Merge branch 'go-next-level' of https://gitty.informatik.th-mannheim.de/steger/pr3-code

go/02-next-level/00-methods.go

@@ -0,0 +1,26 @@
+package main
+
+import "fmt"
+
+type Counter struct {
+	value int
+}
+
+// pointer receiver. The method operates directly on the object c points to
+// Use a pointer receiver whenever the object needs to be modified.
+func (c *Counter) Increment() {
+	c.value++
+}
+
+// value receiver. The method operates on copy of object c. Use a value
+// receiver when the object does not need to be modified in the method.
+func (c Counter) String() string {
+	return fmt.Sprintf("Counter value: %d", c.value)
+}
+
+func main() {
+	c := Counter{}
+	fmt.Println(c)
+	c.Increment()
+	fmt.Println(c)
+}

go/02-next-level/01-interfaces.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type geometry interface {
+	area() float64
+	perim() float64
+}
+
+type rect struct {
+	width, height float64
+}
+
+// Ensure that rect implements the geometry interface
+var _ geometry = rect{}
+
+type circle struct {
+	radius float64
+}
+
+// Ensure that circle implements the geometry interface
+var _ geometry = circle{}
+
+func (r rect) area() float64 {
+	return r.width * r.height
+}
+func (r rect) perim() float64 {
+	return 2*r.width + 2*r.height
+}
+
+func (c circle) area() float64 {
+	return math.Pi * c.radius * c.radius
+}
+func (c circle) perim() float64 {
+	return 2 * math.Pi * c.radius
+}
+
+//Metods demonstrates how to use an interface
+func measure(g geometry) {
+	fmt.Println(g)
+	fmt.Println(g.area())
+	fmt.Println(g.perim())
+}
+
+
+//Method demonstrates casting an interface to a concrete type. 
+//Should be avoided in practice whenever possible.
+func detectCircle(g geometry) {
+	if c, ok := g.(circle); ok {
+		fmt.Println("circle with radius", c.radius)
+	}
+}
+
+func main() {
+	r := rect{width: 3, height: 4}
+	c := circle{radius: 5}
+
+	measure(r)
+	measure(c)
+
+	detectCircle(r)
+	detectCircle(c)
+}

go/02-next-level/02-struct-embedding.go

@@ -0,0 +1,40 @@
+package main
+
+import "fmt"
+
+type Person struct {
+	Name string
+}
+
+func (p Person) Say() {
+	fmt.Println("Hi, my name is", p.Name)
+}
+
+type Student struct {
+	Person //struct Person is embedded
+	Semester int
+}
+
+type Teacher struct {
+	Person //struct Person is embedded
+	Subject string
+}
+
+func main() {
+
+	max := Person{"Max"}
+
+	daniel := Student{Person{"Daniel"}, 3}
+
+	sebastian := Teacher{Person{"Sebastian"}, "PR3"}
+
+	max.Say()
+
+	daniel.Say() //can be invoked directly on Student
+
+	sebastian.Say() //can be invoked directly on Teacher
+
+	max = sebastian.Person
+
+	max.Say()
+}

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

@@ -0,0 +1,62 @@
+package main
+
+import (
+	"errors"
+	"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 {
+		// 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
+}
+
+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 {
+			fmt.Println("f worked:", r)
+		}
+	}
+
+	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) { // 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)
+			}
+			continue
+		}
+
+		fmt.Println("Tea is ready!")
+	}
+}

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

@@ -0,0 +1,28 @@
+package main
+
+import "fmt"
+
+// 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
+	}
+}
+
+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

@@ -2,7 +2,10 @@ package main
 
 import "fmt"
 
-func SlicesIndex[S []E, E int](s []string, v string) int {
+// 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] {
 			return i
@@ -11,27 +14,34 @@ func SlicesIndex[S []E, E int](s []string, v string) int {
 	return -1
 }
 
-type List struct {
+// represents a linked list of any type T. The type parameter T is used in the definition of List and its methods,
-	head, tail *element
+// 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]
 }
 
-type element struct {
+// the generic element of the linked list
-	next *element
+type element[T any] struct {
-	val  int
+	next *element[T]
+	val  T
 }
 
-func (lst *List) Push(v int) {
+// adds an instance of the generic type to the list
+func (lst *List[T]) Push(v T) {
 	if lst.tail == nil {
-		lst.head = &element{val: v}
+		lst.head = &element[T]{val: v}
 		lst.tail = lst.head
 	} else {
-		lst.tail.next = &element{val: v}
+		lst.tail.next = &element[T]{val: v}
 		lst.tail = lst.tail.next
 	}
 }
 
-func (lst *List) AllElements() []int {
+// returns all elements of the list as a slice of the generic type.
-	var elems []int
+func (lst List[T]) AllElements() []T {
+	var elems []T
 	for e := lst.head; e != nil; e = e.next {
 		elems = append(elems, e.val)
 	}
@@ -39,13 +49,21 @@ func (lst *List) AllElements() []int {
 }
 
 func main() {
+
 	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"))
 
-	fmt.Println("index of zoo:", SlicesIndex(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))
 
-	lst := List{}
+	// instantiate a list of float64 elements
+	lst := List[float64]{}
 	lst.Push(10)
 	lst.Push(13)
 	lst.Push(23)
+	lst.Push(23.3)
 	fmt.Println("list:", lst.AllElements())
 }

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

@@ -6,18 +6,22 @@ import (
 )
 
 func main() {
-	f := createFile("/tmp/defer.txt")
-	writeFile(f)
-	closeFile(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) {
@@ -27,8 +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)
 	}