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

[Chris]

On Thursday, 24 October 2013 at 01:48:32 UTC, Jonathan M Davis 
wrote:
> On Tuesday, October 22, 2013 23:40:00 Timothee Cour wrote:
>> Following the recent thread "front evaluated multiple time 
>> with joiner
>> depending on where extra arg given", I'd like to propose the 
>> following
>> addition in std.range :
>> 
>> The goal is to ensure that a given range's 'front' method is 
>> called only
>> once per element, allowing one to handle safely side effects 
>> in 'front'
>> methods
>
> Considering that front really isn't supposed to have side 
> effects, I'm inclined
> to think that putting something like this in the standard 
> library is a bad
> idea. It just encourages bad code. There's certainly no reason 
> why you can't
> implement it for your own stuff if you want to, but I don't 
> think that it's
> something that should be encouraged as it goes against the 
> range paradigm. The
> getter property for front really should be logically const and 
> logically pure,
> and a front with side effects should be something reserved for 
> debugging.
>
> - Jonathan M Davis

How do you define side effects? After all ranges manipulate, 
reformat or restructure data.