Object Inheritance Without Classes Timothy Jones , Michael Homer, - - PowerPoint PPT Presentation

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Object Inheritance Without Classes Timothy Jones , Michael Homer, - - PowerPoint PPT Presentation

Oct 24, 2022 441 likes •1.06k views

Object Inheritance Without Classes Timothy Jones , Michael Homer, James Noble Victoria University of Wellington {tim,mwh,kjx}@ecs.vuw.ac.nz Kim Bruce Pomona College kim@cs.pomona.edu July 21, 2016 Foundations Objects v Classes 1

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Object Inheritance Without Classes

Timothy Jones, Michael Homer, James Noble Victoria University of Wellington {tim,mwh,kjx}@ecs.vuw.ac.nz Kim Bruce Pomona College kim@cs.pomona.edu July 21, 2016

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SLIDE 2

Foundations

Objects v Classes

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SLIDE 3

Foundations

Objects v Classes

Andrew v Kim

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SLIDE 4

Foundations

Objects v Classes

Andrew v Kim Objects-first v Objectdraw

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SLIDE 5

Foundations

Know Thy Self

This problem is solved! ( | parent* = other. | )

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Foundations

Know Thy Self

This problem is solved! ( | parent* = other. | ) ( | parent* = factory new. | )

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SLIDE 7

Foundations

Know Thy Self

This problem was supposed to be solved. . . ( | parent* = other. | ) ( | parent* = factory new. | )

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SLIDE 8

Semantics

Object Inheritance

method graphic(canvas) {

  • bject {

. . . } } def amelia = object { inherit graphic(canvas) . . . }

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SLIDE 9

Semantics

Semantics

What does this mean? inherit graphic(canvas) Do the inherit semantics actually allow us to implement classes?

◮ Let’s investigate different object inheritance semanticses

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Semantics

Semantics

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SLIDE 11

Semantics

Semantics

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SLIDE 12

Semantics

Semantics

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SLIDE 13

Semantics

Semantics

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SLIDE 14

Semantics

Semantics

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SLIDE 15

Semantics

Implementation

Runnable semantics with PLT Redex https://github.com/zmthy/graceless-redex

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Semantics

Reg. Down. Dist. Stable Exist. Mult. Forwarding Delegation Concatenation Merged Uniform

  • Mult. Uniform

Transform U. Positional U. Java yes yes no yes class no (* indicates true for construction, then reversed afterwards)

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Object Inheritance

Object Inheritance

Objects inherit directly from one another Three foundational models:

◮ Forwarding (as in E) ◮ Delegation (as in JavaScript and Self) ◮ Concatenation (as in Kevo)

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Object Inheritance

Forwarding

Requests to inherited methods go directly to inherited object

◮ Simplest semantics

a method m(x) b

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SLIDE 19

Object Inheritance

Forwarding

Requests to inherited methods go directly to inherited object

◮ Simplest semantics

a method m(x) b m(v)

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SLIDE 20

Object Inheritance

Forwarding

Requests to inherited methods go directly to inherited object

◮ Simplest semantics

a method m(x) b m(v) m(v)

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SLIDE 21

Object Inheritance

Forwarding

Requests to inherited methods go directly to inherited object

◮ Simplest semantics

a method m(x) b m(v) m(v) No down-calls (cannot modify existing implementation)

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Object Inheritance

Down-calls

method graphic(canvas) {

  • bject {

method image { abstract } method draw { canvas.render( image ) } } } def amelia = object { inherit graphic(canvas) def image = images.amelia }

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Object Inheritance

Delegation

Requests to inherited methods have self bound to original object

◮ The standard semantics of object inheritance

a method m(x){ self.k(x) } b

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Object Inheritance

Delegation

Requests to inherited methods have self bound to original object

◮ The standard semantics of object inheritance

a method m(x){ self.k(x) } b m(v)

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SLIDE 25

Object Inheritance

Delegation

Requests to inherited methods have self bound to original object

◮ The standard semantics of object inheritance

a method m(x){ self.k(x) } b m(v) m(v)

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SLIDE 26

Object Inheritance

Delegation

Requests to inherited methods have self bound to original object

◮ The standard semantics of object inheritance

a method m(x){ self.k(x) } b m(v) m(v) k(v)

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Object Inheritance

Delegation

Requests to inherited methods have self bound to original object

◮ The standard semantics of object inheritance

a method m(x){ self.k(x) } b m(v) m(v) k(v) Vampire problem

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Object Inheritance

Delegation

Requests to inherited methods have self bound to original object

◮ The standard semantics of object inheritance

a method m(x){ self.k(x) } b m(v) m(v) k(v) Vampire problem Surprising behaviour if you’re used to classes

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Object Inheritance

Action at a Distance

method graphic(canvas) {

  • bject {

var name := "A graphic" } } def parent = graphic(canvas) def amelia = object { inherit parent name := "Amelia" }

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Object Inheritance

Delegation (as in Self)

above = (| value ← 3. run = (|| say). say = (|| ’above’ printLine) |). below = (| parent* = above. say = (|| ’below’ printLine) | run. value: 5).

  • ther = (|

parent* = above. | value print).

