std.range.cacheFront proposal&working code: wraps a range to enforce front is called only once

[Jonathan M Davis]

On Thursday, October 24, 2013 21:10:58 deadalnix wrote:
> On Thursday, 24 October 2013 at 17:57:54 UTC, Jonathan M Davis
> 
> wrote:
> > Now, that being said, I'd argue that map should probably be
> > changed so that it
> > calls its function on popFront rather than front (though that
> > has the downside
> > of requiring that it then hold the current value, which it
> > doesn't currently
> > have to do), but regardless, you're using map for something
> > other than what it
> > was designed to do.
> 
> That would break random access.

No, it wouldn't. It would just require that the function be called directly in 
the case of random access, so you then have the problem with randomly accessed 
values that you really can't implement map in a way that avoids a calling its 
function every time you randomly access a variable. It's one more reason to 
consider not accessing the same element in a range multiple times, but as it's 
pretty much a guarantee that at least some range-based code will access front 
multiple times, it's not a good idea for the range to be designed such that it 
can't reasonably handle having each element accessed multiple times - which in 
the case of map would imply that the programmer using map should make sure 
that the function they give it can handle being called multiple times per 
element without changing the semantics of the code. It also would imply that 
doing something like

range.map!(a => new Aboject(a))

like you do in another post should be done with great care if at all, as there 
is a high risk that you will end up creating multiple objects per element as 
you traverse the range unless you specifically make sure that use foreach on it 
or immediately convert it to an array.

- Jonathan M Davis