Drill 18: Write a test — Structs and interfaces

type Lesson struct {
    Slug  string
    Title string
}

type LessonStore interface {
    Find(slug string) (Lesson, error)
}

type MemoryStore struct {
    lessons map[string]Lesson
}

func (store MemoryStore) Find(slug string) (Lesson, error) {
    lesson, ok := store.lessons[slug]
    if !ok {
        return Lesson{}, ErrNotFound
    }
    return lesson, nil
}