Permissions
In the previous chapter, we saw keywords like my and our attached to variables. These are examples of permissions -- permissions are a key part of Dada. Like Rust, Dada leverages permissions to avoid the need for a garbage collector, while retaining memory safety.
Running example
As we explain permissions both here and over the next several chapters, we're going to work with variations on this example program. It builds on syntax that we introduced previously, so if anything is confusing you may want to check out that chapter.
class Point(our x, our y)
async fn main() {
my p = Point(22, 44)
print("The point is ({p.x}, {p.y})").await
}
my, our, and other permissions
This example already makes use of two permissions, my and our. There are four permissions in total, and they can be divided along two dimensions:
| Unique | Shared | |
|---|---|---|
| Owned | my | our |
| Leased | leased | shleased |
The difference between these dimensions:
- Owned permissions are permanent. They cannot be revoked through access to other variables.
- Leased permissions are temporary -- there is always a lessor (either an owner or another lease), and that lessor can reclaim full access to their object.
- Unique permissions are exclusive. When one variable has unique permission, no other variables can access the object while that variable is in use (without some kind of error occurring).
- Shared permissions can be copied freely, but they require that the object is read-only. In other words, while you are reading from an object with a shared permission, nobody else can come and change it under your feet (except via an atomic field).
Overview of what's to come
Over the next few chapters, we're going to look at each of the permissions in detail: