The Status of Const

[Justin Johansson]

dsimcha wrote:
> This is from a discussion that originated on the Phobos mailing list, but I
> thought I'd bring up the question of what should be done about const on the
> newsgroup to see what others think:
> 
> Despite its theoretical beauty, I find D's const/immutable system to be
> utterly useless for all but the simplest use cases.  I made a serious attempt
> a while back to use it in a real multithreaded program.  In hindsight it was
> more trouble than it was worth, largely for three reasons:
> 
> 1.   It's difficult to create non-trivial immutable data structures, and often
> impossible without relying on either unchecked casts or unnecessary copying.
> 
> 2.  Much generic code in Phobos (even things as simple as std.math.pow()
> before I recently fixed it) behaves incorrectly when given const/immutable
> data.  This also applies to other libraries I use, including ones that I'm the
> main author of, so I'm just as guilty of it as anyone.  Given that noone,
> including me, seems to be able to get const to interact well with generic
> code, perhaps we need a language-level solution.
> 
> 3.  inout is currently so bug-ridden it's not even funny.  (Though this is
> clearly fixable long-term, once we get higher priority stuff off our plates.)
> 
> It would have probably been better if this was brought to a head sooner, but
> it's better late than never.  Do others agree that D's const system is
> difficult to impossible to use properly?  Has anyone successfully used D's
> const system in a non-trivial setting despite these limitations?  If so, was
> it more trouble than it was worth in hindsight?  How can these limitations be
> worked around and/or fixed?

Unfortunately, from my experience in a non-trivial setting, the
work-around seems to be not to use const/immutable.  It is very
very br o  k   e     n.  As the const/immutable implementation
currently stands, it definitely not worth the trouble.  You will
have weeks and weeks of pain only to have to just about throw
your work out and start over.

Sad but true.  Your sentiments are mine also.

Justin Johansson