How to Perform Function Iteration in Golang

The challenge

The purpose of this challenge is to write a higher-order function returning a new function that iterates on a specified function a given number of times. This new function takes in an argument as a seed to start the computation.

For instance, consider the function getDouble. When run twice on value 3, yields 12 as shown below.

getDouble(3) => 6
getDouble(6) => 12

Let us name the new function createIterator and we should be able to obtain the same result using createIterator as shown below:

var doubleIterator = createIterator(getDouble, 2); // Runs *getDouble* twice
doubleIterator(3) => 12

For the sake of simplicity, all function inputs to createIterator would function returning a small number and the number of iterations would always be integers.

The solution in Golang

Option 1:

package solution
func CreateIterator(fn func(int) int,n int) func(int) int {
  return func (i int) int {
    for j := 0; j < n; j++ { i = fn(i) }
    return i
  }
}

Option 2:

package solution
func CreateIterator(fn func(int) int,n int) func(int) int {
  if n == 0 {
    return func(x int) int { return x } // id
  } else {
    return func(x int) int { return CreateIterator(fn, n-1)(fn(x)) }
  }
}

Option 3:

package solution
func CreateIterator(fn func(int) int,n int) func(int) int {
    if n < 1 {
      return nil
    }
    var retFunc = func(r int) int {
      for i := n ; i>0 ; i-- {
        r = fn(r)
      }
      return r
    }
    return retFunc
}

Test cases to validate our solution

package solution_test
import (
    . "github.com/onsi/ginkgo"
    . "github.com/onsi/gomega"
)
var _ = Describe("Iterator for 'getDouble' function",func() {
    var getDouble = func(n int) int {return 2*n}
    It("Running the iterator once",func() {
        doubleIterator := CreateIterator(getDouble,1)
        Expect(doubleIterator(3)).To(Equal(6))
        Expect(doubleIterator(5)).To(Equal(10))
    })
    It("Running the iterator twice",func() {
        getQuadruple := CreateIterator(getDouble,2)
        Expect(getQuadruple(2)).To(Equal(8))
        Expect(getQuadruple(5)).To(Equal(20))
    })
})