This document provides an overview of traits in programming. It discusses the limitations of single and multiple inheritance, as well as mixins. Traits are introduced as a way to share behavior across class hierarchies without these issues. Key points about traits are that they provide methods but no state, and classes have full control over trait composition. The document also presents a case study on refactoring stream classes using traits for improved reuse and design. It concludes with discussion on the implementation of traits in Pharo and potential future directions.

1. 13. Traits

2. Selected literature
> Cook. Interfaces and Specifications for the
Smalltalk-80 Collection Classes. OOPSLA 1992
> Taivalsaari. On the Notion of Inheritance. ACM
Computing Surveys, September 1996.
> Black, et al. Applying Traits to the Smalltalk
Collection Hierarchy. OOPSLA 2003
> Ducasse, et al. Traits: A Mechanism for fine-
grained Reuse. ACM TOPLAS, March 2006.
> Cassou, et al. Traits at Work: the design of a new
trait-based stream library. JCLSS 2009
© Oscar Nierstrasz
Traits
2
http://scg.unibe.ch/scgbib?query=stlit-traits

3. © Oscar Nierstrasz
ST — Smalltalk Basics
2.3
Roadmap
> Why traits?
> Traits in a Nutshell
> Case study — Streams
> Traits in Pharo
> Future of Traits

4. © Oscar Nierstrasz
ST — Smalltalk Basics
2.4
Roadmap
> Why traits?
> Traits in a Nutshell
> Case study — Streams
> Traits in Pharo
> Future of Traits

5. Problem: how to share behaviour across
class hierarchies?
© Oscar Nierstrasz
Traits
5
There are hundreds of methods we would
like RectangleMorph to inherit from
both Rectangle and Morph

6. The trouble with Single Inheritance
> Where to put the shared behaviour?
— Sharing too high inappropriate methods must be “cancelled”⇒
> Duplicating code
— Impacts maintenance
> Delegate
— Ugly boilerplate delegation code
© Oscar Nierstrasz
Traits
6

7. The trouble with Multiple Inheritance
> Conflicts must be resolved
— Implicit resolution leads to
fragility when refactoring
> No unique super class
— Must explicitly name super
methods to compose them
> Diamond problem
— What to do about features
inherited along two paths?
© Oscar Nierstrasz
Traits
7
an
IdentitySet(#topRight
#align:with: #right:
#leftCenter #bottom
#center #height
#right #topCenter
#extent #bottomCenter
#topLeft #width
#printOn:
#containsPoint: #left
#top #intersects:
#bottomLeft #bottom:
#bottomRight #top:
#left: #rightCenter)
an
IdentitySet(#topRight
#align:with: #right:
#leftCenter #bottom
#center #height
#right #topCenter
#extent #bottomCenter
#topLeft #width
#printOn:
#containsPoint: #left
#top #intersects:
#bottomLeft #bottom:
#bottomRight #top:
#left: #rightCenter)
Rectangle selectors select:
[:s | Morph selectors includes: s]
Rectangle selectors select:
[:s | Morph selectors includes: s]

8. Mixins extend single inheritance with
features that can be mixed into a class
© Oscar Nierstrasz
Traits
8

9. The trouble with Mixins
> Mixins are composed linearly to resolve conflicts
— Conflict resolution is sensitive to mixin composition order
— Composing entity has no control!
> Fragile hierarchy
— Changes may impact distant classes
© Oscar Nierstrasz
Traits
9

10. © Oscar Nierstrasz
ST — Smalltalk Basics
2.10
Roadmap
> Why traits?
> Traits in a Nutshell
> Case study — Streams
> Traits in Pharo
> Future of Traits

11. Traits are parameterized behaviours
> A trait
— provides a set of methods
— requires a set of methods
— may be composed of other traits
> Traits do not specify any state!
© Oscar Nierstrasz
Traits
11
= aRectangle
^ self species = aRectangle species
and: [self origin = aRectangle origin]
and: [self corner = aRectangle corner]
= aRectangle
^ self species = aRectangle species
and: [self origin = aRectangle origin]
and: [self corner = aRectangle corner]

12. Class = superclass + state + traits + glue
© Oscar Nierstrasz
Traits
12
The class retains full control of the composition

13. Both traits and classes can be composed
of traits
© Oscar Nierstrasz
Traits
13
Trait named: #NSTPuttablePositionableStream
uses: NSTPuttableStream + NSTPositionableStream
category: 'Nile-Base-Traits'
Trait named: #NSTPuttablePositionableStream
uses: NSTPuttableStream + NSTPositionableStream
category: 'Nile-Base-Traits'
Object subclass: #NSTextStream
uses: NSTPuttablePositionableStream + NSTCharacterWriting
instanceVariableNames: 'collection position writeLimit readLimit'
classVariableNames: ''
poolDictionaries: ''
category: 'Nile-Clients-TextStream'
Object subclass: #NSTextStream
uses: NSTPuttablePositionableStream + NSTCharacterWriting
instanceVariableNames: 'collection position writeLimit readLimit'
classVariableNames: ''
poolDictionaries: ''
category: 'Nile-Clients-TextStream'

