Category theory in scala. This deals with basics of Category Theory and its relation with Scala.

1. Category Theory In Scala
Meetu Maltiar
Principal Consultant
Knoldus

2. Agenda
Category Theory the basics
Category Theory in Scala
Code Examples

3. Category
Pierce defines Category as comprising of:
● A collection of objects
● A collection of arrows (often called morphisms)
● Operations assigning to each arrow f an object dom f, its
domain, and an object cod f, its codomain (f:A → B, where
dom f = A and cod f = B)

4. Category continued..
A composition operator assigning to each pair of arrows f and g with cod f
= dom g, a composite arrow g o f: dom f → cod g, satisfying the following
associative law:
for any arrows f:A → B, g:B → C and h:C → D
h o (g o f) = (h o g) o f
For each object A, an identity arrow idA: A → A satisfying the following
identity law:
for any arrow f:A → B, idB o f = f and f o idA = f

5. Category
Apples Fruit
Banana Vegetable
Brinjal
f: A → B

6. Category Composition
Apples Fruit
Living
Banana Vegetable
Non Living
Brinjal
g o f: A → C

7. Translating to Scala
We can define any function from a type A to type B as:
A => B here we have an example or morphism
For any function val foo: A => B = //Anything
We have type A as domain of function foo and type B as co-domain
of function foo

8. Scala Category Example
We can define composition of arrows or functions in Scala with
following REPL example
scala> val f: Int => String = _.toString
f: Int => String = <function1>
scala> val g: String => Int = _.length
g: String => Int = <function1>
scala> f compose g
res3: String => String = <function1>

9. Scala category identity continued
Identity law is a special version of a composition. Let's define function and play
them in REPL:
scala> val foo: Int => String = _.toString
foo: Int => String = <function1>
scala> val idInt: Int => Int = identity(_: Int)
idInt: Int => Int = <function1>
scala> val idString: String => String = identity(_: String)
idString: String => String = <function1>
scala> idString compose foo
res4: Int => String = <function1>
scala> foo compose idInt
res5: Int => String = <function1>

10. Category theory and PL
Do understanding Category Theory makes us understand PL's better?
If we just do Enterprise software development and do not want to go beyond
our comfort zone then answer is no.
Category Theory provides uniform model of set theory, algebra, logic and
computation.
Many concepts of Category theory maps nicely to structures of PL.
Categorical reasoning helps us to reason about programs. Some basic
structures in PL like product and sum types have their correspondences in
Category Theory.
There is a strong correspondence between typed lambda calculus and
cartesian closed categories

11. Properties of data type in CT
Consider the category of Products of elements. Take for example
of cartesian products from the category of sets.
A cartesian product of two sets A and B is defined by
A X B = {(a, b) | a belongs to A and b belongs to B}
For example if A = {1, 2} and B = {A, B}
A X B = {(1, A), (1, B), (2, A), (2, B)}
So here we have tuples or Pairs as objects in a Category

12. Properties of data type in CT ..
But what will be morphisms?
In case of products, the applicable arrows (or morphisms)
are the projection functions:
π1: A X B → A
π1: A X B → B

13. Properties of data type in CT ..
Now if we draw a Category diagram with C as a product type we
have two functions as projection functions:
1. f: C → A
2. g: C → B
and the product function is represented by: C → A X B It is defined
by:
<F, G>(x) = (f(x), g(x))

14. Diagram of CT
C
f g
! [f,g]
A AXB B
π1 π2

15. Diagram of CT
For every pair of vertices X and Y, all paths in the diagram from X
to Y are equal in the sense that each path forms an arrow and
these arrows are equal in category
For example the path from C to A is: <f,g> and π1 therefore
composition gives us:
π1 o <f,g> = f
Also path from C to B gives us: <f,g> and π2
π2 o <f,g> = g

16. Scala CT example
Lets now see how the laws of commutativity maps to Scala.
As a programmer we use the projection functions (_1 and _2) in
Scala Tuple2 on a regular basis
In the CT diagram we will see that we get additional insights in
abstraction and help understand mathematical properties of how
cartesian product of sets map translates to composition of
functions

17. Scala CT example...
scala> val ip = (10, "meetu")
ip: (Int, java.lang.String) = (10,meetu)
scala> val pi1: ((Int, String)) => Int = (p => p._1)
pi1: ((Int, String)) => Int = <function1>
scala> val pi2: ((Int, String)) => String = (p => p._2)
pi2: ((Int, String)) => String = <function1>
scala> val f: Int => Int = (_ * 2)
f: Int => Int = <function1>
scala> val g: Int => String = _.toString
g: Int => String = <function1>
scala> val `<f, g>`: Int => (Int, String) = (x => (f(x), g(x)))
<f, g>: Int => (Int, String) = <function1>
scala> pi1 compose `<f, g>`
res0: Int => Int = <function1>
scala> pi2 compose `<f, g>`
res1: Int => String = <function1>

18. Scala CT example
So we can claim from the commutativity of the diagram that:
pi1 compose `<f, g>` is type wise equal to f
pi2 compose `<f, g>` is type wise equal to g
Category theory says that morphism between C and A X B is
unique and that A X B is defined upto isomorphism.
Uniqueness is denoted by ! In diagram. This makes sense as well
because a pair can be unique

19. Interface driven modeling
Category Theory maps very closely to PL in the sense that it
focuses on arrows rather than objects corresponding to Interfaces
Pierce: CT typically “abstracts away from elements, treating
objects as black boxes with unimagined internal structure and
focusing attention on the properties of arrows between objects”
Learning CT enriches both CT and PL. For example if we know
what a Functor is in CT then we can easily make it generic enough
so that it can interact with other Functors

20. Thinking Generically
CT talks about objects and morphisms and how arrows compose.
A special kind of morphism is identity function in programming.
It is 0 in addition and 1 in multiplication
CT generalizes this concept by using same vocabulary (morphism)
to denote both stuff that does some operations and those that don't
For every object X, there exists a morphism idX: X → X called
identity morphism on X, such that for every morphism f: A → B we
have:
IdB o f = f = f o idA (used in monoids)

21. Duality
We have seen example of SumTypes for Product. If we look at the
CT diagram of Sum compared to Product we see that arrows are
reversed.
This is called a dual in CT. In Scala we model it by a Union Type
like Either where value of SumType comes either from left or right

22. What's Next
Functors
Monads
Scalaz
Resources:
1. Debasish Ghosh blog “Does Category Theory Makes you A
Better Programmer”
2. Heiko's blog on Category Theory
3. Runar video on Scalaz