[Yaml-core] Mutability

[Oren Ben-K. <or...@ri...>]

I just spent a few hours chasing my tail wrt. the model section, node
identity, and so on. Ouch. BTW, this has made me appreciate Clark's
formulation of the model section even more. This is *hard*.

Anyway, after several unsuccessful trials to put things down on paper so
that they made sense, I came upon the following insight.

The whole meanigful/meaningless alias issue doesn't hang around the matter
of node identity... or rather, the right way to look at it is one level
deeper. The real issue is about node *mutability*.

Obviously in a read-only graph there's no reason to distinguish between
identical and merely equal nodes. And, even in a mutable data structure, the
same holds for type families describing immutable values.

Take integers for example. You can't *change* the value 7; you can just
create a new value (say, 6) and stuff that into your variable instead.
Likewise strings in Perl/Java/Python. You can't change a string (in place).
You can just create a new one and stuff that into your variable instead.

In contrast, if you have an array, you can change the value stored in its
first element. Hence keeping track of identical vs. merely equal arrays is
vital.

So, what I'm going to do is erase a lot of messy wording and replace it with
the following type family property (I'll word it better, promise :-):

mutable: yes/no

- For immutable type families, it is OK to wreak havoc with the alias
structure (make all equal nodes identical, convert identical nodes into
separate equal copies, etc.) without changing the semantics of the native
model (though some representations are obviously more memory efficient than
others). 

- For mutable type families, alias structure is sacred, and must always be
preserved.

- Most collections are mutable and most scalars aren't.

- When in doubt (unknown type family), treat it as if it is mutable, just to
be on the safe side, even if it is a scalar.

Thoughts?

And, BTW, great YAML meeting. I really wish I could have been there.

Have fun,

    Oren Ben-Kiki