The shape of a functor

A functor is an algebraic type that accepts a value (or usually, a list of values) and has a map function that applies to each element in the list to produce a new functor of the same shape. What is a shape?

Let's look at an imperative example:

ints := []int{1,2,3}
impInts := []int{}
for _, v := range ints {
   impInts = append(impInts, v + 2)
}
fmt.Println("imperative loop:", impInts)

Here's the output:

imperative loop: [3 4 5]

The shape in this example means a slice with three ints. We started with a slice with three ints, ran our imperative code, and ended up with a slice with three ints.

A functor gets the same results (three elements in and three elements out) but a functor does it in a different way.

We give our functor the same slice of three ints. The functor executes add2 for each int and returns a slice with three ints (each of which is two greater than before):

add2 := func(i int) int { return i + 2 }
fpInts := Functor(ints).Map(add2)
fmt.Println("fp map:", fpInts...