14. Trait composition rules
1. Class methods take precedence over trait
methods
2. Conflicts are resolved explicitly
3. Traits can be flattened away
© Oscar Nierstrasz
Traits
14

15. Class methods take precedence over trait
methods
© Oscar Nierstrasz
Traits
15
RectangleMorph>>printOn:
prevails over Morph>>printOn:

16. Trait composition rules
1. Class methods take precedence over trait methods
2. Conflicts are resolved explicitly
3. Traits can be flattened away
© Oscar Nierstrasz
Traits
16

17. Conflicts are resolved explicitly
© Oscar Nierstrasz
Traits
17
RectangleMorph subclass: #Morph
uses: TRectangle @ {#rectanglePrintOn: -> #printOn:}
– {#align:with: . #topRight . … }
instanceVariableNames: ''
classVariableNames: ''
poolDictionaries: ''
category: 'Morphic-TraitsDemo'
RectangleMorph subclass: #Morph
uses: TRectangle @ {#rectanglePrintOn: -> #printOn:}
– {#align:with: . #topRight . … }
instanceVariableNames: ''
classVariableNames: ''
poolDictionaries: ''
category: 'Morphic-TraitsDemo'
Aliasing introduces an additional name for a method
Exclusion removes a method from a trait

18. Trait composition rules
1. Class methods take precedence over trait methods
2. Conflicts are resolved explicitly
3. Traits can be flattened away
© Oscar Nierstrasz
Traits
18

19. Traits can be flattened away
© Oscar Nierstrasz
Traits
19
A class using traits is
equivalent to class
that defines those
traits locally

20. © Oscar Nierstrasz
ST — Smalltalk Basics
2.20
Roadmap
> Why traits?
> Traits in a Nutshell
> Case study — Streams
> Traits in Pharo
> Future of Traits
Cassou, et al. Traits at Work: the design
of a new trait-based stream library.
JCLSS 2009.

21. The trouble with Streams
© Oscar Nierstrasz
Traits
21
unused state
methods too high
copying
canceling
reimplementation

22. The Nile core
© Oscar Nierstrasz
Traits
22

23. Nile Stream classes
© Oscar Nierstrasz
Traits
23
no methods too high
no copying
no unused state
no reimplementation
limited canceling

24. Other Nile Stream classes
© Oscar Nierstrasz
Traits
24

25. Assessment
> High reuse achieved
— 40% less code in Stream hierarchy
> More general abstractions
— Streams on any Collection
— With equal or better performance
> Design traits around abstractions, not reuse
— Avoid too fine-grained traits
> Traits or classes?
— Prefer classes — use traits to resolve design conflicts
© Oscar Nierstrasz
Traits
25

26. © Oscar Nierstrasz
ST — Smalltalk Basics
2.26
Roadmap
> Why traits?
> Traits in a Nutshell
> Case study — Streams
> Traits in Pharo
> Future of Traits

27. © Oscar Nierstrasz
ST — Understanding Classes and Metaclasses
12.27
Traits in Pharo
> Language Extension
— Extended the language kernel to represent traits
— Modified the compilation process for classes
built from traits
> No changes to the VM
— Essentially no runtime performance penalty
— Except indirect instance variable access
— But: This is common practice anyway
> No duplication of source code
— Only byte-code duplication when installing methods

28. © Oscar Nierstrasz
ST — Understanding Classes and Metaclasses
12.28
Traits in Pharo 1.0
Object subclass: #Behavior
uses: TPureBehavior @
{ #basicAddTraitSelector:withMethod:
-> #addTraitSelector:withMethod: }
instanceVariableNames: 'superclass methodDict format
traitComposition localSelectors'
classVariableNames: 'ObsoleteSubclasses'
poolDictionaries: ''
category: 'Kernel-Classes'

29. OmniBrowser supports trait browsing and
navigation
© Oscar Nierstrasz
Traits
29
navigation browsing required methods

30. Traits can be manipulated from the browser
© Oscar Nierstrasz
Traits
30

31. Traits and Classes share common
behaviour
Traits
31

32. Can classes share compiled methods from
traits?
© Oscar Nierstrasz
Traits
32
Two problems:
1.super is statically bound
2.compiled methods know their class
⇒methods are copied to method
dictionaries when they are installed

33. © Oscar Nierstrasz
ST — Smalltalk Basics
2.33
Roadmap
> Why traits?
> Traits in a Nutshell
> Case study — Streams
> Traits in Pharo
> Future of Traits

34. The future of Traits
> Stateful traits
— some experimental solutions …
> Tool support
— limited browser support in Pharo
> Automatic refactoring
— some experiments with formal concept analysis
> Pure trait-based language
— can traits and classes be unified?
> Traits in other languages
— Perl, Scala, Fortress, …
© Oscar Nierstrasz
Traits
34

35. © Oscar Nierstrasz
Traits
1.35
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