Run-time Indexing of a Compile-Time Tuple

[John Colvin via Digitalmars-d-learn]

On Tuesday, 12 May 2015 at 09:26:07 UTC, John Colvin wrote:
> On Monday, 11 May 2015 at 22:46:00 UTC, Per Nordlöw wrote:
>> The pattern
>>
>>            final switch (_index)
>>            {
>>                import std.range: empty, front;
>>                foreach (i, R; Rs)
>>                {
>>                    case i:
>>                        assert(!source[i].empty);
>>                        return source[i].front;
>>                }
>>            }
>>
>> occurring in roundRobin() and now also in my merge at
>>
>> https://github.com/nordlow/justd/blob/master/range_ex.d#L604
>>
>> is nor pretty nor concise.
>>
>> Is there a better way of indexing a compile time tuple with a 
>> run-time index?
>>
>> It may have to work together with CommonType somehow as is 
>> shown in my implementation of merge().
>
> I wrote that code in roundRobin to replace a nightmare string 
> mixin. I can't see any way of getting around it, as there is no 
> meaningful CommonType for a tuple of arbitrary ranges. The body 
> of each case statement needs to know the index at compile-time.

Correction: there are ways around it, but none of them are
improvements.