Functional Programming in Go

In programming world, there are basically two types of functions: Pure Functions and Higher-Order Functions.

Pure Functions (first order function)

Function only operate on the parameters; no side effects</br> Predictable, easy to reason about</br> eg:

var DB *sql.DB

func myHandler(w http.ResponseWriter, r *http.Request) {
    res, err := DB.Exec(...)
}

Higher-Order Function

A Higher-Order Funciton is a function that takes a function as an argument, or returns a function.</br> eg:

func myHandler(db *sql.DB) http.HandlerFunc {
    return func(w http.ResponseWriter, r*http.Request) {
        res, err := db.Exec(...)
        //...
    }
}

Functor

Functors are objects that can be treated as thought they are a function or function pointer.

Example

Transforming a Slice

ints := []int{1,2,3,4,5,5,7}
results := make([]int, len(ints))

for i, elt := range ints {
    results[i] = DoSomething(elt)
}

func DoSomething(num int) int {
    return num + 100
}

We can transform above code with functors to abstract the for-loop:

ints := []int{1,2,3,4,5,6,7}
results := magical(ints).Map(DoSomething)

Which magical() makes a Functor that returns a IntSliceFunctor

• this functor is a Go struct that contains the data (i.e: slice)
• Map function that operates on the container

Map(fn func(int) int) IntSliceFunctor