12. Traits and Classboxes (and Changeboxes)

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.2 Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research Selected material courtesy of Nathanael Schaerli and Alexandre Bergel

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.3 Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.4 How Can We Implement This? Circle Visual BorderedCircleBorderedRectangle Rectangle Bordered CircleRectangle Bordered

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.5 Alternative Approaches  Single inheritance leads to code duplication  Alternatives: —Multiple inheritance —Mixins

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.6 Problems with Multiple Inheritance  Multiple Inheritance problems —No access to super (ill-defined) – Workarounds are clumsy or lead to duplicated code —Complex rules for resolving conflicts (esp. state) —Fragile hierarchy – Changes may impact distant classes Ducasse, et al., "Traits: A Mechanism for fine-grained Reuse", TOPLAS 2006

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.7 Problems with Mixins  Mixin problems —Dispersal of glue code – Conflict resolution is sensitive to mixin composition order – Composing entity has no control! —Fragile hierarchy – Changes may impact distant classes Ducasse, et al., "Traits: A Mechanism for fine-grained Reuse", TOPLAS 2006

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.8 Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.9 Traits Circle Rectangle Polygon Colored Bordered Traits = Building Blocks Circle RectanglePolygon CircleColoredBordered RectangleColored + + +

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Traits in a nutshell  Traits are parameterized behaviors —Traits provide a set of methods ( ) —Traits require a set of methods ( ) —Traits do not specify any state TCircle area bounds circumference diameter hash radius radius: center center:

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Using Traits to Compose Classes ColoredCircle Visual TCircle TColor Class = Superclass + State + Traits + Glue methods Class = Class = Superclass Class = Superclass + State Class = Superclass + State + Traits rgbradiuscenter The composing class retains control of the composition.

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Composition Rules 1. Class methods take precedence over trait methods 2. Composition is commutative —Conflict resolution is always explicit 3. Flattening Property —Trait methods have no special semantics

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Rule 1: Precedence of Class Methods ColoredCircle TCircle area hash... radius center... TColor invert red... rgb... rgbradiuscenter radius center... rgb... area hash... radius center... rgb... area hash... radius center rgb hash ^ self radius hash bitXor: self center hash

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Composition Rules 1. Class methods take precedence over trait methods 2. Composition is commutative —Conflict resolution is always explicit 3. Flattening Property —Trait methods have no special semantics

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Rule 2: Explicit Conflict Resolution ColoredCircle TCircle hash... TColor hash... 1) Override the conflict with a glue method —Aliases provide access to conflicting methods 2) Exclude conflicting methods hash hash -> circleHash hash -> colorHash

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Composition Rules 1. Class methods take precedence over trait methods 2. Composition is commutative —Conflict resolution is always explicit 3. Flattening Property —Trait methods have no special semantics

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Rule 3: Flattening Property Circle radius center TCircle radius center... radiuscenter Visual hash area ^ self radius squared area diameter hash ^ super hash bitXor: self radius hash diameter ^ self radius * 2 area ^ self radius squared diameter ^ self radius * 2

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Problems with the Smalltalk Collections Hierarchy  The ST collection hierarchy suffers from: —Duplicated code —Methods implemented too high in the hierarchy —Inappropriate inheritance —Incomplete protocols

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Methods implemented “too high” Collection Sequenceable- Collection Array HeapSet add: addAll: remove: removeAll: add: addAll: remove: removeAll: add: addAll: remove: removeAll: add: remove: Array add: anObject self shouldNotImplement remove: anObject self shouldNotImplement add: addAll: remove: removeAll: addAll: aCollection aCollection do: [:each | self add: each].

