Mini Project: Task Manager (Part 2) - Methods & Error Handling

Let's think about this for a second

Part 1 used a function-based approach, but in Part 2 we'll refactor the code to be more solid using the receiver method pattern you learned in the Methods chapter. We'll turn the tasks slice into a field of a TaskManager struct, and rewrite Add, List, Complete, and Remove as methods on that struct. Since Complete and Remove need to handle what happens when an ID is wrong, we'll put the error return pattern from the Errors chapter into practice. By the end of this stage, the project will follow a more proper Go design, combining struct, methods, and error handling.

Let's build it for real

Define a TaskManager struct with a single tasks []Task field. Using a pointer receiver (m *TaskManager), write four methods: Add(title string), List(), Complete(id int) error, and Remove(id int) error. In both Complete and Remove, use a for loop to find the ID, and if it's not found, return fmt.Errorf("task %d not found", id). In Remove, use the slice trick (append(tasks[:i], tasks[i+1:]...)) to drop the task from the list. In main(), call Complete/Remove, check the error with if err != nil, and print it.

• Wrote the TaskManager struct and 4 pointer-receiver methods (Add, List, Complete, Remove)
• Complete/Remove methods return an error for an invalid ID
• Checked error handling in main() using the if err != nil pattern

Code Example

package main

import "fmt"

type Task struct {
	ID    int
	Title string
	Done  bool
}

type TaskManager struct {
	tasks []Task
}

func (m *TaskManager) Add(title string) {
	t := Task{ID: len(m.tasks) + 1, Title: title}
	m.tasks = append(m.tasks, t)
}

func (m *TaskManager) List() {
	for _, t := range m.tasks {
		status := "[ ]"
		if t.Done {
			status = "[x]"
		}
		fmt.Printf("%s %d. %s\n", status, t.ID, t.Title)
	}
}

func (m *TaskManager) Complete(id int) error {
	for i := range m.tasks {
		if m.tasks[i].ID == id {
			m.tasks[i].Done = true
			return nil
		}
	}
	return fmt.Errorf("task %d not found", id)
}

func (m *TaskManager) Remove(id int) error {
	for i, t := range m.tasks {
		if t.ID == id {
			m.tasks = append(m.tasks[:i], m.tasks[i+1:]...)
			return nil
		}
	}
	return fmt.Errorf("task %d not found", id)
}

func main() {
	manager := &TaskManager{}
	manager.Add("Learn Go basics")
	manager.Add("Build task manager")

	if err := manager.Complete(1); err != nil {
		fmt.Println("Error:", err)
	}
	if err := manager.Remove(5); err != nil {
		fmt.Println("Error:", err)
	}
	manager.List()
}

After completing task 1, trying to remove ID 5 will print "Error: task 5 not found".