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Duck Typing, Compatibility, and the Adaptor Pat...
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Reg Braithwaite
September 12, 2014
Programming
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Duck Typing, Compatibility, and the Adaptor Pattern
Nordic JS, 2014
Reg Braithwaite
September 12, 2014
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Transcript
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the$art$of$the$javascript$metaobject$protocol Duck%Typing,%Compa1bility,%and The$Adaptor$Pa-ern
None
Duck%typing%is%a%style%of%typing%in% which%an%object's%methods%and% proper:es%determine%the%valid% seman:cs,
...rather'than'its'inheritance'from'a' par0cular'class'or'implementa0on'of' an'explicit'interface.
None
Tastes&Like&Fruit
None
Also%Tastes%Like%Fruit
"Write'code'that'depends'upon' tastes1like(fruit)'rather'than' depending'upon'is1a(fruit)."
None
duck%typing%is%a Compa&bility,Technique
None
A"Problem:
None
Conway's)Game)of)Life
"Conway's*Game*of*Life*is*a*two2 state*cellular*automata."
An#Implementa+on 1. Cells,
An#Implementa+on 1. Cells, 2. Universe,
An#Implementa+on 1. Cells, 2. Universe,.and 3. View.
Cell 1. Neighbours,
Cell 1. Neighbours,.and 2. State.
function StandardCell () { this._neighbours = []; this._alive = false;
}
StandardCell.prototype.neighbours = function neighbours (neighbours) { return this._neighbours; }; StandardCell.prototype.setNeighbours
= function setNeighbours (neighbours) { this._neighbours = neighbours.slice(0); return this; };
StandardCell.prototype.alive = function alive () { return this._alive; }; StandardCell.prototype.setAlive
= function setAlive (alive) { this._alive = alive; return this; };
None
moving'through Time
Cell 1. Neighbours, 2. State,2and 3. Transforma9on2of2State.
Universe 1. Neighbourhoods.(not.shown),
Universe 1. Neighbourhoods.(not.shown),.and 2. Transforma:on.of.States.
StandardCell.prototype.nextAlive = function nextAlive () { var alives = this._neighbours.filter(function
(n) { return n.alive(); }).length; if (this.alive()) { return alives === 2 || alives == 3; } else { return alives == 3; } };
Universe.prototype.iterate = function iterate () { var aliveInNextGeneration = this.cells().map(
function (c) { return [c, c.nextAlive()]; } ); aliveInNextGeneration.forEach(function (a) { var cell = a[0], next = a[1]; cell.setAlive(next); }); };
None
drawing Life
View 1. Drawing*a*Cell:
View.prototype.drawCell = function drawCell (cell, x, y) { var xPlus
= x + this.cellSize(), yPlus = y + this.cellSize() this._canvasContext.clearRect(x, y, xPlus, yPlus); this._canvasContext.fillStyle = this.cellColour(cell); this._canvasContext.fillRect(x, y, xPlus, yPlus); return self; };
View.prototype.cellColour = function cellColour (cell) { return cell.alive() ? WHITE
: BLACK; };
None
None
Ch#ch#ch#changes!
None
redesigning(for Colour
function ColourCell () { this._neighbours = []; this._age = 0;
} ColourCell.prototype.neighbours = StandardCell.prototype.neighbours; ColourCell.prototype.setNeighbours = StandardCell.prototype.setNeighbours;
ColourCell.prototype.age = function age () { return this._age; }; ColourCell.prototype.setAge
= function setAge (age) { this._age = age; return this; };
None
moving'through',me in#Colour
ColourCell.prototype.nextAge = function next () { var alives = this._neighbours.filter(function
(n) { return n.age() > 0; }).length; if (this.age() > 0) { return (alives === 2 || alives == 3) ? (this.age() + 1) : 0; } else { return (alives == 3) ? (this.age() + 1) : 0; } };
Universe.prototype.iterate = function iterate () { var ageInNextGeneration = this.cells().map(
function (c) { return [c, c.nextAge()]; } ); ageInNextGeneration.forEach(function (a) { var cell = a[0], next = a[1]; cell.setAge(next); }); };
None
drawing(life in#Colour
var COLOURS = [ BLACK, GREEN, BLUE, YELLOW, WHITE, RED
]; View.prototype.cellColour = function cellColour (cell) { return COLORS[ (cell.age() >= COLOURS.length) ? (COLOURS.length - 1) : cell.age() ]; }; // ...
None
something Doesn't(Fit
Object.keys(StandardCell.prototype) // => [ 'neighbours', 'setNeighbours', 'alive', 'setAlive', 'nextAlive' ]
Object.keys(ColourCell.prototype) // => [ 'neighbours', 'setNeighbours', 'age', 'setAge', 'nextAge' ]
The$state$of$a$cell$is$now$"age"$ instead$of$"aliveness."
None
These%changes%will Ripple&Through&Universe&and&View
our$problem$is$that Universe(is(coupled(to(Cell's( Interface
None
None
how$smart$people Solve&the&Problem
"Make&a&coloured&cell&behave&like&a& standard&cell."
