Applying Traits to the Smalltalk Collection Classes
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1 Applying Traits to the Smalltalk Collection Classes Andrew P. Black Portland State University, Portland, Oregon Joint work with Nathanael Schärli, Stéphane Ducasse, Oscar Nierstrasz Software Composition Group, University of Bern, Switzerland
2 A Play in Two Acts Act I Act II Scene 1, in which we are introduced to the Smalltalk collection classes, and the problems of code reuse therein Scene 2: enter the trait, a solution to those problems Scene 1, in which traits are used to vanquish the wicked and set all right in the world 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 2
3 Act I Scene 1: The Smalltalk Collection Classes
4 The Smalltalk Collection Classes General-purpose library classes Blue Book (1983) classes include 19 subclasses of Collection 9 subclasses of Stream Squeak (2002) classes include 98 subclasses of Collection, of which 39 are Collections 39 subclasses of Stream, of which 10 are Collections Bitmap, FileStream, CompiledMethod etc. are applicationspecific subclasses, not part of the collections library 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 4
5 Protocols in the Collection Classes All collection objects support a uniform set of welldefined protocols. Testing protocol: isempty, includes:, occurencesof: Enumeration protocol: do:, select:, collect:, reject: Copying protocol: copy, copywith:, copywithout: Without uniform protocols: client code depends on the class of the collection 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 5
6 The Varieties of Collection Beyond this uniformity, there are many different kinds of collection Array (indexable, fixed size) OrderedCollection (indexable, extensible) Interval (indexable, immutable) SortedCollection, Heap (indexable, extensible, sorted) Set (unordered, no duplicates) Bag (unordered, duplicates allowed)... and more 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 6
7 Classifying the Collections The differences between the collections manifest themselves in several dimensions Order: unordered (Set), explicitly ordered (LinkedList), or implicitly ordered (SortedCollection) Extensibility: fixed size (Array) or variable size (Heap) Mutability: immutable (Symbol, Interval) or mutable Duplicates: allowed (Bag) or disallowed (Set) Comparison: using identity (IdentitySet) or using the object-level equality operator (Set) etc. 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 7
8 Implementation 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 8
9 Implementation The implementation of the Smalltalk collection hierarchy... has been improved over more than 20 years is often considered a paradigmatic example of objectoriented programming 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 8
10 Implementation The implementation of the Smalltalk collection hierarchy... has been improved over more than 20 years is often considered a paradigmatic example of objectoriented programming But: the implementation is still far from optimal! 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 8
11 Implementation The implementation of the Smalltalk collection hierarchy... has been improved over more than 20 years is often considered a paradigmatic example of objectoriented programming But: the implementation is still far from optimal! Single inheritance is not powerful enough to model a hierarchy of classes that can be categorized in so many dimensions 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 8
12 Classification: the Problem 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
13 Classification: the Problem Collection 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
14 Classification: the Problem Collection Order 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
15 Classification: the Problem Collection Sequenced Unsequenced Order 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
16 Classification: the Problem Collection Sequenced Unsequenced Order Extensibility 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
17 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Extensibility 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
18 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Extensibility 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
19 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Extensibility Array 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
20 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Extensibility Array Ordered- Collection 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
21 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Extensibility Array Ordered- Collection FixedSet 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
22 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Fixed Extensibility Array Ordered- Collection FixedSet 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
23 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Fixed Extensibility Array Ordered- Collection FixedSet Set 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
24 Classification: the Problem Collection Sequenced Unsequenced Order Fixed Variable Fixed Variable Extensibility Array Ordered- Collection FixedSet Set 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 9
25 Classification: the Problem Single inheritance is not up to the job 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 10
26 Consequences As a consequence, the Squeak collection hierarchy suffers from: Methods implemented too high Code duplication Unnecessary inheritance Deficiencies in Modeling 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 11