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Inappropriate Inheritance Set size union: remove: removeAll:... Dictionary remove: removeAll:... size union:... remove: anObject self shouldNotImplement removeAll: anObject self shouldNotImplement size union: remove: removeAll:...  Hard to see the real interface of a class  Unexpected runtime errors

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Classifying the Collection Classes  Many different classification criteria —Order – unordered ( Set ) – explicitly ordered ( LinkedList ) – implicitly ordered ( Heap ) —Extensibility – fixed size ( Array ) – variable size ( OrderedCollection ) —Mutability – immutable ( String) – mutable (the others) —Comparison – using “identity” ( IdentitySet ) – using “equals” ( Set )

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses ImplicitlyOrdered Linked Extensible ExplicitlyOrdered ArrayBased Refactoring Strategy  Create traits for the identified properties  Combine them to build the collection classes Extensible ExplicitlyOrdered ArrayBased Collection Functional properties Implementation properties Extensible ExplicitlyOrdered ArrayBased

Facilities supported by all collection classes Concrete classes adding implementation traits Common combinations of functional traits

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses What Did We Gain?  Consistent hierarchies —No inappropriate inheritance —No methods implemented too high (before ~10%) —Protocols are uniform and complete  Less code —Refactored 38 classes with 1252 methods —Created a total of 67 traits —After the refactoring ~ 10% less source code > 20% fewer methods  Improved Reusability —“Toolbox of traits” makes it easy to build new classes Black, et al., “Applying Traits to the Smalltalk Collection Hierarchy,” OOPSLA 2003

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Traits in Squeak  Language Extension —Extended the language kernel to represent traits —Modified the compilation process for classes built from traits  No changes to the Squeak VM —Essentially no runtime performance penalty —Except indirect instance variable access —But: This is common practice anyway  No duplication of source code  Small amount of byte-code duplication —Only methods with super-sends

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Traits in Squeak 3.9 Object subclass: #Behavior uses: { #basicAddTraitSelector:withMethod: -> #addTraitSelector:withMethod: } instanceVariableNames: 'superclass methodDict format traitComposition localSelectors' classVariableNames: 'ObsoleteSubclasses' poolDictionaries: '' category: 'Kernel-Classes'

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Practical Impact of Traits  Perl —Traits were ported to Perl 5 (by Stevan Little) —Traits are planned as a standard feature of Perl 6  VisualWorks Smalltalk —Traits were ported to VW (by Terri Raymond)  Scala (developed at EPFL) —Traits are a built-in language feature of Scala —Fully integrated into the static type system .NET —Microsoft Research Project to bring traits to C#

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Class Extension  A Class Extension is the ability to add or redefine methods on a class after its creation —Solves a different problem than subclassing – Clients of existing classes see the extensions —Common in dynamic languages like CLOS and Smalltalk

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Visitor example BlackPackage Opr AddMult acceptVisitor() BluePackage Visitor doAdd() doMult()

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Subclassing does not Solve it Opr Add Mult Parser parse() Add.new() … … Subclasses are not referenced by the tree constructor Add’Mult’ acceptVisitor()

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses In Smalltalk  Class extensions are global —Any application can modify any class in the system Consequences: —Conflicts may arise – e.g., two applications may add the same method —System fragility – e.g., an application may redefine a critical method Problems: —How to control class extensions? —How to apply a set of unanticipated changes without breaking existing clients?

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Classboxes  A Classbox —Is a unit of scoping – I.e., a namespace —Can define classes —Can import class definitions from other classboxes —Can define class extensions – I.e., methods on classes in scope  Definitions are local to the classbox that introduces them —Changes only impact the classbox itself and classboxes that explicitly import its definitions

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Classboxes: Avoiding Conflict Since class extensions local to a classbox, conflicts are avoided TextClassbox String import WWWClassbox WWWURL String asUrl WWWURL.new() … URLClassbox URL String asUrl URL.new() …

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Classboxes: Import and Visibility Extensions can be imported by other classboxes TextClassbox String import URLClassbox URL String asUrl RedClassbox … url = “

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Classbox as a Sandbox CollectionClassbox OrderedCollection import do() A classbox establishes a purely local scope for extensions, so they cannot break applications that do not import the extensions. C foo() aCollection.do(…) C.new().foo() GreenClassbox OrderedCollection do()

