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--- go:love_of_go [2025/10/15 14:14] – [TESTING] v1ctor
+++ go:love_of_go [2025/11/18 10:39] (current) – [ERRORS] v1ctor
@@ Line 83: / Line 83: @@
 </code>
-===== ERRORS =====
+==== ERRORS ====
-Creating a custom error:
+Go has a type to communicate errors - **error**. Example usage:
 <code go>
-return 0, errors.New("division by zero not allowed")
+func (book *Book) SetCopies(copies int) error {
+  if copies < 0 {
+    return fmt.Errorf("negative number of copies: %d", copies)
+  }
+  book.Copies = copies
+  return nil
+}
+</code>
+<code go>
+err := book.SetCopies(-1)
+if err != nil {
+  fmt.Println("Oh dear, something went wrong:", err)
+}
 </code>
-===== VARIABLES =====
+==== VARIABLES ====
 Go assigns a default **zero** value to any variable, that is declared but not assigned a value.
@@ Line 108: / Line 121: @@
-===== SLICES =====
+==== SLICES ====
 Slice can be declared as a **nil** or as an **empty** slice:
@@ Line 125: / Line 138: @@
 </code>
-===== MAPS =====
+**Sorting a Slice of Struct**
+<code go>
+type Person struct {
+  Name string,
+  Age int,
+}
+people := []Person{
+  {"Alice", 25},
+  {"Bob", 30},
+}
+slices.SortFunc(people, func(a,b Person) int {   // slice.SortFunc - generic function for sorting slices
+  return cmp.Compare(a.Age, b.Age)               // with a custom comparison.
+})                                               // cmp.Compare - a helper that returns -1, 0, or 1
+</code>
+==== MAPS ====
 Map is a reference type - when we pass it to the function, we are passing a reference, not a copy.
@@ Line 182: / Line 213: @@
 book, ok := catalog["some-key]"]   // maps support "comma, ok" pattern. ok == false if key is missing
 </code>
-===== STRUCTS =====
+==== STRUCTS ====
 Empty literal:
@@ Line 204: / Line 236: @@
 Instead, we can create a **struct** type, with a field of type we want.
 ==== COMMA, OK PATTERN ====
@@ Line 218: / Line 251: @@
   return Book{}, false
 }
+</code>
+==== POINTERS ====
+<code go>
+x := 5
+y := &x           // `y` is a pointer to `x`. `&` is an address operator
+fmt.Println(*y)   // *y dereferences y - it retrieves the value that y “points” to
+</code>
+==== OBJECTS ====
+In order to create a Method, we need to specify the **receiver** parameter. It tells Go that this method should be called on a specific object value, and that value is available inside the method:
+<code go>
+func (book Book) String() string {            // (book Book) - method's receiver
+  return fmt.Sprintf("%v by %v (copies: %v)",
+    book.Title, book.Author, book.Copies)
+}
+</code>
+There are two types of receivers - **Value** and **Pointer** receivers. Think of it as //how the function gets access to the object://
+  * By Copy (value receiver) - this **copies the data** into the method
+  * By Value (pointer receiver) - this gives the method a **reference to the original object**
+<code go>
+func (book Book) SetCopies(copies int) {
+  book.Copies = copies                    // only affects local `book`, not the original
+}
+</code>
+And the //pointer// example:
+<code go>
+func (book *Book) SetCopies(copies int) {
+	book.Copies = copies               // Go provides an automatic de-referencing when we use
+                                           // pointers to struct. Pointers can't have fields, so there
+                                           // is no ambiguity here. Otherwise it would look like:
+	                                   // (*book).Copies = copies
+}
+book := books.Book{
+	Copies: 5,
+}
+book.SetCopies(12)                         // That's here, when we pass `value` or `reference` to
+                                           // the method
 </code>