### Kleisli Categories

NOTE: I have decided to skip the Haskell portion of this chapter being Section 4.2 as there was not a full implementation of the Writer Monad and Kleisli Category examples provided. As I am not an expert in Haskell, I have decided to move forward to focus on the math over Haskell. If you have an implementation, feel free to open a PR on this repo!

What is an example of a non-pure function?

global logger = ""

function negate(x::Bool)
global logger = "Negated!"
return !x
end

negate (generic function with 1 method)


This function also modifies logger as a side effect and as such negate is a non-pure function.

What is the Writer Category?

The Writer Category is a category that allows one to track the execution of functions.

What are examples of the Writer Category involving Strings?

function toUpper(str::String)::Vector{Union{String, String}}
return [uppercase(str), "toUpper "]
end

toUpper (generic function with 1 method)

words(w::String)::Vector{Union{Vector{String}, String}} = [string.(split(w)), "toWords "]
toWords(w::String)::Vector{Union{Vector{String}, String}} = words(w)

toWords (generic function with 1 method)

function process(sentence::String)::Vector{Union{Vector{String}, String}}
p1 = toUpper(sentence)
p2 = toWords(first(p1))
return [first(p2), p1 .* p2]
end

process (generic function with 1 method)


What are examples of the Writer Category involving Numerics?

isEven(x::Int)::Vector{Union{Bool, String}} = [x % 2 == 0, "isEven "]
negate(x::Bool)::Vector{Union{Bool, String}} = [!x, "Negated "]

negate (generic function with 1 method)

function isOdd(x::Int)::Vector{Union{Bool,String}}
p1 = isEven(x)
p2 = negate(first(p1))

return [p2, p1 * p2]
end

isOdd (generic function with 1 method)


What is a generic composition function of the Writer Category?

function compose(f::Function, g::Function)
function _compose(x; f = f, g = g)
p1 = f(x)
p2 = g(p1)

return [p2, p1 * p2]
end
end

compose (generic function with 1 method)


What is a generic identity function of the Writer Category?

function identity(x)
return [x, ""]
end

identity (generic function with 1 method)


What are Kleisli Categories?

A working definition is that it is a category that has as objects the types of the underlying programming language.

What are the characteristics of Kleisli Categories?

• It is based on the writer monad

• Its objects are the types of an programming language

• Kleisli categories define their own compositions

What are examples of morphisms in the Kleisli Category?

An arrow from some type $A$ to some type derived from $B$.

A writer monad is used for logging or tracing the execution of functions. General process to embed effects in pure computations.

They log or trace the execution of functions as well as execute a procedure.

What is a partial function?

A function that is not defined for all possible values of its argument.

### Challenge

1. Construct the Kleisli category for partial functions (define composition and identity).

function compose(f::Function, g::Function)
function _compose(x; f = f, g = g)
p1 = f(x)
p2 = g(p1)

return [p2, p1 * p2]
end
end

compose (generic function with 1 method)

function identity(x)
return [x, "" * x]
end

identity (generic function with 1 method)

1. Implement the embellished function safe_reciprocal that returns a valid reciprocal of its argument, if it’s different from zero.

function safe_reciprocal(x)
x != 0.0 ? [1.0 / x, "Safe Reciprocal "] : [Real[], "Safe Reciprocal "]
end

safe_reciprocal (generic function with 1 method)

1. Compose the functions safe_root and safe_reciprocal to implement safe_root_reciprocal that calculates sqrt(1/x) whenever possible.

function safe_root(x)
x >= 0.0 ? [sqrt(x), "Safe Root "] : [Real[], "Safe Root "]
end

safe_root (generic function with 1 method)

function safe_root_reciprocal(x)
p1 = safe_root(x)
p2 = identity(safe_reciprocal(p1))

return [p2, p1 * p2]
end

safe_root_reciprocal (generic function with 1 method)