The Philosophy of Domain Driven Design

training@instil.co
DDD Belfast - February 2019
© Instil Software 2018
The Philosophy of
Domain Driven Design
garth.gilmour@instil.co
@GarthGilmour

How I Got Started
“Why, it’s wonderful – a perfect little city – and I kept
thinking: ‘Why did no-one tell me about this?’ I knew, of
course, that it had a ﬁne Norman cathedral but I had no idea
that it was so splendid. I couldn’t believe that not once in
twenty years had anyone said to me, ‘You’ve never been to
Durham? Good God, man, you must go at once! Please – take
my car’.”
Bill Bryson

Why Does Philosophy Matter?
“One guy said in exasperation. A very famous
neurobiologist. Look in my discipline its ok to be
interested in consciousness. But. Get. Tenure.
First.”
John Searle

All academic fields refuse to answer certain questions
•  That’s how they delimit themselves and do useful work
•  Much like we learn to talk by ignoring certain sounds
Philosophy is what you do before you can experiment
•  We can examine our own preconceptions of the thing long
before we can examine the thing itself (e.g. mind and time)

Philosophy asks the questions above science
•  When should be regard something as being proven?
Philosophy also watchdogs the different sciences
•  When they tie themselves in metaphysical knots
•  Such as Psychology and The Rules Of Logic
Certain questions will always be philosophical
•  Such as questions about the ethical use of science
•  For example the now famous ‘trolley problem’

A Very Real Use Of Philosophy - Rhetoric

//Under what circumstances will 'foobar' return true?
fun main(args: Array<String>) {
fun foobar(a: Boolean, b: Boolean) : Boolean {
if(a) {
if (b) {
return a && b
}
}
return false
}
println(foobar(true, true)) //A
println(foobar(false, false)) //B
println(foobar(true, false)) //C
println(foobar(false, true)) //D
}
Program.kt
A Coding Quiz…

A Coding Quiz…
‘A’ is correct Robin.
But only according to the rules of logic.

Mathematical: 2 + 2 = 4
Religious: “I am the way”
Moral: “Killing people is bad”
Observational: “I am sitting down”
Legal: “Possession is 9/10 of the law”
Aesthetic: “The Mona Lisa is beautiful”
Counterfactual: “Hitler caused World War 2”
Some Examples of Truths

Combining Truths
The set of integers is infinite
&&
Street Fighter is a cinematic masterpiece

Combining Truths
Did anything about that seem strange to you?

Correspondence Theory
•  To be true is to correspond to facts in the world
•  But most truths cannot be tested in this way
Coherence Theory
•  Truth is the degree to which we fit within a larger scheme
•  But then LOTR is more truthful than Quantum Physics
Pragmatic Theory
•  Something is proved true by its utility
•  But then “the ends justify the means”
Redundancy Theory
•  “It is true that it is true that it is true that it is raining outside”
•  But this leads to an infinite recursion
Different Theories of Truth

We don’t know what truth
is. Or what it means for
something to be true…
Boolean b; //WTAF???
We Don’t Know what Truth Means

Proof That Developers Don’t Understand Booleans…

Introducing Wittgenstein
Wittgenstein is one of the most celebrated philosophers
•  Whose life was remarkable in many different ways
He created two different and opposing philosophies
•  Both concerning the meaning of utterances in language
•  How is it that we can communicate ideas via sounds?
His contemporaries were involved in Logicism
•  The attempt to give mathematics a firm foundation
•  By showing that it extends, or can be reduced to, logic
Frege, Russell and Whitehead were the main proponents
•  Gödel's incompleteness theory ultimately devastated it

Initially Wittgenstein wanted to bring logic to language
•  To establish logical foundations for the use of language
He believed words had meaning by denoting things
•  Language works by triggering pictures of how things are
•  They represent arrangements of things in the world
•  All words are descriptions (perhaps in disguise)
In his view words enable us to paint pictures with facts
•  This is how we communicate ideas with each other
•  Language is ultimately and literally pictorial in nature
This view also enabled him to preserve bivariance in logic
•  Which as we have seen is very convenient to have
Wittgenstein's First Philosophy

This first philosophy was the
basis of basis of a book
•  The only writing to be
published in his lifetime
He believed that he had
solved all of philosophy
•  By proving that it was
ultimately meaningless
•  Which is not to say that
ethics, art etc. don’t matter
The book ends famously:
•  “Whereof one cannot
speak, thereof one must
be silent.”
Wittgenstein's First Philosophy

“The Tractatus is a work of
genius, but it otherwise satisﬁes
the requirements for a Ph.D.”
G.E.Moore

After a 10 year gap Wittgenstein began philosophy again
•  Based on talks with what would become the ‘Vienna Circle’
He ended up opposing his original view
•  Coming to view language as a social instrument
Now language is a tool we use to play ‘games’
•  The idea of a ‘game’ was not meant frivolously
•  But to empathise how they are based on rules
Wittgenstein’s Second Philosophy

Meaning arises from the use we give language in games
•  The meaning of an expression is its usage within a game
Confusion occurs when we mistake the game being played
•  Problems of philosophy arise from incorrectly importing
expressions from one game into another
•  In other words they were being used in the wrong context
Games have a ‘family resemblance’ to one another
•  But there is no single characteristic that they all share

Wittgenstein's 2nd philosophy
was published posthumously
•  In the book ‘Philosophical
Investigations’ (1953)
There is a one continuous
thread with his earlier work
•  Philosophy remains as a
pursuit that one engages in
•  Its an activity not a theory
A philosopher:
•  Reminds people of all the
games they participate in
•  Exposes confusion when it
occurs (esp. in psychology)

Bounded Contexts and Language Games
But how many kinds of sentence are there?
Say assertion, question, and command?---
There are countless kinds: countless
different kinds of use of what we call
"symbols", "words", "sentences". And this
multiplicity is not something ﬁxed, given
once for all; but new types of language, new
language-games, as we may say, come into
existence, and others become obsolete and
get forgotten. (We can get a rough picture of
this from the changes in mathematics.)

Here the term "language-game" is meant to
bring into prominence the fact that the
speaking of language is part of an activity, or
of a form of life.

How Does This Relate To Domain Driven Design?

Everything is within a
language game and /
or bounded context.
Words have no
absolute meaning
A Bold Statement To Finish…

Kripkes Sceptical Paradox
Are you an adder or a quadder?
How do you know?

class Quint(private val value: Int) {
operator fun plus(other: Quint): Quint {
val total = value + other.value
if(total < 57) {
return Quint(total)
}
return Quint(5)
}

override fun toString(): String {
return value.toString()
}
}
Program.kt

fun test(input1: Quint, input2: Quint) {
println(input1 + input2)
}

fun test(input1: Int, input2: Int) {
println(input1 + input2)
}

fun foo(value: Int) = value
fun bar(value: Int) = Quint(value)
Program.kt

test(foo(10), foo(20))
test(bar(10), bar(20))
test(foo(50), foo(70))
test(bar(50), bar(70))
}
Program.kt

test(foo(10), foo(20)) //30
test(bar(10), bar(20)) //30
test(foo(50), foo(70)) //120
test(bar(50), bar(70)) //5
}
Program.kt

The Philosophy of Domain Driven Design

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