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Object Inheritance

Concatenation

Copy the methods and fields from the inherited object

◮ Removes direct relationship between inheritor and inheritee

a method m(x) method k(y)

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Object Inheritance

Concatenation

Copy the methods and fields from the inherited object

◮ Removes direct relationship between inheritor and inheritee

a method m(x) b method m(x); method k(y)

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Object Inheritance

Concatenation

Copy the methods and fields from the inherited object

◮ Removes direct relationship between inheritor and inheritee

a method m(x) b method m(x); method k(y) Changes to inherited object are not reflected in inheriting object

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Object Inheritance

Registration

method graphic(canvas) {

  • bject {

canvas.register( self ) } } def amelia = object { inherit graphic(canvas) }

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Emulating Classes

Emulating Classes

Objects inherit from calls to constructor methods Two class-like models

◮ Merged Identity (as in C++) ◮ Uniform Identity (as in Java)

Cannot inherit from preëxisting objects

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Emulating Classes

Merged Identity

Inheriting object ‘becomes’ the inherited object

◮ Registered identities eventually resolve to the intended object

a method m(x) method k(y)

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Emulating Classes

Merged Identity

Inheriting object ‘becomes’ the inherited object

◮ Registered identities eventually resolve to the intended object

a method m(x); method k(y)

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Emulating Classes

Merged Identity

Inheriting object ‘becomes’ the inherited object

◮ Registered identities eventually resolve to the intended object

a method m(x); method k(y) Body-snatchers problem

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Emulating Classes

Merged Identity

Inheriting object ‘becomes’ the inherited object

◮ Registered identities eventually resolve to the intended object

a method m(x); method k(y) Body-snatchers problem Objects not stable during construction

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Emulating Classes

Stability

method graphic(canvas) {

  • bject {

image method image { abstract } } } def amelia = object { inherit graphic(canvas) def image = images.amelia }

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Emulating Classes

Uniform Identity

Inherited initialisation code runs as the inheriting object

◮ Basically magic

method m(x) b method k(y)

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Emulating Classes

Uniform Identity

Inherited initialisation code runs as the inheriting object

◮ Basically magic

b method m(x); method k(y)

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Emulating Classes

Uniform Identity

Inherited initialisation code runs as the inheriting object

◮ Basically magic

b method m(x); method k(y)

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Emulating Classes

Uniform Identity

Inherited initialisation code runs as the inheriting object

◮ Basically magic

b method m(x); method k(y) Uninitialised state during construction

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Emulating Classes

Emulating Classes

Not very satisfactory as foundational models

◮ No inheritance from preëxisting objects

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Emulating Classes

Emulating Classes

Not very satisfactory as foundational models

◮ No inheritance from preëxisting objects

Other languages (JavaScript, E) achieve this using other features

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Emulating Classes

Classes in JavaScript

function Above() { this.value = 3; this.say(); } Above.prototype.run = function () { this.say(); }; function Below() { Above.call(this); } Below.prototype.say = function () { console.log("hello"); }; new Below().run();

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Emulating Classes

Classes in E

def makeAbove(self) { def above { to run() { self.say() } } self ← say() return above } def below extends makeAbove(below) { to say() { println("hello") } } below.run()

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Conclusion

Multiple Inheritance

Every model except merged identity Various different conflict resolution schemes

◮ Named supers ◮ Method transformations ◮ Positional inheritance

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Conclusion

Reg. Down. Dist. Stable Exist. Mult. Forwarding no no yes yes yes can Delegation no no* yes no yes can Concatenation no no* no no yes can Merged yes no* no no* fresh can’t Uniform yes yes no yes fresh no

  • Mult. Uniform

yes yes no yes fresh yes Transform U. yes yes no no fresh yes Positional U. yes yes no no fresh yes Java yes yes no yes class no (* indicates true for construction, then reversed afterwards)

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Conclusion

Reg. Down. Dist. Stable Exist. Mult. Forwarding no no yes yes yes can Delegation no no* yes no yes can Concatenation no no* no no yes can Merged yes no* no no* fresh can’t Uniform yes yes no yes fresh no

  • Mult. Uniform

yes yes no yes fresh yes Transform U. yes yes no no fresh yes Positional U. yes yes no no fresh yes Java yes yes no yes class no (* indicates true for construction, then reversed afterwards)

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Conclusion

Conclusion

No obviously superior semantics for object inheritance Emulating classes requires magic or complicated language features Ultimately depends on the design goals for the language

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Conclusion

Lessons

OO language designers

◮ Simple foundations do not imply simple design

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Conclusion

Lessons

OO language designers

◮ Simple foundations do not imply simple design

Everyone else

◮ Problems are hidden in solved designs

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Extra Slides

Semantics

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Extra Slides

Forwarding (as in E)

def above { to run() { above.say() } to say() { println("above") } }

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Extra Slides

Forwarding (as in E)

def above { to run() { above.say() } to say() { println("above") } } def below extends above { to say() { println("below") } } below.run()

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Extra Slides

Delegation (as in Self)

above = (| value ← 3. run = (|| say). say = (|| ’above’ printLine) |). below = (| parent* = above. say = (|| ’below’ printLine) | run).

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Extra Slides

Delegation (as in Self)

above = (| value ← 3. run = (|| say). say = (|| ’above’ printLine) |). below = (| parent* = above. say = (|| ’below’ printLine) | value: 5).

  • ther = (|

parent* = above. | value print).

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Extra Slides

Delegation (as in JavaScript-ish)

let above = {}; above.value = 3; above.run = function () { this.say(); }; above.say = function () { console.log("above"); }; let below = Object.create(above); below.say = function () { console.log("below"); }; below.value = 5; below.run(); console.log(above.value = below.value);

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