Strong Duck Type Driven Development

John Cinnamond // panagile ltd

Strong Duck Type Driven Development

strong duck type driven development

Let’s build a duck

How do you build a duck?

What is a duck?

Let’s deconstruct a duck

quacky bit

walking bit

floaty bit

flying bit

quacky bitwalking bitfloaty bitflying bit

This is a pretty rubbish duck

too much negative space

This is still a pretty rubbish duck

The bits don’t fittogether very well

floaty bit

This is still a pretty rubbish duck

quacky bit

walking bit

floaty bit

flying bit

quacky bit

this is worse

floaty bit

something isn’t quite right

I changed the quacky bit

I was required to change the floaty bit too

quacky bit

neck bit

brain bit eye bit

floaty bit

this is going to

hurt

floaty bit

We focus on the bits we’re building

We don’t think about how the bits fit together

this causes

problems




class QuackyBit

def quack puts “quack” end

end

class QuackyBit

def soft_quack puts “quack” end

end

class QuackyBit


def loud_quack puts “QUACK” end

end

bundle exec rspec ..........................................................................

Finished in 43.32027 seconds 73 examples, 0 failures

bundle exec rspec ..........................................................................

Finished in 43.32027 seconds 73 examples, 0 failures

wat?

either

the code is not used

or

the tests are wrong

class DuckBrain

def say_hello if other_duck.nearby? @quacky_bit.quack end end

end

class DuckBrain

def say_hello if other_duck.nearby? @quacky_bit.quack end end

end

it “greets other ducks” do expect(quacky_bit).to receive(:quack)

duck_brain.say_hello end

it “greets other ducks” do expect(quacky_bit).to receive(:quack)

duck_brain.say_hello end

How would we write it in Go?

type DuckBrain struct { quackyBit Quacker }

func (b DuckBrain) sayHello() { b.quackyBit.quack() }



type Quacker interface { quack() }

type Beak struct { }

func (b Beak) quack() { fmt.Println(“quack”) }

beak := Beak{}

brain := DuckBrain.new { quackyBit: beak, }

brain.sayHello()


func (b Beak) quack() { fmt.Println(“quack”) }


func (b Beak) softQuack() { fmt.Println(“quack”) }


func (b Beak) softQuack() { fmt.Println(“quack”) }

func (b Beak) loudQuack() { fmt.Println(“QUACK”) }

./duck.go:4:

cannot use beak (type Beak) as type Quacker in field value:

Beak does not implement Quacker (missing quack method)

type Quacker interface { softQuack() loudQuack() }

./duck_brain.go:6:

db.quackyBit.quack undefined

(type Quacker has no field or method quack)



The compiler tells me when I break stuff

The compiler hints about how to fix it

DuckBrain knows nothing about Beak

Beak knows nothing about DuckBrain

Interfaces are no longer intangible

Hey Jakub

wouldn’t it be great if we had

interfaces in ruby?

in rubythis is easy

just write it

word

class DuckBrain

def initialize(quacky_bit) @quacky_bit = quacky_bit end

end

class DuckBrain

def initialize(quacky_bit) if !quacky_bit.respond_to?(:quack) raise InterfaceError end @quacky_bit = quacky_bit end

end

We just invented NoMethodError

There are contracts between objects

One object promises to provide some methods

The other promises to only use these methods

require 'lawyer'

class Quacker < Lawyer::Contract

end

require 'lawyer'

class Quacker < Lawyer::Contract confirm :quack => 0 end

class QuackyBit


end

class QuackyBit


end

QuackyBit.implements(Quacker)

class QuackyBit


end


class QuackyBit


def loud_quack puts “QUACK” end

end


QuackyBit does not implement <Quacker>

QuackyBit does not implement <Quacker>

(Lawyer::BrokenContract) (1 method missing) [missing] quack

class Quacker < Lawyer::Contract confirm :quack => 0 end

class Quacker < Lawyer::Contract confirm :soft_quack => 0 confirm :loud_quack => 0 end

it “greets other ducks” do

end

it “greets other ducks” do

duck_brain.say_hello expect(<something>). to have_received(:quack) end

it “greets other ducks” do brain = DuckBrain.new(quacker)

duck_brain.say_hello expect(quacker). to have_received(:quack) end

it “greets other ducks” do quacker = contract_double( Quacker )

brain = DuckBrain.new(quacker)


it “greets other ducks” do quacker = contract_double( Quacker )

brain = DuckBrain.new(quacker)


Failure/Error: duck_brain.say_hello

Double "Quacker" received unexpected message :quack with (no args)

The compiler tells me when I break stuff

toolchain

This is still ruby

This is just duck typing

But it’s strong duck typing

The toolchain helps enforce the types

We check the type before runtime

We have timeto fix type problems

Is this actually useful?

