On Sunday, 25 October 2015 at 05:37:02 UTC, deadalnix wrote:
On Saturday, 24 October 2015 at 19:33:03 UTC, David Nadlinger
wrote:
On Friday, 23 October 2015 at 17:44:55 UTC, deadalnix wrote:
Collection!T and Collection!const(T) are 2 completely
different types.
Isn't this also required anyway because of covariance vs.
contravariance considerations?
— David
It is close, but not exactly the same.
Covariance/contravariance can be emutalted via alias this
without too much trouble for a container (however, it is hard
to ensure correctness, but I'd assume not too hard). On the
other hand, the qualifier thing turtle from the collection to
the element in the collection, which is not that easy to
achieve.
The bigger problem is probably ranges rather than containers. We
get tail-const slices from arrays all the time, but opSlice with
no arguments isn't really a range operation (none of the range
traits require it), and it's pretty painful to get it to make
tail-const work with opSlice anyway (though I know that the EMSI
guys were jumping through all kinds of hoops to make it work, so
their containers make actually handle it reasonably well). Plus
there's the fun problem of how declaring opSlice on a range
causes foreach to call it, which essentially means that your
range can get saved accidentally, which can be problematic
(especially for ranges like RefRange).
It's been a while since I sat down and worked through the various
issues that we have here. I should probably do that soon and see
if I can distill them down well enough to come up with a DIP to
resolve them - or at least to clearly present them so that they
can be discussed. What we have right now mostly works, but it
falls apart in some corner cases - especially if you want to be
able to operate on a range like you would an array.
- Jonathan M Davis
