# 群论笔记 09

## Cat 9.1 Natural transformations

3 most important things in category: 1. definition of category 2. definition of functor 3. definition of natrual transformation

the most most important is (3), (1)(2) is just nessary background knowledge.

category - structure functor - preserve the structure, model a cat in another cat.

compare diff image given by the functors.

natural trans are mappings between functors, — alse preserve the structure.

take into these 2 functor's structure and presever.

so ,what is mapping between functors( remember that, functor is a type constructor with fmap inside )

1. for objects in Cat C Figure 1: mapping between 2 functors

for other object in C I do the same thing. I find the images of this object and pick a morphism between them.

then I create a whole family of morphisms, these individual morphism is called the component of the natural transformation, — component of /alpha at object of a, notice that object a is in the Cat C, but component /alpha is in Cat D. Figure 2: component of natrual trans

2. for morphism in Cat C

is there some relation between the Ff and Gf?

[notice] natrual trans is not have to exist between any functors.

we are now pay more attention to the 2 kinds of morphism in Cat D: * the component of NT(relation between two type constructor of same object in C) - in picture below, /alpha\_a : Fa->Ga and /alpha\_b : Fb->Gb * the mophism of original Cat and its mapping in target Cat - in picture below, Ff and Gf Figure 3: the 'beautiful' Rectangle

Natural transformation describe a mapping between 2 functors, means these two functors are somehow similar, somehow related.So there must be some relations between these 4(2 components and 2 morphism mappings) — THEY ARE COMPOSABLE

`α_b * Ff = Gf * α_a`

is called Naturality conditoin, and this rectangle is called Naturality Square.

α\_b and α\_a is called Natural Transformation

### How Strong is the naturality is ?

that's depends very much on the structure of the category.In some categories there are lots of morphisms between objects like set — morphism rich category.

If we draw these 4 ojbects( set, type constructor ): Fa, Fb, Ga, Gb Figure 4: 4 type costructors

### Natural trans vs. commutative graph

In mathematics, we often say that: from Cat C, we can find the object a can be mapping to a morphism of Cat D; and similarly, morphism of Cat C can be mapping to a Naturality Squre in Cat D.

natural transformation can be something like higher-level thant the commutative graph. So you just can say "there is a Natural trans between 2 functors", then this means that "there are some commutative graphs,these Naturality squares"

and we will have a second semaster lectures, introducing "limit, colimit" — defined on the basis of Natural trans, of course they also can be defined by the commutative graphs. But Natural Trans is a higher-level 'language' to describe. And we can find that product and coproduct are just special cases of limit and colimit.

components of a Natural Trans are morphisms, and you know that morphisms are functions, function can shrunk information — can lose information. If you map one functor into another functor using a natural trans, there maybe lot of information will lost. Figure 5: Natural Trans will shrink information

Natural Trans acts like sampling from a high resolutin picture, and get a low resolution picture.

### Natural Transformation can invertibel

seriously :| then you find that: you have some mapping, and you have a invertible mapping:

• function and invertible function will lead to a definition of isomorphism, and a isomorphic objects.
• Natural Trans and invertible Natural Trans must will lead to somethingisomorphic functors.

so natural transfomation, is also natural isomorphism.

Natural transformation is just a bunch of morphisms, that means there will be bunch of isomorphism.

Move further, means that all of components are invertible morphism, means every component is isomorphism.

Natural isomorphism are really important. Figure 6: Natural isomorphism

### Natural Trans in programming

Functor is mapping between categories, and in Haskell it's endofunctor mapping from ONE category to itself.

Natural transformation is a family of morphisms between two endofunctors.

Family of morphisms, morphism is functin, family of functions that parameterize by a type — which is called polymorphic function.

seriously :|

So,

naturally transformation is a polymorphic function

```alpha:: forall a. Fa -> Ga
```

Notice the difference between polymorphic function and ad-hoc polymorphism. * polymorphic function : apply one function for all type.

## ad-hoc polymorphism : (overriding) can apply different function for

different type.

Why I emphasize "apply one function for all types."? Figure 7: Natural Transform illustration

`fmap :: (a->b) -> (Fa->Fb)`

`alpha:: forall a. Fa -> Ga`

`α_b * Ff = Gf * α_a`

`α_b * fmap_F f = fmap_G f α_a` is the interpretation of the Naturality Condition.

