Transforming Programming · 2019. 9. 23. · When faced with a coding decision, choose the path that makes future change easier 5

Copyright © 2018 David Thomas

Transforming Programming

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Dave [email protected]

@pragdave

Copyright © 2018 David Thomas�2

Why bother with design?

Design is what lets us change code

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Well designed code is easier to change

than poorly designed code

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When faced with a coding decision, choose

the path that makes future change easier

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Decoupling

http://happypontist.blogspot.com/2014/06/sydney-harbour-bridge.html�6

S O L I D

ingle Responsibilitypen/closediskov substitutionnterface separationependency inversion

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But…

design principles are

misunderstood

(just like design patterns)

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So…

Don’t sweat the

principles

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When faced with a coding decision, choose

the path that makes future change easier

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Copyright © 2018 David Thomas�11

Why bother with design?

How can we remember to design?

HabitWe first make our habits and then our habits make us. – unknown

All habits gather, by unseen degrees, as brooks make rivers, rivers run to seas. – Dryden

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My HabitBefore coding something

1. write single sentence–same level of abstraction

2. clauses (commas, and, then, …)

3. split and repeat

4. discover state

deliverable component

module function

data structure

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State

functioninput output

• stateless• pure transformation

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Statefunction

state

• stateful

• state is opaque and local

• stateful

• state is opaque and local

• yes, it’s an object�15

The Essence of OO

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Simula

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SimulaOriginal idea of passive customers moving through a

network of active components (activity approach) became

a limiting factor. Other way round possible - active

customers moving through a passive network. This

resulted in a unifying general process concept:

System = set of interacting quasiparallel processes.

1964

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set of interacting quasiparallel processes.

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Class Rectangle (Width, Height); Real Width, Height; Begin Real Area, Perimeter; Procedure Update; Begin Area ":= Width * Height; Perimeter ":= 2*(Width + Height) End of Update; Boolean Procedure IsSquare; IsSquare ":= Width=Height; Update; OutText("Rectangle created: "); OutFix(Width,2,6); OutFix(Height,2,6); OutImage End of Rectangle;

http://staff.um.edu.mt/jskl1/talk.html�21


http://staff.um.edu.mt/jskl1/talk.html

parameters used to instantiate instances of

this class

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instance variables

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instance method

instance function

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lifecycle code

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Lifecycle Code

• Here used to initialize the instance (like a constructor would)

• But commonly continues to run alongside instance in pseudo-parallel to other instances

(because simulation language)

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Simula Instances

• Are created by factories (classes)

• Have per-instance state

• Have methods and functions that operate on that state

• Have an pseudo-independent execution existence

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Smalltalk

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Smalltalk

• Inspired by Simula, Lisp, and others

• Objects are little independent computers that communicate by sending each other messages

• 1st class objects

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The mental image was one of separate computers

sending requests to other computers that had to be

accepted and understood by the receivers before

anything could happen. In today's terms every

object would be a server…

The Early History Of Smalltalk Alan C. Kay Apple Computer [email protected]#

HOPL-II/4/93/MA, USA © 1993 ACM 0-89791-571-2/93/0004/0069

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The mental image was one of separate computers

sending requests to other computers that had to be

accepted and understood by the receivers before

anything could happen. In today's terms every

object would be a server…


HOPL-II/4/93/MA, USA © 1993 ACM 0-89791-571-2/93/0004/0069

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The last thing you wanted any programmer to do is

mess with internal state even if presented

figuratively. Instead, the objects should be

presented as sites of higher level behaviors more

appropriate for use as dynamic components.


HOPL-II/4/93/MA, USA © 1993 ACM 0-89791-571-2/93/0004/0069

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Smalltalk Instances

• Are created by factories (objects of #Object)

• Have per-instance state

• Have functions that operate on that state

• Should be considered to be independent processes

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C++

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C++• Single-handedly influenced the next 20 years of

programming language development

• Classes are code, instances are allocated memory

• Not just inheritance; multiple inheritance

• Class-orientation; objects are second class

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Honorable Mentions

• Beta

• Ruby

• JavaScript

• and a secret surprise…

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The Tao ofObject Orientation

work is done by independent, opaque, processing units that communicate by passing each

other messages

道

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Functional Programming

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• Origins in lambda calculus (Church, 1930s)

• Elements in early Lisp systems (1950s)

• ML (1973) -> OCaml -> F# / Reason

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Functional Programming• 1980s saw resurgence in research around program

proving

• refinement of type systems

• lazy/non-strict evaluation

• Miranda -> Haskell

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• immutability

• 1st class, pure functions

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Immutability

• values never change

• changed values are never shared

implementation detail

design paradigm

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1st Class Functions

• Functions are values

• (in lambda calculus they’re the only values)

• Program is a composition of functions

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The Tao ofFunctional Programming

immutable data transformed by pure functions, often in

the presence of an algebraic type system and lazy

evaluation

道 apparently

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The Great Conflict

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immutable data transformed by pure

functions, often in the presence of an

algebraic type system

independent, opaque, processing units that communicate by passing each other messages

OO FP— vs —

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Except…

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immutable data transformed by pure

functions, often in the presence of an

algebraic type system

independent, opaque, processing units that communicate by passing each other messages

OO FP

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OO FPnon shared state

transformed by functionsinvoked by message

passing

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Elixir

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ElixirNot an OO language

Not (really) a functional language

(Joe calls Erlang Concurrency-Oriented)

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ElixirNot an OO language

Not (really) a functional language

(Joe calls Erlang Concurrency-Oriented)

NO MORE LABELS�52

Elixir

Is just a programming language and environment.

Let’s steal the best ideas from OO and FPlearn from

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non shared statetransformed by functions

invoked by message passing

OO FP

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OK, Functions ᴙ Us

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smells like a process to me

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Processes Unify Functional

and OO Code

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Actors Unify Functional

and OO Code

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CSP Unifies Functional

and OO Code

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So I No Longer Care

• it’s just coding

• sometimes with state

• sometimes without

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My HabitBefore coding something



3. split and repeat

4. discover state

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X X X X

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XX

XX

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XX

XX

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“Web app that waits for two players to join, then mediates

a game of 4⨉4⨉4 naughts and crosses between them”

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2. clauses (commas, and, then, …) 3. split and repeat 4. discover state

MatchMaker Controller

Game Controller

Player Pair

View View

Client JavaScript

Game Model

rails new sixty_four mix phx.new sixty_four

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“Web app waits for two players to join, then mediates

a game of 4⨉4⨉4 naughts and crosses between them”



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“Web app waits for two players to join, then mediates a game of 4⨉4⨉4

naughts and crosses between them”

MatchMaker GameWeb apps

Not

hing

sh

ared

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MatchMaker

“when a player arrives make them wait until the next player arrives, then pass the two to the game”



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MatchMaker


let player join

pass to game



waiting room

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MatchMaker


let player join

pass to game



pending player

waiting room

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let player join

(web UI)

pass to game

pending player

waiting room

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pubsub / events

Copyright © 2018 David Thomas

and so on…

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Before coding something



3. split and repeat

4. discover state

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The Goal

Maximize cohesion

Minimize coupling

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The Goal

Make it easy to change

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The Goal

Stop using labels

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The Goal

Use the past (but don’t be a slave to it)

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The Goal

Have fun!

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Transforming Programming · 2019. 9. 23. · When faced with a coding decision, choose the path that makes future change easier 5