybe Jack Stouffer's PR is a 
better start.

-Steve