added comments

go/02-next-level/03-errors.go

@@ -5,23 +5,28 @@ import (
 	"fmt"
 )
 
+// a function that can fail, so it returns an error as the second return value
 func f(arg int) (int, error) {
 	if arg == 42 {
-
-		return -1, errors.New("can't work with 42")
+		// in case of an error, return the zero value for the result and a non-nil error
+		return 0, errors.New("can't work with 42")
 	}
 
+	// simply return the result and a nil error in the success case
 	return arg + 3, nil
 }
 
+// define some errors to use in makeTea
 var ErrOutOfTea = fmt.Errorf("no more tea available")
 var ErrPower = fmt.Errorf("can't boil water")
 
+// a function that can fail in multiple ways, so it returns an error as the second return value
 func makeTea(arg int) error {
 	if arg == 2 {
+		// return the error directly, so we can check for it later with errors.Is
 		return ErrOutOfTea
 	} else if arg == 4 {
-
+		// use %w to wrap the error, so we can check for it later with errors.Is
 		return fmt.Errorf("making tea: %w", ErrPower)
 	}
 	return nil
@@ -30,6 +35,7 @@ func makeTea(arg int) error {
 func main() {
 	for _, i := range []int{7, 42} {
 
+		//typical pattern for checking errors in Go: if the error is not nil, handle it and return or continue; otherwise, use the result
 		if r, e := f(i); e != nil {
 			fmt.Println("f failed:", e)
 		} else {
@@ -40,9 +46,10 @@ func main() {
 	for i := range 5 {
 		if err := makeTea(i); err != nil {
 
+			// use errors.Is to check for specific errors, even if they are wrapped
 			if errors.Is(err, ErrOutOfTea) {
 				fmt.Println("We should buy new tea!")
-			} else if errors.Is(err, ErrPower) {
+			} else if errors.Is(err, ErrPower) { // this error is wrapped, but errors.Is can still check for it
 				fmt.Println("Now it is dark.")
 			} else {
 				fmt.Printf("unknown error: %s\n", err)

go/02-next-level/04-closure.go

@@ -2,9 +2,11 @@ package main
 
 import "fmt"
 
-func intSeq() func() int {
-	i := 0
-	return func() int {
+// a closure is a function value that references variables from outside its body.
+// The function may access and assign to the referenced variables; in this sense the function is "bound" to the variables.
+func intSeq() func() int { //returns a function that returns an int
+	i := 0              // i is a variable that intSeq's function value will reference. It continues to exist even after intSeq returns.
+	return func() int { // the anonomous function is returned here. It is not executed yet.
 		i++
 		return i
 	}
@@ -12,12 +14,15 @@ func intSeq() func() int {
 
 func main() {
 
+	//functions are first class citizens in Go, so we can assign them to variables,
+	// pass them as arguments to other functions, and return them from functions.
 	nextInt := intSeq()
 
 	fmt.Println(nextInt())
 	fmt.Println(nextInt())
 	fmt.Println(nextInt())
 
+	//a second function value from intSeq, with its own i variable.
 	newInts := intSeq()
 	fmt.Println(newInts())
 }

go/02-next-level/05-generics.go

@@ -2,6 +2,9 @@ package main
 
 import "fmt"
 
+// a generic function that takes a slice of any type and a value of that type, and returns the index of the value in the slice, or -1 if it is not found.
+// The type parameters S and E are declared in square brackets before the function name, and they can be used in the function signature and body.
+// The constraint comparable means that the type E must support the == operator, which is necessary for comparing the value with the elements of the slice.
 func SlicesIndex[S []E, E comparable](s S, v E) int {
 	for i := range s {
 		if v == s[i] {
@@ -11,15 +14,21 @@ func SlicesIndex[S []E, E comparable](s S, v E) int {
 	return -1
 }
 
+// represents a linked list of any type T. The type parameter T is used in the definition of List and its methods,
+// and it can be instantiated with any type when we create a List value. The type any is in fact defined as
+// an empty interface. Thanks to duck-typing it can be used to represent any type, but it does not provide any
+// operations on the values of that type.
 type List[T any] struct {
 	head, tail *element[T]
 }
 
+// the generic element of the linked list
 type element[T any] struct {
 	next *element[T]
 	val  T
 }
 
+// adds an instance of the generic type to the list
 func (lst *List[T]) Push(v T) {
 	if lst.tail == nil {
 		lst.head = &element[T]{val: v}
@@ -30,7 +39,8 @@ func (lst *List[T]) Push(v T) {
 	}
 }
 
-func (lst *List[T]) AllElements() []T {
+// returns all elements of the list as a slice of the generic type.
+func (lst List[T]) AllElements() []T {
 	var elems []T
 	for e := lst.head; e != nil; e = e.next {
 		elems = append(elems, e.val)
@@ -39,14 +49,17 @@ func (lst *List[T]) AllElements() []T {
 }
 
 func main() {
-	var s = []string{"foo", "bar", "zoo"}
-	fmt.Println("index of zoo:", SlicesIndex(s, "zoo"))
 
-	_ = SlicesIndex[[]string, string](s, "zoo")
+	var s = []string{"foo", "bar", "zoo"}
+	// The types S and E are explicitly specified as []string and string respectively.
+	fmt.Println("index of zoo:", SlicesIndex[[]string, string](s, "zoo"))
 
 	var s2 = []int{2, 4, 5, 6}
+	// The type E is inferred from the type of the value 4, which is int,
+	// and the type S is inferred from the type of the slice s2, which is []int.
 	fmt.Println("index of 4: ", SlicesIndex(s2, 4))
 
+	// instantiate a list of float64 elements
 	lst := List[float64]{}
 	lst.Push(10)
 	lst.Push(13)

go/02-next-level/06-defer.go

@@ -7,18 +7,21 @@ import (
 
 func main() {
 
-	f := createFile("/tmp/defer.txt")
-	defer closeFile(f)
-	writeFile(f)
-}
-
-func createFile(p string) *os.File {
 	fmt.Println("creating")
-	f, err := os.Create(p)
+	f, err := os.Create("/tmp/defer.txt")
+
+	// defer the closing of the file until the surrounding function returns.
+	// This ensures that the file will be closed even if there is a panic or an early return in the function.
+	defer closeFile(f)
+
 	if err != nil {
 		panic(err)
 	}
-	return f
+
+	//will not be called in case of a panic before
+	writeFile(f)
+
+	// closeFile will be called here due to the defer-statement, even in case of a panic before.
 }
 
 func writeFile(f *os.File) {
@@ -28,8 +31,10 @@ func writeFile(f *os.File) {
 
 func closeFile(f *os.File) {
 	fmt.Println("closing")
-	err := f.Close()
-
+	var err error
+	if f != nil {
+		err = f.Close()
+	}
 	if err != nil {
 		panic(err)
 	}