Uniform and Safe Metaclass Composition. Stéphane Ducasse, Nathanael Schaerli and Roel Wuyts (ESUG 2004, Koethen)

1. S. Ducasse 1
Uniform and Safe Metaclass
Composition
Stéphane Ducasse, Nathanael Schaerli and Roel
Wuyts
ducasse@iam.unibe.ch
http://www.iam.unibe.ch/~ducasse/

2. S. Ducasse 2
Why Metaclasses?
• Classes are objects too
• A class is an instance of a class called a
metaclass
• Uniformity: Classes are just objects
• Control: Instance creation behavior
• Reuse of class behavior

3. S. Ducasse 3
Implicit vs. Explicit
• Implicit: programmer cannot specify the class
• Safe but
• Limited or no reuse
• Explicit:
• Composition metaclass is not safe
• Ad-hoc composition mechanisms (for example:
strings manipulation of neoclasstalk and dynamic
class change)
• Safe?: upward and downward compatible

4. S. Ducasse 4
Upward Compatible
• aB i-foo => no error
Meta A
c-bar
A
i-foo
A>>i-foo
^ self class c-bar
Meta B
B
inherits from
instance of
?

5. S. Ducasse 5
Downward Compatible
• B c-bar => no error
Meta A
c-bar
A
i-bar
Meta A>>c-bar
^ self new i-bar
Meta B
B
inherits from
instance of
?

6. S. Ducasse 6
Smalltalk-80
• Implicit
• Safe
• Limited reuse
Meta A>>c-foo
^ self new i-bar Meta A
c-bar
c-foo
A
i-foo
i-bar
Meta B
B
A>>i-foo
^ self class c-bar
inherits from
instance of

7. S. Ducasse 7
Existing Solutions
• Smalltalk-80
• Implicit, safe
• But limited reuse
• Clos
• Explicit but help yourself!
• NeoClasstalk
• Dynamic class changes
• Metaclass behavior specified as strings
• MetaClasstalk
• Double meta-layer with mixin composition

8. S. Ducasse 8
Our approach: traits
• Implicit metaclasses
• But traits to compose classes
• Reuse of traits
A B
inherits from
instance of
Meta A
Class
Property2
Meta B
Class
Property1

9. S. Ducasse 9
What are Traits?
• Traits are parameterized behaviors
• Traits provide a set of methods
• Traits require a set of methods
• Traits are purely behavioral
• Traits do not specify any state
radius
radius:
center
center:
TCircle
area
bounds
diameter
hash

10. S. Ducasse 10
How are Traits Used?
• Traits are the behavioral building blocks of
classes
• Class = Superclass + state + traits + glue
methods
• Class methods take precedence over trait
methods
Object
ColoredCircle
TColor
TCircle

11. S. Ducasse 11
Conflicts Explicitly Resolved
I) Override the conflict with a glue method
 Aliases provide access to the conflicting methods
II) Avoid the conflict
 Exclude the conflicting method from one trait
ColoredCircle
TColor
hash
TCircle
hash
hash
hash -> circleHash
hash -> colorHash

12. S. Ducasse 12
Hierarchies Revisited...
Boolean
False
True
False class
True class
TSingleton
Boolean class
TAbstract
TSingleton
Behavior
TCreator
new
basicNew
Boolean class
TAbstract
new
False class
TSingleton
new
default
reset
basicNew
uniqueInstance
uniqueInstance:

13. S. Ducasse 13
Metalevel Reengineering
TRememberInstances
new
rememberInstance
instances
reset
rememberedInstances
rememberedInstances:
TInstantiator
new
TAllocate
basicNew
TInitInstantiator
new initialize
TAbstract
new
TFinal
subclass:
TSharedInstance
reset
sharedInstance
directSharedInstance
directSharedInstance:
TDefault
TSingleton
new
sharedNew ! new
default !
sharedInstance uniqueInstance !
sharedInstance
nonRememberingNew ! new
is composed from
TSingleton
new
Trait
m1 ! m2
creates an alias
m2 referring to m1
Legend

14. S. Ducasse 14
Traits Implementation
• Available in Squeak 3.7
• Bootstrapping the kernel
PureBehavior
BehaviorTraitBehavior
Class
Description
Class Metaclass
Trait
Description
Trait

15. S. Ducasse 15
Conclusion
• Uniform
• Traits are instance and class level!
• Explicit Composition
• Reuse but Safe