Ur Domain Haz Monoids?

Cyrille Martraire - @cyriux

"My first encounter with FP concepts was from

DDD"

A matter of Taste

http://rosshirt.blogspot.fr/

The CodeGourmet

(dedicated to @ziobrando)

FP

Passionate developer

PARISSince 1999

@cyriuxCyrille Martraire

Paris Software Craftsmanship Community

http://www.meetup.com/paris-software-craftsmanship/

TDDBDDDDDLegacy

WARNINGThe following show features no spectacular stunt, no live coding, only trivial Java code. You can re-create or re-enact at home with no danger.

a bit personal

Adopted 2005Still in love

...

What do DDD & FP have in common?

FP = ?

http://www.jaider.net/archives/609-intro-to-functional-programming/

PURE

No StateNo War

So what do DDD & FP

have in common?

Value Objects................................................................................................. 19

Learn oneand get the other

one for FREE!

DDDFP

(OO)

NICE STYLE

of code

DDD+FP = ?

Example PLZ?

MONOIDZ!

@cyriux

ClosureAssociativity

Neutral Element

only 3 numbers in

programming

0, 1, MANY

Monoid:encapsulate

diversity inside

0, 1, MANY

Neutral Element

Element

Operation

Battle against complexity

Encapsulate details (unit...)

→ simple again

For a given interface I find myself doing often 0, 1, Many

OO FTW!

0, 1, MANY

NullObject

Implementation(s)

Composite

Identity Element

= NullObject

VatCalculation.NONE

Applied often:

→ scalable process

→ can grow to very complex complexity

Example PLZ!

Numbersint+int=int

(3+5)+2=3+(5+2)0

Lists

(.)+(.,.)=(.,.,.) ︎(a ︎)+(b,︎c)=(a ︎, b)+(c) ︎

()

Strings

"hello"+"world""cy"+"ri"+"lle"

""

look simplistic;the key to very

complex behavior

The key to infinite scalability!

The key to infinite incremental computing!

Hadoop x Storm

Composeability

Monoids ☛ reduce

Monoids ☛ domain

Monoids are typical FP

FP: everything is a value

Therefore:Monoids are

values!

VALUEOBJECTS

ImmutableEquals by value

Not Anemic

18°C+ 16°C= 34°C

returns  a  new  instance

Change -> new instance

SIDE-EFFECT -free

FUNCTIONS

Immutability&

Side-effect-free functions!

Value Object

A DDD patternto import FP-ish

values in OO languages

“Functional-First” style

90%“Functional-First” style

Value Objects

(Ok, gut feeling, I did’nt measure)

Money

(25, EUR)+

(30, EUR)=

(55, EUR)

(25, EUR)+

(30, USD)=

exception

Money

amountcurrencyadd(Money): Money

<<ValueObject>>

Cashflows (Payments)

(25, EUR, today)+

(30, EUR, today)=

(55, EUR, today)

(25, EUR, today)+

(30, EUR, next day)=

exception

CashFlow

amountcurrencydate

add(CashFlow):CashFlow

<<ValueObject>>

CLOSURE of

OPERATION

Cashflows Sequences

(10, EUR, 20/03)

(30, EUR, 21/03)

(25, EUR, 21/03)

(12, EUR, 22/03)

(10, EUR, 20/03)

(55, EUR, 21/03)

(12, EUR, 22/03)

+

=

Cashflow Sequence +

Cashflow Sequence =

Cashflow Sequence

ObjectArithmetics

This is how domain experts THINK

about itSAY

SKETCH

Ranges

[1, 3]Union [2, 4]

= [1, 4]

[1, 3]Union [2, 4]

= [1, 4] ][

Predicates

FilterAND Filter

= Filter

FilterAND Filter

= Filter

Always

True

FilterOR

Filter=

Filter

FilterOR

Filter=

FilterAlways

False

Grants

Read, Write, Execute

"most secure wins"

r + w = rw + x = w

Configuration Maps

+

=

Color BLUE

Enable TrueTimeout 30 +

=

Color RED

Enable FalseTimeout 25

Desk FX

Color RED

Enable TrueTimeout 25

Desk FX

Default Desk-specific

+

=

Color BLUE

Enable TrueTimeout 30 +

=

Color RED

Enable FalseTimeout 25

Desk FX

Color RED

Enable TrueTimeout 25

Desk FX

Default Desk-specific

overw

rite

logic

al OR

min

+

=

Color BLUE

Enable TrueTimeout 30 +

=

Color RED

Enable FalseTimeout 25

Desk FX

Color RED

Enable TrueTimeout 25

Desk FX

Default Desk-specific

overw

rite

logic

al OR

min

Values are monoids too!

