Lecture 12
Category Theory
Proposed title: Do You Even Lift?
...allows one to see the forest rather than the individual trees, and offers the possibility for study of the structure of the entire forest, in preparation for the next stage of abstraction  comparing forests.
Roots in algebraic topology in the early 1940s by Eilenberg and Mac Lane
It provides a powerful language.
It can translate difficult problems to easy ones.
It features strong abstractions and attempts to unify separate ideas.
A continuous function from a unit circle to itself must have a fixed point.
(Image © Jack E.)
If you crumple up one sheet of paper and place it on top of a flat sheet, then at least one point will be directly over it's corresponding point.
After sloshing around a cup of coffee, at least one point will always remain in the same spot.
Brouwer's Theorem assures existence of solutions to some differential equations.
Brouwer's Theorem assures existence of equilibria in Game Theory.
Let D be the unitcircle disk.
Let S be the surface of the unitcircle.
Lemma: There is no continuous function
h: D → S that leaves each point on S fixed.
Using a Functor, we can transform one Category to another by preserving identities and compositions.





By instead examining the Category of Groups, we can conclude that no such group homomorphism g: 0 → Z can exist.
We have proved the Lemma.
GIVEN: Lemma: There is no continuous function
F: D → S that leaves each point on S fixed.
SHOW: A continuous function f: D → D must have a fixed point
...and now, THE DROP!
By way of contradiction, assume ∀x∈D: x ≠ f(x)
Then we can always form the function F: D → S that leaves each point on S fixed.
Contradiction. GG.
A Category C is a collection of Ob(C) and Ar(C).
Ob(C) are the objects of C. Ar(C) are the "arrows" or morphisms of C.Each f:A→B ∈ Ar(C) has it's A and B chosen from Ob(C).
If f:A→B and g:B→C, then there always exists h = g∘f: A→C For every A ∈ Ob(C), there is an identity function id_{A}: A→A.Left and right identity: f∘id_{A} = id_{A}∘f
Associativity: h∘(g∘f) = (h∘g)∘f
Ob(Hask) = the Haskell types. (Bool, [Char], ...)
Ar(Hask) = the Haskell functions. (head, not, ...)
The identity function is id :: a > a
The axioms are satisfied
Left and right identity: f∘id = id∘f
Associativity: h∘(g∘f) = (h∘g)∘f
Functor F: C→D is a transformation from Category C to Category D
It maps objects in C to objects in D, and functions in C to functions in DFunctor Axioms:
In Haskell, a Functor is a typeclass for things that can be mapped over.
Prelude> fmap odd (Just 3)  Maybe is a Functor
Just True
Prelude> fmap odd [1..5]  a list is a Functor
[True,False,True,False,True]
Maybe
Functor1. The type constructor transforms anything of type a
to Maybe a
Like transforming an object in C to an object in D.
Maybe
derives the Functor
typeclass as follows:
instance Functor Maybe where
fmap f (Just x) = Just (f x)
fmap _ Nothing = Nothing
2. fmap transforms a function f: a→b
to Maybe a → Maybe b.Maybe
Functor (Cont.)We just showed that Maybe transforms objects and functions over from the Hask category to the Maybe subcategory.
The Functor Axioms are satisfied:
A Monad is a functor from a Category to itself: M: C→C
And, for every X ∈ Ob(C)
unit: X→M(X)
join: M(M(X))→M(X)
class Functor m => Monad m where
return :: a > m a
(>>=) :: m a > (a > m b) > m b
Prelude> import Control.Monad
Prelude Control.Monad> :t join
join :: Monad m => m (m a) > m a
join ∘ M(join) = join ∘ join
Collapsing the inner two layers first, then that with the outer layer is exactly the same as collapsing the outer layers first, then that with the innermost layer.
join ∘ M(unit) = join ∘ unit = id
Applying return to a monadic value, then joining the result should have the same effect whether you perform the return from inside the top layer or from outside it.
Category theory powers Haskell's generalizability.
Homework: Fill out the Course Evaluation