◦ ◦◦◦ TECHNISCHE UNIVERSITAT¨ MUNCHEN¨ ◦◦◦◦ ◦ ◦ ◦◦◦ ◦◦◦◦ ¨ ¨ ◦ ◦◦ FAKULTAT FUR INFORMATIK Programming Languages Mixins Dr. Michael Petter Winter 2014 Mixins 1 / 30 “What advanced techiques are there besides multiple implementation inheritance?” Mixins Introduction 2 / 30 Outline Weak implementation inheritance 1 Decorator Problem 2 Wrapper Problem Inheritance in Detail 1 Models for single inheritance 2 Introducing Mixins 3 Modelling Mixins Mixins in the wild 1 Mixins as C++-Pattern 2 Native Mixins Mixins Introduction 3 / 30 The Adventure Game Door ShortDoor LockedDoor canPass(Person p) canOpen(Person p) ? ShortLockedDoor canOpen(Person p) canPass(Person p) Mixins Introduction The Adventure Game 4 / 30 The Adventure Game Door <interface>Doorlike canPass(Person p) canOpen(Person p) Short canPass(Person p) Locked canOpen(Person p) ShortLockedDoor ! Door implements empty methods canOpen(Person p) ! Doorlike must anticipate wrappers canPass(Person p) Mixins Introduction The Adventure Game 5 / 30 The Wrapper FileStream SocketStream read() read() write() write() ? SynchRW acquireLock() releaseLock() ! Cannot inherit from both seperately ! Creating new wrapping Classes duplicates code Mixins Introduction The Wrapper 6 / 30 The Wrapper Stream read() write() FileStream SocketStream read() read() write() write() SynchRW read() ! Undoes specialization write() ! acquireLock() Needs common ancestor releaseLock() Mixins Introduction The Wrapper 7 / 30 “Let’s go back to the basics of inheritance” Mixins Mixins Modelling Inheritance 8 / 30 Classes and Methods The building blocks for classes are a countable set of method names N a countable set of method bodies B Classes map names to elements from the flat lattice B (called bindings), consisting of: method bodies 2 B or classes 2 C attribute offsets 2 N+ ? (yet) undefined > in conflict and the partial order ? v m v > for each m 2 B Definition (Abstract Class 2 C) A partial function c : N 7! B is called abstract class. Definition (Interface and Class) An abstract class c is called (with pre beeing the preimage) interface iff 8n2pre(c) : c(n) = ?. (concrete) class iff 8n2pre(c) : ? @ c(n) @ >. Mixins Mixins Modelling Inheritance 9 / 30 Computing with Classes and Methods Definition (Family of classes C) We call the set of all maps from names to bindings the family of abstract classes C := N 7! B. Several possibilites for composing maps C C: the symmetric join t, defined componentwise: 8 >b2 if b1 = ? or n2 = pre(c1) > <b1 if b2 = ? or n2 = pre(c2) (c1 t c2)(n) = b1 t b2 = where bi = ci(n) >b2 if b1 = b2 > :> otherwise in contrast, the asymmetric join t, defined componentwise: ( c1(n) if n 2 pre(c1) (c1 t c2)(n) = c2(n) otherwise Mixins Mixins Modelling Inheritance 10 / 30 Example: Smalltalk-Inheritance Smalltalk inheritance is the archetype for inheritance in mainstream languages like Java or C# inheriting smalltalk-style establishes a reference to the parent Definition (Smalltalk inheritance (.)) Smalltalk inheritance is the binary operator . : C × C 7! C, definied by c1 . c2 = fsuper 7! c2g t (c1 t c2) Example: Doors Door = fcanP ass 7! ?; canOpen 7! ?g LockedDoor = fcanOpen 7! 0x4204711g . Door = fsuper 7! Doorg t (fcanOpen 7! 0x4204711g t Door) = fsuper 7! Door; canOpen 7! 0x4204711; canP ass 7! ?g Mixins Mixins Modelling Inheritance 11 / 30 Excursion: Beta-Inheritance In Beta-style inheritance the design goal is to provide security from replacement of a method by a different method. methods in parents dominate methods in subclass the keyword inner explicitely delegates control to the subclass Definition (Beta inheritance (/)) Beta inheritance is the binary operator / : C × C 7! C, definied by c1 / c2 = finner 7! c1g t (c2 t c1) Example (equivalent syntax): class Person{ String name= "Axel Simon"; public String toString(){ return name+inner.toString();}; }; class Graduate extends Person{ public extension String toString(){ return ", Ph.D.";}; }; Mixins Mixins Modelling Inheritance 12 / 30 Extension: Attributes Remark: Modelling attributes is not in our main focus. Anyway, most mainstream languages nowadays are designed so that attributes are not overwritten: Definition (Mainstream inheritance (.0)) The extended mainstream inheritance .0 : C × C 7! C binds attributes statically: 8 >c2 if n = super > + 0 <> if n 2 pre(c1) ^ c2(n) 2 N (c1 . c2)(n) = >c1(n) if n 2 pre(c1) > :c2(n) otherwise Mixins Mixins Modelling Inheritance 13 / 30 “So what do we really want?” Mixins Mixins Introducing Mixins 14 / 30 Adventure Game with Code Duplication Door ShortDoor LockedDoor canPass(Person p) canOpen(Person p) ShortLockedDoor canOpen(Person p) canPass(Person p) Mixins Mixins Introducing Mixins 15 / 30 Adventure Game with Mixins <mixin>Locked Door canOpen(Person p) canOpen(Person p) canPass(Person p) <mixin>Short mixin canPass(Person p) compose <mixin>ShortLocked Mixins Mixins Introducing Mixins 16 / 30 Adventure Game with Mixins class Door{ boolean canOpen(Person p){ return true;}; boolean