Implementing an Iterator in Go

I was asked to implement an iterator in python for an interview once. For a little quick practice I thought it might be interesting to try the same question in Go.Below is the simplest possible implementation just to get things started. Then we can add some simple features to it as we go.

package main

import (
	"fmt"
)


// Create a Counter type
// that we can later implement Iterator interface on.
type Counter struct {
	Value int
}

// Iterator interface requires one method with no params
// and no return value.
type Iterator interface {
	next()
}


// Implementing the Iterator interface on Counter.
// calling next will mutate Value and add one.
func (c *Counter) next() {
	c.Value++
	return c
}


func main() {
	counter := Counter{Value: 0}
	counter.next()
	// Wow Counter.Value is now 1. Hoorray.
	fmt.Println(counter)
}

Another example, this time with a second type called Doubler implementing Iterator to demonstrate the usefulness of the Interface.

package main

import (
	"fmt"
)

// Create a Counter type
// that we can later implement Iterator interaface on.
type Counter struct {
	Value int
}

// Create a Doubler type
// that we can later implement Iterator interaface on.
// With a different behavior from Counter.
type Doubler struct {
	Value int
}

// Iterator interface requires one method with no params
// and no return value.
type Iterator interface {
	next()
}

// Implementing the Iterator interface on Counter.
// calling next will mutate Value and add one.
func (c *Counter) next() {
	c.Value++
}

// Implementing the Iterator interface on Doubler.
// calling next will mutate Value and double it.
func (c *Doubler) next() {
	c.Value = c.Value * 2
}

// Iterate function that can iterate any iterator
// We can use this on Doubler or Counter or any future type we implement next() on
func iterate(i Iterator) {
	i.next()
}

func main() {
	counter := Counter{Value: 0}
	doubler := Doubler{Value: 2}
	// You need to pass a pointer because next()
	// has a pointer receiver.
	iterate(&counter)
	fmt.Println(counter)
	iterate(&doubler)
	fmt.Println(doubler)
}

Now what if we wanted to chain calls to next for some reason? We can make the next() method on interface return an Iterator. Then update the implementation to return its self.

package main

import (
	"fmt"
)

// Create a Counter type
// that we can later implement Iterator interaface on.
type Counter struct {
	Value int
}

// Create a Doubler type
// that we can later implement Iterator interaface on.
// With a different behavior from Counter.
type Doubler struct {
	Value int
}

// Iterator interface requires one method with no params
// To have types calling next() return themselves
// we can have a method signature that returns an Iterator..
// Even from inside the interface definition.
type Iterator interface {
	next() Iterator
}

// Implementing the Iterator interface on Counter.
// calling next will mutate Value and add one.
// It will now also return its self cast as an Iterator.
func (c *Counter) next() Iterator {
	c.Value++
	return c
}

// Implementing the Iterator interface on Doubler.
// calling next will mutate Value and double it.
// It will now also return its self cast as an Iterator.
func (c *Doubler) next() Iterator {
	c.Value = c.Value * 2
	return c
}

// Iterate function that can iterate any iterator
func iterate(i Iterator) Iterator {
	// Now we can chain calls to next.
	i.next().next()
	return i
}

func main() {
	counter := Counter{Value: 0}
	doubler := Doubler{Value: 2}
	// You need to pass a pointer because next()
	// has a pointer receiver.
	iterate(&counter)
	fmt.Println(counter)
	// next also works on the return value of iterate
	// because it is also an Iterator
	iterate(&doubler).next()

	// Notice the output for doubler has been doubled
	// three times. Twice inside the iterator function
	// and once more when we chained a call to its return value.
	fmt.Println(doubler)
}