27 Methods implemented too high Collection Sequenceable- Collection Set add: addall: remove: removeall: Heap add: addall: remove: removeall: Array 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
28 Methods implemented too high Collection Sequenceable- Collection Set add: addall: remove: removeall: Heap add: addall: remove: removeall: Array 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
29 Methods implemented too high Collection Sequenceable- Collection Set Heap Array 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
30 Methods implemented too high Collection add: addall: remove: removeall: Sequenceable- Collection Set Heap Array 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
31 Methods implemented too high Collection add: addall: remove: removeall: Sequenceable- Collection Set Heap Array add: addall: remove: removeall: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
32 Methods implemented too high Collection add: addall: remove: removeall: Sequenceable- Collection add: anobject self shouldnotimplement Set Heap Array add: addall: remove: removeall: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
33 Methods implemented too high Collection add: addall: remove: removeall: addall: acollection acollection do: [ : each self add: each ] Sequenceable- Collection add: anobject self shouldnotimplement Set Heap Array add: addall: remove: removeall: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
34 Methods implemented too high Collection add: addall: remove: removeall: addall: acollection acollection do: [ : each self add: each ] Sequenceable- Collection add: anobject self shouldnotimplement Set Heap Array add: addall: remove: removeall: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
35 Methods implemented too high Collection add: addall: remove: removeall: Sequenceable- Collection add: anobject self shouldnotimplement Set Heap Array add: addall: remove: removeall: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 12
36 Methods implemented too high (2) In the Collection Classes, we found 131 methods (more than 10%) are implemented too high 106 methods (80%) are not explicitly cancelled Consequences Hard to see the real interface of a class Unexpected runtime errors when an inappropriate method is called 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 13
37 Code Duplication Alternative to implementing methods too high Often used when implementing methods too high is hard or impossible Set scanfor: Dictionary IdentityDictionary scanfor: IdentitySet scanfor: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 14
38 Unnecessary Inheritance Inheritance is often used when only a few of the superclass methods should be inherited Many methods are inherited unnecessarily Makes understanding a class much harder Instead of improving understandability, this form of inheritance hinders understanding of the code Example: Dictionary inherits from Set, but presents a very different interface Methods such as remove:, remove:ifabsent: need to be disabled 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 15
39 Deficiencies in Modeling For Sets and Dictionaries there are different classes that exhibit different ways of comparing elements Set (uses method =) IdentitySet (uses method ==) But: PluggableSet (uses pluggable comparsion block) Corresponding variants are not available for other classes such as Bag, Heap, etc. Implementing them is cumbersome because there is no way to factor-out and reuse pluggability 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 16
40 Missing Properties Other properties such as immutability are not captured in a general way Two ad-hoc implementations in Interval and Symbol The interfaces of some classes are missing methods that one would expect: String adds 142 methods to the protocol of its superclass (searching for substrings, regular expressions) Text, which represents a string with visual attributes, implements only 15 of them. The other 127 are missing! 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 17
41 But that s not all! Suppose that one would like to implement collection-like behavior outside of the collection hierarchy Path, a subclass of DisplayObject, contains an ordered sequence of points It would be useful to be able to do: and collect: over them How can we re-use code from the Collection Classes? We can copy and paste it! 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 18
42 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
43 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing SyncColoredCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
44 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing SyncColoredCircle BorderedColoredPolygon 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
45 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing SyncColoredCircle BorderedColoredPolygon SyncDrawing 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
46 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing SyncColoredCircle BorderedColoredPolygon SyncDrawing Colored 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
47 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing SyncColoredCircle SyncDrawing BorderedColoredPolygon Bordered Colored 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
48 Wouldn t it be nice if... Hierarchy of graphical objects GraphicalObject Generic properties Colored Circle Polygon Bordered SyncDrawing SyncColoredCircle SyncDrawing Colored BorderedColoredPolygon Bordered Colored 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 19
