Why D const is annoying

[Chris Cain]

On Wednesday, 2 May 2012 at 03:22:40 UTC, Mehrdad wrote:
> Yes, that's **exactly** why I've said 'const is broken' for at 
> least a year now, but it seemed like people objected to my 
> conclusion...
>
> The trouble, as I just showed, is that it isn't.
> "Lazy-loading" and caching aren't exactly obscure or 
> rarely-used concepts --  which seems to be the way they are 
> portrayed here. They are *bound* to be used in any nontrivial 
> program.
> So, if D doesn't let you use those techniques without 'hacking 
> around' the language, then it doesn't matter how awesome D is 
> otherwise -- C++ programmers _simply won't_ switch to D, and 
> other programmers will definitely have trouble using const, 
> immutable, shared, etc...

I've never commented on this issue, but here's my viewpoint:

If you intend to change something, it's NOT const. Don't make an 
object and say you won't change it then change it. It's 
nonsensical. 9 times out of 10, what you really mean is "I have 
an object that has some immutable/const data in it".

A clear way to understand what I mean is a Matrix. Some people 
want "immutable matrices" which cache their determinant when it's 
time to calculate it. What they REALLY want is a matrix of 
immutable data. So, instead of a "immutable Matrix!int", they 
really need to be "Matrix!immutable(int)". Effectively it's the 
same thing, only it's more precisely defined and it's actually 
possible to do caching with it.

The way C++ works is simply incorrect, and yes it might take some 
time to get used to the fact that const means const and immutable 
means immutable, but if you don't want to respect those types in 
the first place, there's no sense in using them.