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Local consistency Any expression executed in a classbox will be evaluated from the perspective of that classbox alone. CollectionClassbox OrderedCollection import do() copy() OrderedCollection do() C 3 1 aCollection.copy(…) bar() GreenClassbox 2 self.do(element.copy()) C.new().bar()

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses From within the Green Classbox aCollection.copy(…) C.new().bar() self.do(element.copy()) OrderedCollection do() C 1 3 bar() GreenClassbox 2 copy()

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Implementation  Implemented in Squeak, an open-source Smalltalk —New method lookup to take classboxes into account – Originating classbox – Collection of all the traversed classboxes – Import before inheritance —Intensive use of reflection: – Reification of the method calls stack – Message passing control

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Traits + Classboxes  Collaborating traits should be applied to a set of classes —Use classboxes to encapsulate and apply Bergel and Ducasse, “Supporting Unanticipated Changes with Traits and Classboxes,” NODE 2005

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Cross-Cutting Collaborations  Used to describe cross-cutting features by layering an application into collaborations [Holland, ECOOP 92]. —A role is a set of features intended to be applied to a class. —A collaboration is a set of roles intended to be applied to another collaboration.  With Mixin-layers [Batory, ECOOP 98]: —a role is a mixin —a collaboration is a parametrized class  With Traits and Classboxes —A role is encapsulated as a trait —A collaboration is encapsulated as a classbox

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Example: Graph Application using Inheritance UndirectedGraph GraphVertex CycleChecking GraphCycleVertexCycleWorkspace ConnectedRegion GraphRegionsVertexRegionsWorkRegions

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses UndirectedGraph GraphVertex TUGraph TVertexAdj The Graph Application using Traits/Classboxes CycleChecking GraphVertex Workspace GraphDFT VertexDFT WNumber ConnectedRegion GraphVertexWorkspace WRegionsGraRegionsVerRegions

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Gain with Traits and Classboxes  No subclasses —Identity of participant preserved.  A collaboration can be used to defined unanticipated change —Scope of change is limited to extending classbox and clients  Bounded visibility of extensions —Multiple versions of classes can be simultaneously active in the system

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Roadmap  Problems with Inheritance  Traits  Refactoring Collections with Traits  Traits in Squeak  Classboxes  Traits and Classboxes  Ongoing Research

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Ongoing research  Scoped reflection —Restricting both availability and effect of reflection to given scope  Changeboxes —Encapsulating changes, not just extensions —For both forward- and reverse-engineering  Context-oriented programming —Software systems whose behaviour is context-dependent

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Changeboxes  Encapsulate change —Capture changes to methods, state, class hierarchy – Express semantics of refactoring and other changes  Delimit the scope of change —Restrict scope of changes to specific clients – Delay global impact  Enable changes depending on context —Dynamically extend the scope of changes – Manage system evolution  Dispatch on dynamic context —Dynamically apply or disable changes – Temporally or contextually scope changes

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses 12.51

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses What you should know!  Why does single inheritance lead to duplicated code?  How does the composing class retain control of trait composition?  What do “glue” methods do for traits?  What is the “flattening property” and why is it important for traits?  Why is there “inappropriate inheritance” in the Smalltalk Collections hierarchy?  What is a “class extension”?  In what way to classboxes ensure locality of changes?  What problems are solved by combined traits and classboxes?

© Oscar Nierstrasz ST — Understanding Classes and Metaclasses Can you answer these questions?  Why do multiple inheritance and mixins leads to “fragile class hierarchies”?  C++, Eiffel and Python all offer multiple inheritance – are they broken?  Why don’t traits specify any state?  How much code is duplicated in the standard Java libraries?  What problems occur in Java due to the lack of class extensions?  Can classboxes be “flattened” in the same way that traits can? Why or why not?

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