49 Existing reuse technologies are inadequate Single inheritance is not expressive enough Language designers proposed alternatives Multiple inheritance Mixin inheritance However, both of these approaches have drawbacks but that s another talk [Schärli et al, ECOOP 2003] 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 20
50 Act I Scene 2: Enter Traits which solve all of these problems, without the drawbacks
51 Goals for Traits Improve code reuse Maintain understandability No surprises Avoid fragile hierarchies 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 22
52 What are Traits? Traits are parameterized behaviors Traits provide a set of methods ( ) Traits require a set of methods ( ) Traits are purely behavioral just methods, no instance variables (no state) area bounds diameter hash... TCircle radius radius: center center: 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 23
53 How are Traits Used? 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
54 How are Traits Used? Traits are the behavioral building blocks for classes 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
55 How are Traits Used? Traits are the behavioral building blocks for classes Class = 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
56 How are Traits Used? Traits are the behavioral building blocks for classes Class = ColoredCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
57 How are Traits Used? Traits are the behavioral building blocks for classes Class = Superclass Object ColoredCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
58 How are Traits Used? Traits are the behavioral building blocks for classes Class = Superclass + State Object ColoredCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
59 How are Traits Used? Traits are the behavioral building blocks for classes Class = Superclass + State + Traits Object ColoredCircle TColor TCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
60 How are Traits Used? Traits are the behavioral building blocks for classes Class = Superclass + State + Traits + Glue methods Object ColoredCircle TColor TCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
61 How are Traits Used? Traits are the behavioral building blocks for classes Class = Superclass + State + Traits + Glue methods Object ColoredCircle TColor TCircle 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
62 How are Traits Used? Traits are the behavioral building blocks for classes Class = Superclass + State + Traits + Glue methods Object ColoredCircle TColor TCircle Traits do not replace single inheritance Traits provide modularity within classes 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 24
63 Composition Rules Class methods take precedence over trait methods ColoredCircle draw radius hue rgb TCircle TColor 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 25
64 Composition Rules Class methods take precedence over trait methods ColoredCircle draw TCircle draw radius hue rgb TColor 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 25
65 Composition Rules Class methods take precedence over trait methods ColoredCircle draw TCircle draw radius hue rgb TColor 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 25
66 Composition Rules Class methods take precedence over trait methods ColoredCircle draw TCircle draw radius hue rgb TColor 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 25
67 Conflicts Must be Resolved Explicitly ColoredCircle TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
68 Conflicts Must be Resolved Explicitly ColoredCircle TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
69 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
70 Conflicts Must be Resolved Explicitly hash self traitconflict ColoredCircle hash TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
71 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
72 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
73 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle circlehash -> hash TColor colorhash -> hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
74 Conflicts Must be Resolved Explicitly hash self colorhash bitxor: ColoredCircle [self circlehash] TCircle circlehash -> hash hash TColor colorhash -> hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
75 Conflicts Must be Resolved Explicitly hash self colorhash bitxor: ColoredCircle [self circlehash] TCircle circlehash -> hash hash TColor colorhash -> hash I) Override the conflict with a glue method 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
76 Conflicts Must be Resolved Explicitly hash self colorhash bitxor: ColoredCircle [self circlehash] TCircle circlehash -> hash hash TColor colorhash -> hash I) Override the conflict with a glue method Aliases provide access to the conflicting methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
77 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle circlehash -> hash TColor colorhash -> hash I) Override the conflict with a glue method Aliases provide access to the conflicting methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
78 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle circlehash -> hash TColor colorhash -> hash I) Override the conflict with a glue method Aliases provide access to the conflicting methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
79 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle TColor hash hash I) Override the conflict with a glue method Aliases provide access to the conflicting methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