Initially I was

🙋

“I have more confidence that my production code won’t break”

I’ve only seen interface problems a few times…

…in 8 years of writing ruby code

I’ve solved the wrong problem

#fml

The compiler hints about how to fix it

The object don’t know about each other

Interfaces are no longer intangible




Visual Cortex

Brain

Lungs Tongue Beak

duck nearby

describe “Duck.new” do

it “creates a VisualCortex” do

end

end


it “creates a VisualCortex” do expect { Duck.new }

end

end


it “creates a VisualCortex” do expect { Duck.new }.to create(VisualCortex)

end

end


it “creates a VisualCortex” do expect { Duck.new }.to create(VisualCortex). with(no_args) end

end

class Duck

end

class Duck

def initialize VisualCortex.new end

end

Visual Cortex

Brain

duck nearby

it “notifies the brain” do

end

it “notifies the brain” do visual_cortex = VisualCortex.new() visual_cortex.run

end


expect(brain). to have_received(:duck_nearby) end

class VisualCortex

def run brain.duck_nearby end

end

class VisualCortex


end



it “notifies the brain” do visual_cortex = VisualCortex.new(brain) visual_cortex.run


it “notifies the brain” do brain = ?

visual_cortex = VisualCortex.new(brain) visual_cortex.run


it “notifies the brain” do brain = contract_double(

)



it “notifies the brain” do brain = contract_double( Contracts::Greeter )



module Contracts class Greeter < Lawyer::Contract end end

module Contracts class Greeter < Lawyer::Contract confirm :duck_nearby => 0 end end







class VisualCortex


end

class VisualCortex

def initialize(brain) @brain = brain end

def run @brain.duck_nearby end

end

VisualCortex when there is duck nearby notifies the brain

Finished in 0.00082 seconds 1 example, 0 failures

We haven’t created the brain yet



end



end

ArgumentError: wrong number of arguments (0 for 1)



end


it “creates a VisualCortex” do expect { Duck.new }.to create(VisualCortex). with(Contract::Greeter) end

end

class Duck

def initialize VisualCortex.new(?) end

end



end


it “creates a Brain” do expect { Duck.new }.to create(Brain). with(no_args) end


end

class Duck

def initialize VisualCortex.new(?) end

end

class Duck

def initialize brain = Brain.new VisualCortex.new(brain) end

end

class Brain end

Lawyer::BrokenContract: Brain does not implement<Contracts::Greeter> (1 method missing) [missing] duck_nearby

Lawyer::BrokenContract: Brain does not implement<Contracts::Greeter> (1 method missing) [missing] duck_nearby

describe Brain do

it “greats nearby ducks” do

end

end

describe Brain do

it “greats nearby ducks” do brain = Brain.new brain.duck_nearby

end

end

describe Brain do


expect(quacker).to receive(quack) end

end

describe Brain do


expect(quacker).to receive(quack) end

end

The tests tell us what to write next

Writing new code is easy

Change is hard

We should focus on making change easier

We use TDD to make refactoring easier

We use TDD to make refactoring objects easier

Contracts make refactoring messages easier

Think about the messages

Change the contract

Fix the failing specs

Change the messages between objects

module Contracts class Greeter < Lawyer::Contract confirm :duck_nearby => 0 end end

module Contracts class Greeter < Lawyer::Contract confirm :duck_nearby => [:distance] end end

Lawyer::BrokenContract:

Brain does not implement <Contracts::Greeter> (1 method with the wrong signature) [wrong signature] duck_nearby (missing [:distance])

Failure/Error: visual_cortex.run

Double "Contracts::Greeter" received :duck_nearby with unexpected arguments

expected: ({:distance=>AnyArgMatcher}) got: (no args)

Replace implementations

Replace implementations(as long as they implement the same interface)

class FrontalLobe def duck_nearby(distance:) end end

FrontalLobe.implements(Contracts::Greeter)

class Duck

def initialize brain = Brain.new VisualCortex.new(brain) end

end

class Duck

def initialize fl = FrontalLobe.new VisualCortex.new(fl) end

end

The tests tell us what to write next

We can trust the tests totell us what to write next

We focus on isolated parts of the system

Then we focus on messages

between parts of the system

– Alan Kay

The big idea is "messaging"

http://lists.squeakfoundation.org/pipermail/squeak-dev/1998-October/017019.html

Recap

Built a duck(badly)

Noticed accidental coupling…

…caused by ignoring the connections between objects

…caused by ignoring the messaging between objects

duck typing…Supercharged

…to focus on connections between objects

…to generate errors if we break those connections

Use errors to drive our development

Reduces accidental coupling

Avoidsball-of-mud code

🙋

(strong (duck type)) driven development

John Cinnamond @jcinnamond

panagile.com/talks/rubyconf-2014

http://panagile.com/talks/rubyconf-2014

Attributions

Picture of Jakub Oboza used with permission and taken from http://lambdacu.be

The following images are all available under a CC Attribution 2.0 Generic licence. Many thanks to the original photographers for sharing these images.

‘~ Duck Dribble ~’ by Stuart Williams https://www.flickr.com/photos/viamoi/3336548665

‘Morgan 8’ Plus by Stewart Cambers https://www.flickr.com/photos/stewc/4392996817

‘Champion Bodybuilders in kuwait 2 June2010’ by Ra'ed Qutena https://www.flickr.com/photos/raedqutena/4665308585

‘duck pair walking’ by Sebastian Ziebell https://www.flickr.com/photos/zebel/2399225416

http://lambdacu.be

https://www.flickr.com/photos/viamoi/3336548665

https://www.flickr.com/photos/stewc/4392996817

https://www.flickr.com/photos/raedqutena/4665308585

https://www.flickr.com/photos/zebel/2399225416

Strong Duck Type Driven Development

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