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# Category Theory

This is the first in a series of presentations designed to summarize the somewhat esoteric "Category Theory" of Computer Science research for Developers. These presentations are not definitive but try to show how CT can be used in daily development.
by

## Kevin Quick

on 14 February 2017

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#### Transcript of Category Theory

Quick Category Theory Intro for Software Developers, part 1
Quick Category Theory Intro for
Software Developers (1)

What is Category Theory and how does
it help me with day-to-day coding activities?
What is it?
Somewhat esoteric math study that is difficult to state in layman's terms, but as applied to computer science, it is roughly:

The study of functions and how they can be transformed and combined.
Function Domains
and Codomains
In Algebra, the domain of a function is the set of inputs it is valid for, and the range is the set of outputs it can generate.

In Category Theory, the Range is called the CoDomain.

For function composition, the range of the first function should match the domain of the second, or at least be a subset. If this isn't the case,
there will be trouble.
Function Classification
Describes Domain and CoDomain.
Referential Transparency
A function has no side effects (i.e. it is "pure").

It takes input (from its domain) and generates an output (in its codomain) and
does nothing else
.

It does not modify global variables, update databases, read stdin, write to stdout, or launch missiles.

Simple examples:

def f(x) = x * 2 // int-to-int
def g(x: Int): Int = x + 5

strlen(s: String): Int

Even polymorphic functions are morphisms (i.e. "generics", "templates", etc.)

Composition Notation
When combining two functions together, there is a special notation using a dot separator, where:

f(g(x)) = (f.g)(x)

[The dot is usually in the middle height-wise, but in ascii just a period is used.]

"f applied to the result of: g applied to x"

(right to left)
Domain Mismatch Example
f(x) = sqr_root(x)
g(x) = x * (-1)

f(64) = 8
(g.f)(64) = -8
(f.g)(64) = ??!

Injective
All inputs generate a
unique
output, but not all
possible
outputs. A monomorphism.

f(x) = x * 2 // injective
g(x) = abs(x) // no: abs(1) == abs(-1)

Implies that the function is reversible, but there may be more values in the output type than can be generated from the inputs.

h(x) = ord(x) // injective
Surjective
All possible outputs
can
be generated, although some outputs may be generated from
multiple
different inputs. An epimorphism.

f(x) = x * 2 // surjective
g(x) = abs(x) // not surjective (no negatives)
h(x) = sqr_root(x) // not surjective (no negatives)

cos(x) // surjective in CoDomain of -1..1

Not reversible.
Bijective
Every input value has exactly one output value (i.e. it is Injective and Surjective). An isomorphism.

Same number of inputs and outputs. Frequently reversible.

f(x) = x * 2 // bijective
g(x) = abs(x) // not bijective
cos(x) // not bijective (multiple inputs -> same output)
Uses?
Simplifies code (prevents violation of the "Area of Concern" rule).
Allows function composition.
Makes operations idempotent.
Allows operations to be temporally shifted (i.e. delayed or pre-computed as needed), which helps optimizers.
Databases and I/O are Useful!
Yes, they are. Any "real world" interaction is not referentially transparent, including:
Keyboard input
Database values
Disk I/O
Network traffic
Random numbers
All of these change the state of "stuff" outside the code.
Any function you can't call an infinite number of times is not referentially transparent).
So, which is it?
Keep the two separated!

Any non-referentially-transparent code should be separated out as independent functions which should have the minimum possible functionality.

Most changes occur in the referentially transparent code!

Testing
Testing referentially transparent code is easy!
No side effects, no external dependencies.
Just a set of inputs and outputs.
Same transformation.
Every time.

Testing
Injective: focus on inputs, for each input output must be unique.
Surjective: focus on outputs and verify surjections
Bijective: random sample testing useful, especially if reversible.
Some functions do not fit any of these 3 types.

f(x) = if (x < 0) 0 else (x mod 10)

Multiple inputs generate a single output (all negative values return 0), so not injective.

There is no input that can generate an output value > 10, so it is not surjective.
Category of Sets
A Class of objects is mapped to a new class of objects by a transformation, known as a "morphism".

A morphism is a function.
Composition
Single functions are easy. Putting them together is more interesting. This is called "composition".

def f(x) = x * 2
def g(x) = x + 5

Composing our two functions together:

(f.g)(x) = f(g(x)) = (x+5)*2
Why Use Composition?
Compiler optimization
f(x) = x * 2
(f.f) = (x * 2) * 2 = x * 4
Build complex systems from simple parts (maintains Separation of Concerns).
Increase both extensibility and re-useability.
Graphically
(sqr_root . strlen)
STRING
INTEGER
FLOAT
strlen
sqr_root
Full transcript