Fb->Gb * Fa->Fb = Ga->Gb * Fa->Ga

but α can be applied for all types

or we can say: 1. because of parametric polyphism, Fa->Fb; Ga->Gb; - like some scala codes `List[Int].map() = List[Double]` or `Option[String].map() = Option[Int]` - every kind of `type constructor` will have its `own implementation` of `fmap` 2. because of polymorphic function, Fb->Gb; Fa->Ga - for all type a

```alpha:: forall a. Fa -> Ga
alpha * fmap f = fmap f * alpha
```

by the TWO KINDS OF POLYMORPHISM, the naturality condition is automatically satisfied

### example

```safeHead :: [a]->Maybe a //this is a F[a]->G[a] a NaturalTransformation
```

and it'll automatically find the different implemention of `fmap` by different invocator, this scenario `List` and `Maybe`. So the whole naturality condition will automatically satisfied. Figure 8: naturally condition

### How Natural Transformation save lots of computing time

This will give you a intuition that category theory can be used in programming in a very practical way. You see that from picture above, apply `fmap` to a `List` is very expensive. But if compiler knows natural transformation, then convert `fmap` of `List`, to `fmap` of a `Maybe`, this will save a lot of computing resource

### Intuition of functor

So we know that why call a functor a container, because fmap apply to a container will ONLY modify content inside of the container, it will NEVER change the shape of container.

### Intuition of natural transformation

On the opposite side, the natural trans NEVER modify the contents of the container, what it ONLY dose is change the shape of the container, repackages the container.

`fmap :: (a->b) -> (Fa->Fb)`

`alpha:: forall a. Fa -> Ga`

### The ubiquitous of natural transformation

we use polymorphic function a lot, is it a natural transformation?

### `fn:: a->[a]`

function from `a` to `List[a]`, yes it's a natural transformation:

• type a to type a: is just a identity functor on a

so `fn:: a->[a]` is truely a natural transformation

### `fn:: [a]->Int`

Like computing the length of a List, it's also a natural transformation, because the `a` is constant functor, it will ignore its type argument.

So, in general if you have a polymorphic function from an `ADT` to another `ADT`, it's a natural transformation.

because ADT as showed before, are functors.

## Cat 9.2 bicategories

Giving you a large large view of category, as far as professor can do. Diving into deep math.

### Natural transformation composition Figure 9: multi-natural transformation

before that we must check the Naturality condition:

`Hf * (β * α_a) ?` (β * α_b) * Ff=

Diagram chasing, you must make sure all the Naturality condition satisfied.

### Identity Figure 10: Identity natural transformation

### Composition

of course, NT is bunch of morphism, morphism of course is composable, so dose the NT.

But, hold on, what are we talking about? Mappings of Functors, and Functor is mapping of 2 category.

*when refer to a mapping of some thing, you must ask yourself, can I take this "something" back to the basic concept of "object" of category.*

In this scenario, yes we can:

• NT -> morphism
• functor -> object
• ??? -> category

??? = [C, D] Figure 11: [C,D] = D\^C

So this scenario: * NT is morphism; * functor is object;

remeber "Category of Category" is a "Cat". remeber that, * functor is the morphism of Cat. * category is the object of Cat.

### Cells and 2-category

2-category : has move further one step than "Cat"—category of category, which is the pair category as object.

Hom-set of Cat is set of Functors; Cat often think as a Category of small Category.

Hom-set are trully a set. And then set can be seen as a Category, then we get [C,D] — 2-category.

but wait a minite, in the Category of Category, we have 2 objects: C and D, then [C,D] must also be in the same category with C and D. So: Figure 12: [C,D] in same Cat with C and D

something like the set: * set = {a,b,c,d} * a ∈ set * c ∈ set * {a,c} ∈ set

### Exponential ? Figure 13: [C,D]

but functor of category, is a exponential ADT,

?????? why [C,D] = D\^C Figure 14: why [C,D] = D\^C

by the partial function, we can see some glue of [C,D] = D\^C, this should be marked as TODO, I ignore the details of this, when this taught at previous lectures, need reviewing.

categorical products as pairs of elements and categorical products with universal construction and projections, they will give you the same result.

the category of functor category[C,D] is same as the D\^C, we start with a product category CXD,and then do the universal construction and we get the exponential objects.

Can we build something instead hom-sets of hom-objects

vertical composition Figure 15: vetical composition

Is there the natural trans between G * F and G'*F' Figure 16: G/F and G'/F' Figure 17: Natural trans between G/F and G'/F'

and it's called `β * α`.

### Interchange law

horizon composition and vertical compoisition Figure 18: composition of vertical and relation with horizon composition

skipped, too long to draw

but definition of 2-category involves both vertical composition and horizon composition of two cells ,and the interchanging law.

we can get isomorphism from the vertical composition ??? They are called left and right unitor.

### 2-category vs. bicategory

So, if we start defining the stuff up to isomorphsim, then suddenly from a 2-category we are getting into something called bicategory.

the diff between 2-category and bicategory: * in a bicategory categorical laws are up to isomorphism. * 2-category is more strict than bicategory, becasue categorical laws is equale * bicategory is relaxed version of 2-category.

### coherence law

can not understand, so skipped!!