80 Conflicts Must be Resolved Explicitly ColoredCircle hash TCircle TColor hash hash I) Override the conflict with a glue method Aliases provide access to the conflicting methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
81 Conflicts Must be Resolved Explicitly ColoredCircle TCircle TColor hash hash I) Override the conflict with a glue method Aliases provide access to the conflicting methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
82 Conflicts Must be Resolved Explicitly ColoredCircle TCircle TColor hash hash 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
83 Conflicts Must be Resolved Explicitly ColoredCircle TCircle TColor hash hash II) Avoid the conflict Edit one of the traits Exclude one of the conflicting methods from the composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
84 Conflicts Must be Resolved Explicitly ColoredCircle TCircle hash TColor II) Avoid the conflict Edit one of the traits Exclude one of the conflicting methods from the composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
85 Conflicts Must be Resolved Explicitly ColoredCircle TCircle hash TColor II) Avoid the conflict Edit one of the traits Exclude one of the conflicting methods from the composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
86 Conflicts Must be Resolved Explicitly ColoredCircle TCircle hash TColor II) Avoid the conflict Edit one of the traits Exclude one of the conflicting methods from the composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
87 Conflicts Must be Resolved Explicitly ColoredCircle TCircle TColor hash hash II) Avoid the conflict Edit one of the traits Exclude one of the conflicting methods from the composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
88 Conflicts Must be Resolved Explicitly ColoredCircle uses: TCircle + TColor {#hash} TCircle TColor hash hash II) Avoid the conflict Edit one of the traits Exclude one of the conflicting methods from the composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 26
89 Flattening Property (Two views on code) Structured view Object ColoredCircle draw draw radius TCircle hue rgb TColor 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 27
90 Flattening Property (Two views on code) Structured view Object Object ColoredCircle ColoredCircle draw draw radius TCircle draw radius hue rgb TColor hue rgb 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 27
91 Flattening Property (Two views on code) Structured view Object Flat view Object ColoredCircle ColoredCircle draw draw radius TCircle draw radius hue rgb TColor hue rgb 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 27
92 Flattening Property (Two views on code) Structured view Object equivalent Flat view Object ColoredCircle ColoredCircle draw draw radius TCircle draw radius hue rgb TColor hue rgb 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 27
93 Composition of multiple properties Circle BorderedColoredCircle TBordered TColored 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 28
94 Composition of multiple properties Circle BorderedColoredCircle TBordered TColored Live illustration Traits implementation on top of Squeak Smalltalk 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 28
95 Conceptual Evaluation of Traits Improve code reuse Composing classes from reusable building blocks Maintain Improve understandability Flattening property Two views on the code Flat View Ordinary Smalltalk (as if no traits were used) Structured View Separating glue methods from reusable behavior No surprises Simple composition rules Explicit conflict resolution Avoid fragile hierarchies Composite entity is in full control 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 29
96 Current Status Fully implemented in the Smalltalk dialect Squeak No performance penalty for method lookup Indirect instance variable access Used to refactor the Smalltalk collection hierarchy Used to build reusable metaclass properties Used to rebuild the trait implementation! Formal model 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 30
97 Act II Scene 1: Using Traits to Refactor the Collection Classes
98 Refactoring Strategy Create traits for the identified collection properties Functional properties (e.g., extensible, explicitly ordered, implicitly ordered) Implementation properties (e.g., array based implementation, linked implementation) Combine them to build the required collection classes 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 32
99 Refactoring Process Iterative and bottom up Basic steps: Pick an existing class (e.g., SequenceableCollection) Create a new (empty) trait for one aspect of its behaviour (e.g., TEnumeration) Make the class use this new trait and move the corresponding methods into the trait Iterate th October 2004 Applying Traits to the Smalltalk Collection Classes 33
100 Refactoring Process (2) Refinement steps: Making the traits more fine-grained Breaking traits into multiple subtraits Juggling with classes and traits Renaming classes and traits Introducing new abstract classes Replacing inheritance with trait composition 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 34
101 Resulting Class Hierarchy Resulting class hierarchy consists of 3 parts Abstract root class Collection contains only those methods supported by all collection classes Layer of abstract classes providing the public functionality of the different kinds of collection Layer of concrete collection classes that: inherit the public functionality from one of the functionality classes use a trait that adds a specific implementation 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 35
102 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 36
103 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 36
