Persistent list

Andrei Alexandrescu via Digitalmars-d digitalmars-d at
Sat Nov 14 14:37:41 PST 2015

On 11/14/2015 05:04 PM, Timon Gehr wrote:
> We have had the discussion you are asking for before, and you have
> decided to ignore it, with the justification that this was how you
> decided and a vague appeal to emotion. I usually don't operate under
> the assumption that identical experiments lead to different outcomes
> without a good reason.

Yah, of course I remember that discussion. Thing is I remained
unconvinced after said discussion.

There's no technical argument being made here. It's a judgment call.

Clearly allowing List!int with the same codebase as is possible, and it would allow a
number of additional uses at the cost of somewhat murky semantics (that
list is not a value and not a reference; defining other containers with
similar semantics is tricky).

But there's a distinct possibility: List!(immutable int) is
indistinguishable from a value type - each value is independent from all
others, up to taking address of elements in the list.

So what we could do is define List!int to implement value semantics with
a completely different implementation - COW. That's a very appealing
equation for the user: "List!T is always a value regardless of T."
Optimizations apply depending on T, but that's transparent and the user
doesn't need to care. A List!T is a value. Boom. Done.

Per what you propose, we'd have "List!T is a Lisp-style list. It has
cons cells and atoms, and cons(head, list) creates a new list that
shares its tail with list." etc.

You seem to assert there's no contest, and anyone choosing (1) over (2)
is seriously incompetent. I seem to think there is a real choice there,
and I want to keep it open for the time being. Is this reasonable?

> Maybe not, but what is? It can't be laying down a well-reasoned
> argument, because as recent history shows it will just be ignored
> without a reasonable justification.
> I guess one other good way to proceed would be to just not have the
> dialog for now and instead wait until people who are actually trying
> to use the containers start complaining in blog posts and on reddit.
> Feel free to voice any better suggestions you may have.

One interesting problem Walter and I have as leaders of this community 
is to attract within the core circle people who are literally better 
than ourselves. People whom we can trust with using good judgment in 
complex situation and can lead themselves entire features and parts of 
the language (such as compiler-supported RC etc).

Amaury and you (Timon) may as well be the smartest people hanging out in 
this forum. You both are also very generous with your time. I can't talk 
for Walter, but for all I can tell you two are better than I'll ever be 
at all metrics that matter to language design. The challenge, then, 
becomes in convincing you to put that good expertise and good will 
toward something positive; because, to be blunt, a large chunk of your 
and Amaury's energy is dedicated to picking fights and proving just what 
chowderheads Walter and I are for not doing as you say.

I'm very happy to admit - you're a whole lot better than myself. So 
let's do something constructive.


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