The lines of code below:

type MathExp = string

const (
	addExp = MathExp("add")
	subExp = MathExp("subtract")
	mulExp = MathExp("multiply")
	divExp = MathExp("divide")
)

Is simply a refactor of:

const addExp = "add"
const subExp = "subtract"
const mulExp = "multipy"
const divExp = "divide"

We need the codes above to be able to compare with the cases in the switch statement of the mathExpression function:

func mathExpression(exp MathExp) func(float64, float64) float64 {
	switch exp {
	case addExp:
		return simplemath.Add
	case subExp:
	 	return simplemath.Subtract
	case mulExp:
		return simplemath.Multiply
	case divExp:
	 	return simplemath.Divide
	default:
		return func(f1 float64, f2 float64) float64 {
			return 0
		}
	}
}

These codes simply return a function which takes 2 float64's and returns a float64 value based on the case.

In the fmt.Printf function of the main function, we call the double function. as an parameter along with two numbers and performs specified maths operations.

func main() {
	fmt.Printf("%f\n", double(3, 2, mathExpression(addExp)))
	fmt.Printf("%f\n", double(3, 2, mathExpression(mulExp)))
}

The double function takes the mathExpression as parameter alongside two float64 numbers. The double function doubles the outcome of the specified mathematical expression in the mathExp and then returns the result.

func double(f1, f2 float64, mathExp func(float64, float64) float64) float64 {
	return 2.0 * mathExp(f1, f2)
}