104 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 36
105 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 36
106 Resulting Trait Hierarchies Two trait hierarchies Functional Traits Implementation Traits Very fine-grained Most traits consist of multiple subtraits 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 37
107 Functional Traits 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 38
108 Let s summarize... We refactored 30 concrete and 8 abstract classes 29 subclasses of Collection 9 subclasses of Stream The refactored classes are built from 67 traits The average number of traits used to build a class is more than 5 The maximum number of traits per class is th October 2004 Applying Traits to the Smalltalk Collection Classes 39
109 What did we gain? Improved Understandability No unnecessary inheritance Every inheritance relationship is conceptually sound No abuse of inheritance for the purpose of reusing only a fraction of the superclass methods No methods implemented too high Every class has a clean interface More Uniform Protocols No missing methods Classes provide all the methods in the appropriate protocols Collections can be used more uniformly 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 40
110 What did we gain? (2) Less code! Elimination of code duplication Eliminates potential replicated bug problems No need to disable methods that are implemented too high In numbers: About 10% less source code More than 20% fewer methods 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 41
111 What did we gain? (3) Improved reusability The toolbox of traits makes it much easier to create new collection classes e.g., a class PluggableBag can be created as a subclass of Bag by using the trait TPluggableAdaptor Traits can be reused outside of the collection hierarchy e.g., the trait TEnumeration can be used to add the enumeration protocol to the class Path newpath := path collect: [:each rect containspoint: each] 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 42
112 Why Traits (vs. Mixins and Multiple Inheritance)? Main reason: flattening property Although classes are built from up to 22 traits, we can still view and work with these classes in the ordinary (i.e., single inheritance) way There is no trade-off between fine grained components and understandability We introduced more traits than were strictly necessary in order to improve understandability and unanticipated reuse This is not so for mixins and multiple inheritance 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 43
113 Why Traits (vs. Mixins and Multiple Inheritance)? (2) Traits Collection Extensible- Sequenced- Explicitly Ordered- Collection 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 44
114 Why Traits (vs. Mixins and Multiple Inheritance)? (2) Traits Collection Mixins Collection Extensible- Sequenced- Explicitly 22 Ordered- Collection Ordered- Collection 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 44
115 Why Traits (vs. Mixins and Multiple Inheritance)? (2) Traits Collection Mixins Collection Multiple Inheritance Collection Extensible- Sequenced- Explicitly 22 Ordered- Collection Ordered- Collection Ordered- Collection 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 44
116 Related Work on Refactoring, Reuse and Collections William Cook s study of conformance and inheritance in the Smalltalk-80 collection classes Other languages with more sophisticated collection hierarchies Self (also using a construct called traits ) Eiffel (using multiple inheritance) Animorphic Smalltalk (using mixins) these approaches are all coarse-grained compared to traits A lot of work on refactoring Semi-automated refactoring 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 45
117 Related Work on Languages Xerox Star project in Mesa [Curry 82] really an MI framework Larch specification language [Guttag 85] adding a trait constrains behavior of methods Jigsaw modularity framework [Bracha 92] completeness vs. simplicity Caesar [Mezini 97, 02] Combination layer between objects and classes Logtalk [Unpublished] reusable categories, but no composition of categories 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 46
118 Related Work at OGI [WoDiSEE 04] Multiview: multiple views on a program moving away from linear text to more structured and expressive views of programs Refactorings considered as defining equivalence relations programs as the equivalence classes that they induce As the language becomes more expressive: more refactorings become available more equivalence relations exist more views are can be utilized 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 47
119 Limitations Unintended name captures This is a limitation of Smalltalk No visibility/scoping mechanism for methods We are working on an extension of Smalltalk/Traits that tackles this problem Traits cannot specify state Slots and accessor methods are automatically generated 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 48
120 Conclusion Traits have nice theoretical properties These properties of traits paid off in practice! Flattening property Sum operation with explicit conflicts Migrating to traits is easy No change in method level syntax/semantics Ordinary Smalltalk programmer can understand, use, and work with the new hierarchy Fine-grained trait structure has no disadvantages Refactoring with traits was fun! Tools are important 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 49
121 Thank you Questions? 29 th October 2004 Applying Traits to the Smalltalk Collection Classes 50
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