+

=

Color BLUE

Enable TrueTimeout 30 +

=

Neutral Element

Color BLUE

Enable TrueTimeout 30

+

=

Color BLUE

Enable TrueTimeout 30 +

=Color BLUE

Enable TrueTimeout 30

Value Objects may be

*BIG* object trees!

(DOM)

Non-Linear Stuff(average, std dev,

K-clustering, barycenters...)

Average + Average = WRONG

Average + Average Not Composeable!

avg = sum / count

avg = sum / count

avg = sum / count

avg = sum / count+

avg = sum / count

avg = sum / count+ +

avg = sum / count

avg = sum / count

avg = sum / count

+

=

+

=

(sum, count)

(sum, count)

(sum, count)

+

=

Average

(sum, sum2, count)

(sum, sum2, count)

(sum, sum2, count)

+

=

Std deviation

Can model as a monoid even non-

linear stuff!

MOARMATHS

PLZ!

Monoid several times...

toString(): Stringunion(MailingList): MailingListintersection(MailingList): MailingListnobody(): MailingListeverybody(): MailingList

MailingList

Space Vectors

average temperature = t1.add(t2) .scale(1/2)

returns  a  new  instance

Change -> new instance

Space Vector

toCelsius(): TemperaturetoFarenheit(): Temperatureadd(Temperature): Temperaturescale(double): Temperature

Temperature

Why is it useful?

(10, EUR, 20/03)

(30, EUR, 21/03)

(25, EUR, 21/03)

(12, EUR, 22/03)

(10, EUR, 20/03)

(55, EUR, 21/03)

(12, EUR, 22/03)

+

=

Cashflow Sequence +

Cashflow Sequence =

Cashflow Sequence

This is how domain experts THINK

about itSAY

SKETCH

DECLARATIVE STYLE

Much less codeMuch less bugs

// without monoidsPaymentsFees(...)PaymentsFeesWithOptions(...)PaymentsFeesWithInsuranceAndOptions(...)PaymentsFeesWithInsurance(...)NoFeesButInsurance(...)...

// with monoidsfees(...) : Paymentsoptions(...) : Paymentsinsurance(...) : Payments

Payments.add(Payments) : Payments

side-effect-free

operation

Very easy to test

input output

Much less stuff to learn

1 interface to rule them all

Cashflow Sequence +

Cashflow Sequence =

Cashflow Sequence

ESTABLISHEDFORMALISMS

Monoid/Vector Spaces/Cyclic Group

LiteratureDocumented

@annotations

@Monoid()intersection(MailingList): MailingList

@NeutralElement("intersection")nobody(): MailingList

MailingList

LIVINGDOCUMENTATION

Signal to Noise ratio

http://www.flickr.com/photos/28471130@N07/2666802097

Signal to Noise Ratio

• SNR ≥ 80 %

• Signal (VO): CashFlow.multiply(), CashFlowSequence.add(), .negate(), truncate(), Money.add(), .times(), .opposite(), Fixing, FinancialProduct, BankHolidays, ReferenceData

• Noise: CashFlowBuilder, CompositeEngine, ProductFactory, BankHolidaysDecorator

Write code that tells the business

domain stories

@annotations

@Monoid()intersection(MailingList): MailingList

@NeutralElement("intersection")nobody(): MailingList

MailingList

Domain-Specific

@Monoid()overlaping(MailingList): MailingList

@NeutralElement("overlaping")nobody(): MailingList

MailingList

Value Objects have domain-specific

flavors too:

Convention, Policy, Status, Quantity,

Observation...

- Annotate domain-relevant classes

- Custom Doclet to export Excel-formatted glossary of every domain concept

Glossary from Code

Sent directly to end customers

Glossary from Code

SELF-EXPLAINING VALUES

We Want:Traceability of processing

No worry!

Just enrich our types

Just enrich our types

Just enrich our types

label field

Monad-ishNo logging neededEach value stores

its history

http://stuartcook.files.wordpress.com/2010/11/happy-monkey.jpg

In Closing

Invest time:

Learn DDD, and get free FP exposure

A paradox:

FP influence helps craft better Object-

Oriented code!

Monoids are good: Eat Them!

LOOK 4 Ur DOMAIN MONOIDZ!

Also learn other maths structures

Wikipedia is your friend!

DDD+FP=

http://wadler.blogspot.fr/2008/04/functional-programming-is-beautiful.html

Taste-Driven Development

TDD

Questions? Did you try similar things too?

Let’s discuss!

@cyriux

Follow me @cyriux

Slides: slideshare.net/cyriux

Blog: cyrille.martraire.com

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Merci