On 10/30/2011 01:53 PM, David Rajchenbach-Teller wrote:
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Dear Rusties (?),
I am in the process of designing a small rope library for Rust and I
have a few issues wrt `str`. I am a newbie at Rust, so I may be wrong,
but it is my understanding that `str` supports a form of mutability
through an in-place append operation `+=`.
The mutability story for strings continues to make me uncomfortable. We
tend to assert that strings are immutable, but they often behave as
though they are mutable and they are implemented as mutable.
The reason we can consider them immutable (I think) is that they have
unique ownership, so no two parties can have a view of the same string
and see it mutate. When we mutate a string in Rust, what is actually
happening is that the 'slot' containing the string changes to hold a
(conceptually) different string. So when you do
let a = "foo";
a += "bar";
you aren't (conceptually) mutating the "foo" string to be "foobar", you
are mutating the slot named 'a' to hold the string "foobar" instead of
the string "foo". Because we have unique semantics for strings we
actually just go ahead and mutate "foo" into "foobar". The net effect is
that strings behave like scalar value types - either mutable or
immutable depending on the nature of the slot they reside in. Local
strings are mutable and strings inside of aggregates are only mutable
when preceded with the 'mutable' keyword. What tends to happen is two
parties will have a reference to the same 'slot', one will change the
string in the slot and then the other party is looking at a different
string.
So if you want to have a bunch of ropes looking at the same immutable
string you need to copy it into a shared, immutable box. In an immutable
box, you won't be able to perform append operations on it.
Someone please correct me. Obviously I spend a lot of time being
confused about the situation.
Regards,
Brian
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