Functional solid

Matt Stine

Functional Soλid

• Senior Consultant, PSOvFabric, Cloud Application PlatformVMware

• Speaker (JavaOne, SpringOne/2GX, CodeMash, NFJS, RWX, PAX, UberConf)

• Author (GroovyMag, NFJS the Magazine, Selenium 2.0 Refcard)

• Founder of the Memphis/Mid-South Java User Group

• Former Agile Zone Leader @ DZone

About your speaker...

SOLIDPrinciples

FunctionalProgramming

SOLIDPrinciples

FunctionalProgramming

Pitch

• Software designs tend to rot.

• Design rot impedes our effectiveness.

• SOLID principles help treat design rot.

• Achieving SOLID designs is another reason to learn FP!

Motivation

Trends

• How Software Systems Evolve

• Programming Paradigms

• The Quest for “Best Practices”

The evolution of software...

http://www.flickr.com/photos/unloveable/2400895854

http://www.flickr.com/photos/billselak/427719926

From simplicity to complexity...

Software starts to rot like a

bad piece of meat.

“Uncle Bob” Martin

http://www.flickr.com/photos/thejcgerm/379082415

We don’t handle change well...

Unclear or incomplete change specifications...

Extreme time pressure...

Team members unfamiliar with original design/architecture...

It smells!

Rigidity

Fragility

Immobility

Viscosity

Needless Complexity

Needless Repetition

Opacity

Programming Paradigms

Programming eras...

• Structured Programming(late 1960’s - late 1980’s)

• Object Oriented Programming(mid 1970’s - present day)

• Functional Programming(early 1990’s - ???)

TH

E C

HA

SM

Accumulation of Knowledge?

Accumulation of Knowledge?

Paradigm Shifts

http://www.gotw.ca/publications/concurrency-ddj.htm

The Quest for

“Best Practices”

• Which web framework should I use?

• Why are there so many persistence API’s?

• Which is better: Scala or Clojure (or Erlang)?

• How should I use design patterns?

• How do I build a good architecture?

http://en.wikipedia.org/wiki/No_Silver_Bullet

What are the FP best practices?

The Perfect Storm

TheSOLID

Principles

SRP OCP LSP

ISP DIP

http://clojure.org

http://www.infoq.com/presentations/SOLID-Clojure

The Single Responsibility Principle

A class (module) should have only one reason

to change.

Tom DeMarco Meilir Page-Jones

COHESION

Rectangle

+ draw()+ area() : double

GraphicalApplication

Computational Geometry Application

GUI

Rectangle

+ draw()

GraphicalApplication

Computational Geometry Application

GUI

GeometricRectangle

+ area() : double

Metaresponsibilities of an OO class...

• Model state

• Model behavior

State

• car.color = black• elevator.floor = 10• account.balance = 2500

Behavior

• car.drive()• elevator.climb()• account.deposit(500)

How many metaresponsibilities?

It is better to have 100 functions operate on one data structure than to have 10 functions operate

on 10 data structures.

Alan J. Perlis

Metaresponsibilities of an OO class...

• Model state

• Model behavior

• Model identity

IDENTITY

VALUE

car1 != car2

car1 == car2

Concurrent System

• Automated “Needs Service” Notification System

• Thread 1: Update mileage and “ready for service indicators”

• Thread 2: Harvest cars ready for service and send notifications

The Open Closed Principle

Bertrand Myer

Software entities should be open for extension but closed for modification.

Open for extension...

...closed for modification.

Abstraction!

• Java:

• interface

• abstract class

(pause OCP)

Let’s do design!

• OO language (Java)

• We want to use the OCP

• We’ll create one or more inheritance hierarchies!

• Well...

Inheritance Hierarchies?

• What do good ones look like?

• Are there rules we can follow (best practices even)?

• Are there traps we can fall in to?

Barbara Liskov

The Liskov Substitution Principle

What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is

substituted for o2, then S is a subtype of T.

Subtypes must be substitutable for their

base types.

f(B b) { }

D extends B

B d = new D();f(d)

f(B b) { if (b instance of D) { //deal with D } else { //continue as before }}

(resume OCP)

CompositionThe Functional OCP

First-class Functions

Higher-order Functions

The Interface Segregation Principle

Clients should not be forced to depend on

methods they do not use.

<<interface>>TimerClient

+ timeout()Timer

0..*

Door

TimedDoor

<<interface>>TimerClient

+ timeout()Timer

0..*Door

DoorTimerAdapter

+ timeout()

<<creates>>

TimedDoor

+ doorTimeOut()

<<interface>>TimerClient

+ timeout()Timer Door

TimedDoor

+ timeout()

Thinking in Verbs

Verbs

• Open

• Close

• Register Timeout

• Trigger Timeout

OK...what about that unique ID problem?

The Dependency Inversion Principle

Abstractions should not depend upon details. Details

should depend upon abstractions.

High-level modules should not depend on

low-level modules.

Policy Layer

Mechanism Layer

Utility Layer

Policy Layer

Mechanism Layer

Utility Layer

<<interface>>Policy Service

Interface

<<interface>>Mechanism

Service Interface

Our verbs have been taken captive by

nouns...

http://steve-yegge.blogspot.com/2006/03/execution-in-kingdom-of-nouns.html

http://www.infoq.com/presentations/Simple-Made-Easy/

Complectedness

SRP

Complecting responsibilities leads to rigid and/or fragile design.

OCP

Problematic:Complecting concretions of an

abstraction in such a way that new concretions adversely affect

existing, working concretions.

LSP

Reuse via inheritance is dangerous. Often complects entities not in a true “is-a” relationship. Leads to non-

substitutability.

ISP

Don’t complect unrelated operations in a single entity!

DIP

Transitive dependency leads to transitive complectedness!

Design is the art of breaking things apart.

- Rich Hickey

Please fill out your evaluations!

Matt [email protected]

Twitter: mstinehttp://www.mattstine.com