canPass(Person p){ returnp.size()< 210;}; } mixin Locked{ boolean canOpen(Person p){ if(!p.hasItem(key)) return false; else return super.canOpen(p); } } mixin Short{ boolean canPass(Person p){ if(p.height()>1) return false; else return super.canPass(p); } } class ShortDoor= Short(Door); class LockedDoor= Locked(Door); mixin ShortLocked= Short o Locked; class ShortLockedDoor= Short(Locked(Door)); class ShortLockedDoor2= ShortLocked(Door); Mixins Mixins Introducing Mixins 17 / 30 Back to the blackboard! Mixins Mixins Modelling Mixins 18 / 30 Abstract model for Mixins A Mixin is a unary second order type expression. In principle it is a curried version of the Smalltalk-style inheritance operator. In certain languages, programmers can create such mixin operators: Definition (Mixin) The mixin constructor mixin : C 7! (C 7! C) is a unary class function, creating a unary class operator, defined by: mixin(c) = λx : c . x ! Note: Mixins can also be composed ◦: Example: Doors Locked = fcanOpen 7! 0x1234g Short = fcanP ass 7! 0x4711g Composed = mixin(Short) ◦ (mixin(Locked)) = λx : Short . (Locked . x) = λx : fsuper 7! Lockedg t (fcanOpen 7! 0x1234; canP ass 7! 0x4711g . x) Mixins Mixins Modelling Mixins 19 / 30 Wrapper with Mixins FileStream SocketStream read() read() write() write() mixin <mixin>SynchRW acquireLock() releaseLock() mixin SynchedFileStream SynchedSocketStream read() read() write() write() Mixins Mixins Modelling Mixins 20 / 30 Mixins on Implementation Level Short class Door{ boolean canOpen(Person p)... canPass() boolean canPass(Person p)... } super mixin Locked{ boolean canOpen(Person p)... Locked ... } canOpen() mixin Short{ boolean canPass(Person p)... super } class ShortDoor ... = Short(Door); Door class ShortLockedDoor = Short(Locked(Door)); ... ! non-static super-References ShortDoor d = new ShortLockedDoor(); dynamic dispatching without precomputed virtual table Mixins Mixins Modelling Mixins 21 / 30 “Surely multiple inheritance is powerful enough to simulate mixins?” Mixins Mixins Simulating Mixins in C++ 22 / 30 Simulating Mixins in C++ template<class Super> class SyncRW: public Super { public: virtual int read(){ acquireLock(); int result= Super::read(); releaseLock(); return result; }; virtual void write(int n){ acquireLock(); Super::write(n); releaseLock(); }; // ... acquireLock & releaseLock }; Mixins Mixins Simulating Mixins in C++ 23 / 30 Simulating Mixins in C++ template<class Super> class LogOpenClose: public Super { public: virtual void open(){ Super::open(); log("opened"); }; virtual void close(){ Super::close(); log("closed"); }; protected: virtual void log(char*s) { ... }; }; class MyDocument: public SyncRW<LogOpenClose<Document>> {}; Mixins Mixins Simulating Mixins in C++ 24 / 30 True Mixins vs. C++ Mixins True Mixins super natively supported C++ Mixins Mixins as Template do not offer composite mixins Mixins reduced to templated superclasses C++ Type system not modular Can be seen as coding Mixins have to stay source pattern code Hassle-free simplified version of multiple inheritance Common properties of Mixins Linearization is necessary Exact sequence of Mixins is relevant Mixins Mixins Simulating Mixins in C++ 25 / 30 “Ok, ok, show me a language with native mixins!” Mixins Mixins Native Mixins in Python 26 / 30 Ruby module Short def canPass(p) class Person p.size< 160 and super(p) attr_accessor :size end def initialize end @size= 160 module Locked end def canOpen(p) def hasKey p.hasKey() and super(p) true end end end end class Door class ShortLockedDoor< Door def canOpen(p) include Short true include Locked end end def canPass(person) person.size< 210 p= Person.new end d= ShortLockedDoor.new end putsd.canPass(p) Mixins Mixins Native Mixins in Python 27 / 30 Ruby class Door module ShortLocked def canOpen(p) include Short true include Locked end end def canPass(person) class Person person.size< 210 attr_accessor :size end def initialize end @size= 160 module Short end def canPass(p) def hasKey p.size< 160 and super(p) true end end end end module Locked def canOpen(p) p= Person.new p.hasKey() and super(p) d= Door.new end d.extend ShortLocked end putsd.canPass(p) Mixins Mixins Native Mixins in Python 28 / 30 Lessons Learned Lessons Learned 1 Formalisms to model inheritance 2 Mixins provide soft multiple inheritance 3 Multiple inheritance can not compensate the lack of super reference 4 Full extent of mixins only when mixins are 1st class language citizens Mixins Mixins Native Mixins in Python 29 / 30 Further reading... Gilad Bracha and William Cook. Mixin-based inheritance. European conference on object-oriented programming on Object-oriented programming systems, languages, and applications (OOPSLA/ECOOP), 1990. James Britt. Ruby 2.1.5 core reference, December 2014. URL https://www.ruby-lang.org/en/documentation/. Matthew Flatt, Shriram Krishnamurthi, and Matthias Felleisen. Classes and mixins. Principles of Programming Languages (POPL), 1998. Mixins Further materials 30 / 30.