1. Design of the Parrot Virtual Machine The OO-ish bits
Dan Sugalski

2. Important notes The OO parts of the design aren’t complete
This is not a pure OO system
I am not an OO guy
We care about speed
We don’t care about theoretical purity
July 22, 2019
Design of the Parrot VM

3. Quick parrot design recap
Register based
Bytecode driven
Mostly language neutral
July 22, 2019
Design of the Parrot VM

4. Multiple language support
Perl 6
Perl 5
Ruby
Python
JVM
.NET
Z-Machine
July 22, 2019
Design of the Parrot VM

5. Challenges of the design
Several conflicting object systems
Runtime instantiation of much of the system
Dynamic redefinition of classes, methods, and types of objects
Dynamic redefinition of inheritance hierarchies
Object system differences must be invisible to user code
July 22, 2019
Design of the Parrot VM

6. Programmer view of objects
Programmer shouldn’t care about the source of an object
Programmer shouldn’t have to make any allowances for the source of an object when calling its methods
It all ought to just work
July 22, 2019
Design of the Parrot VM

7. Required end-user functionality
Check for
Class parentage
Method existence
Call method
Get property
Set property
July 22, 2019
Design of the Parrot VM

8. Implementation of objects
Objects are an opaque and fundamental variable type
All variables have a core set of vtable functions
Call method and the other minimal functionality is shared across all variables
Since it all looks the same at this level, usage is consistent regardless of object type
July 22, 2019
Design of the Parrot VM

9. Usage example my $foo; $foo = Some::Class.new();
$foo.a_method(12, 15);
July 22, 2019
Design of the Parrot VM

10. Advantages of this scheme
Programs are isolated from the implementation details
Doesn’t place many real restrictions on the implementation of objects
Offers class authors plenty of flexibility
July 22, 2019
Design of the Parrot VM

11. Disadvantages of this scheme
Programs are isolated from the implementation details
Doesn’t place many real restrictions on the implementation of objects
Offers class authors plenty of flexibility
July 22, 2019
Design of the Parrot VM

12. Parrot’s object model requirements
Support at least two wildly different implementation models
Be more efficient than perl 5’s object system
July 22, 2019
Design of the Parrot VM

13. Required functionality
Untyped variables
Runtime class changes on objects
Runtime class mutation
Runtime inheritance hierarchy changes
Multiple inheritance
Signature-based dispatch
Cross-class-scheme inheritance
July 22, 2019
Design of the Parrot VM

14. Requirements for classes
More stringent
Need:
List of methods in class
List of immediate parent classes
Parent class method search list
Duplication capabilities
Parent class method search scheme
Redispatch scheme
Attribute count
Duplication capabilities mean whether the class can be really in the class hierarchy twice, or if multiple occurrences are thrown away so only one is in there
July 22, 2019
Design of the Parrot VM

15. Easy Answer Do all inheritance with delegation At least conceptually
Does allow for inheritance from disparate object systems
Performance is potentially an issue
Shortcuts are definitely in order
July 22, 2019
Design of the Parrot VM

16. Calling convention provides
Real self
Current self
Class offset
Attribute start offset
July 22, 2019
Design of the Parrot VM