ColourCell.prototype.alive = function alive () { return this._age > 0;
};
ColourCell.prototype.setAlive = function setAlive (alive) { if (alive) { this.setAge(this.age()
+ 1); } else this.setAge(0); return this; };
ColourCell.prototype.nextAlive = StandardCell.prototype.nextAlive;
Object.keys(ColourCell.prototype) // => [ 'neighbours', 'setNeighbours', 'age', 'setAge', 'nextAge', 'alive',
'setAlive', 'nextAlive' ]
Presto!(Backwards(Compa2bility. (it$"tastes$like$a$standard$cell")
None
universe(and(cell(are No#Longer#Coupled
None
At#What#Cost?
None
Increasing*Flexibility Increases(Complexity
None
None
there%is Another(Way
"Make&a&cell&that&behaves&like&a& Standard&Cell,&but&is&implemented&in& terms&of&a&Coloured&Cell."
function AsStandard (colouredCell) { this._colouredCell = colouredCell; } var tastesLikeAStandardCell
= new AsStandard(aColourCell);
AsStandard.prototype.neighbours = function neighbours () { return this._colouredCell.neighbours(); }; AsStandard.prototype.setNeighbours
= function setNeighbours (neighbours) { this._colouredCell.setNeighbours(neighbours); return this; };
AsStandard.prototype.alive = function alive () { return this._colouredCell.age() > 0;
};
AsStandard.prototype.setAlive = function setAlive (alive) { if (alive) { this._colouredCell.setAge(this._colouredCell.age()
+ 1); } else this._colouredCell.setAge(0); return this; };
AsStandard.prototype.nextAlive = function nextAlive () { return this._colouredCell.nextAge() > 0;
}
Object.keys(AsStandard.prototype) // => [ 'neighbours', 'setNeighbours', 'alive', 'setAlive', 'nextAlive' ]
When%we%Upgrade%to%ColourCell 1. View'can'depend'upon'ColourCell,
When%we%Upgrade%to%ColourCell 1. View'can'depend'upon'ColourCell,'and 2. Universe'can'depend'upon'AsStandard.
Sweet! 1. Universe*is*decoupled*from*changes*to*Cell,
Sweet! 1. Universe*is*decoupled*from*changes*to*Cell,*and 2. ColourCell*isn't*burdened*with*backwards*compa>bility.
None
AsStandard)is)an)Adaptor
The$adapter$(sic)$pa/ern$is$a$ so2ware$design$pa/ern$that$allows$ the$interface$of$an$exis8ng$class$to$ be$used$from$another$interface...
It#is#o'en#used#to#make#exis0ng# classes#work#with#others#without# modifying#their#source#code.
TIMTOWTDI
None
good$programmers$borrow$from$smalltalk, Great&Programmers&Steal&from&C++
function colourFromStandard (standard) { return new ColourCell() .setNeighbours(standard.neighbours()) .setAge(standard.alive() ?
1 : 0); }
function standardFromColour (colour) { return new StandardCell() .setNeighbours(colour.neighbours()) .setAlive(colour.age() >
0); }
None
None
Coproxies
None
None
Approaching+the+End
None
adaptors Separate'Concerns
None
and$thus,$adaptors Isolate(Change
None
and$they Decouple(Modules
None
adaptors Solve&Compa+bility&Problems
None
but$they$also$gives$us$the$freedom$to Make%Changes,%Safely
None
when%we're%using%teams%to%build%so3ware%at%scale Adaptors)are)an)Important)Tool
None
None
one$more$thing:
None
None
when%you're%holding%a Version(Shaped.Hammer
None
all#changes#look#like Migra&on)Shaped/Nails
None
Packages(Have(Interlocking( Dependencies
None
Migra&ons*Can*Cascade you$never$know$how$much$work$a$single$upgrade$can$cause
None
migra&on)is)expensive,)so)semver)values Backwards)Compa.bility
None
"duck&typed"&backwards&compa2bility Holds&Progress&Back
None
breaking)compa.bilty Exposes'Clients'to'Bugs
None
adaptors(decouple Compa&bility,from,Progress
None
adaptors(provide Safe%Compa*bility
None
What%would%an%adaptor.aware% package%manager%look%like?
An#Adaptor*Aware#Package#Manager 1. Interfaces,and,Implementa1ons,would,be,independantly, versioned,
An#Adaptor*Aware#Package#Manager 1. Interfaces,and,Implementa1ons,would,be,independantly, versioned, 2. Interfaces,would,have,a,many<to<many,rela1onship,with, Implementa1ons,,mediated,by,Adaptors,
An#Adaptor*Aware#Package#Manager 1. Interfaces,and,Implementa1ons,would,be,independantly, versioned, 2. Interfaces,would,have,a,many<to<many,rela1onship,with, implementa1ons,,mediated,by,adaptors, 3. Clients,would,specifiy,the,required,interface,and, implementa1on,
An#Adaptor*Aware#Package#Manager 1. Interfaces,and,Implementa1ons,would,be,independantly, versioned, 2. Interfaces,would,have,a,many<to<many,rela1onship,with, implementa1ons,,mediated,by,adaptors, 3. Clients,would,specifiy,the,required,interface,and, implementa1on,,and
4. Old,clients,can,benefit,from,bug,fixes,without,rewrites.
None
an#adaptor)aware#package#manager#could Change'the'Way'We'Grow'So0ware
None
and$that$provides Food$for$Thought for$a$very$long$,me$indeed
None
Tack%Så%Mycket!
Reg$Braithwaite,$ GitHub,$Inc. raganwald.com,•,@raganwald Ar#pelag,*Stockholm,* September*19,*2014