compile-time opIndex

[Dicebot via Digitalmars-d]

On Thursday, 18 December 2014 at 16:15:09 UTC, Steven 
Schveighoffer wrote:
> On 12/18/14 10:44 AM, Dicebot wrote:
>> Have you seen my http://wiki.dlang.org/DIP63 ?
>
> I admit this is somewhat over my head, but it seems to be more 
> focused on types.

In D there is no special built-in value tuple concept. It is all 
same thing but emulating this behavior for symbols and types is 
much more difficult compared to values. This is the reason for 
such focus - rest is not really a problem.

> I will note that your example:
>
> struct Pack(T...)
> {
>    alias expand = T;
>    alias expand this;
> }
>
> I don't think this would work, since you can't alias a value to 
> a type?

Try this and be surprised :) `alias this` affects any symbol 
lookup and this pattern results in pretty interesting behavior. 
But it is still not good enough as described in the DIP.

> But I don't use tuples regularly, so I am not sure where 
> exactly expand is used. I thought it was to convert a concrete 
> struct into a tuple of values, not a type tuple.

You can ignore expansion for now, it was motivating use case but 
DIP itself defines tools of defining user types that act like 
built-in tuples - which fits the theme of your original proposal. 
It also briefly speculates why adding such features to structs 
does not seem a good idea.

Also there is no difference between tuple of values and type 
tuple, it is same entity.

> I don't disagree with the idea of having operators inside 
> templates, I just don't know if it solves the problem I was 
> looking at.

Your original snippet seems to implementable within proposed DIP 
if you replace struct with raw template. You will lose struct 
properties like having actual instance but those don't exist for 
built-in tuples either.