Permissions
Dada hopefully feels familiar to you thus far, but if you played a lot with the programs, you may have noticed some errors you didn't expect. Consider this program...what do you expect it to print? Take a guess, and then hit the "Run" button to see what happens...
Surprise! It gets an error! What is going on? The answer lies in the key Dada concept of permissions.
What is a permission?
In Dada, variables don't just store a reference to an object, like they do in Python or Java. Instead, they store a reference to an object with some permission. These permissions determine whether you can read or write to the object.
Permissions in Dada can be divided across two axes. We'll cover those two axes separately:
- Read vs write -- covered now!
- Owned vs leased -- covered later, in the chapters on ownership
Read permission is the default
When you write something like let q = p in Dada, the default is that you get a leased, read permission. Leasing will be covered in more detail later, but for now it suffices to say that the permission for q is tied to the permission from p; when p goes out of scope, for example, then q's permission will also be canceled.
As the name suggests, read permissions can only be used to read fields. This is why we get an error!
Dada comes equipped with a visual debugger that lets you visualize permissions. To see how it works, try hitting the "Debug" button and then position your cursor write after the line for let q = p:
Requesting write permission
You can explicitly request write permission by using the lease keyword, like p.lease. If you use the debugger and position it after let q = p.lease, you will see that q is given write permission this time. As a result, q.x = 23 succeeds and, when we print the variable p, we see the new value.