Demonstrate a custom iterator from go1.23

Author: quii

.github/workflows/go.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v5
       with:
-        go-version: 1.22
+        go-version: 1.23
 
     - name: Build
       run: ./build.sh

go.mod

@@ -1,6 +1,6 @@
 module github.com/quii/learn-go-with-tests
 
-go 1.22
+go 1.23
 
 require (
 	github.com/approvals/go-approval-tests v0.0.0-20211008131110-0c40b30e0000

intro-to-acceptance-tests.md

@@ -298,7 +298,7 @@ import (
 
 const (
 	port = "8080"
-	url  = "<http://localhost:" +port
+	url  = "<http://localhost:" + port
 )
 
 func TestGracefulShutdown(t *testing.T) {

iterators/iterators_test.go

@@ -0,0 +1,55 @@
+package iterators
+
+import (
+	"iter"
+	"slices"
+	"testing"
+)
+
+func Concatenate(seq iter.Seq[string]) string {
+	var result string
+	for s := range seq {
+		result += s
+	}
+	return result
+}
+
+// annoyingly, there is no builtin way to go from seq2, to seq (e.g just get the values)
+func Values[K, V any](seq iter.Seq2[K, V]) iter.Seq[V] {
+	return func(yield func(V) bool) {
+		for _, v := range seq {
+			if !yield(v) {
+				return
+			}
+		}
+	}
+}
+
+// WIP!
+func TestConcatenate(t *testing.T) {
+	t.Run("values of a slice", func(t *testing.T) {
+		got := Concatenate(slices.Values([]string{"a", "b", "c"}))
+		want := "abc"
+		if got != want {
+			t.Errorf("got %q want %q", got, want)
+		}
+	})
+
+	t.Run("values of a slice backwards", func(t *testing.T) {
+		backward := slices.Backward([]string{"a", "b", "c"})
+
+		got := Concatenate(Values(backward))
+		want := "cba"
+		if got != want {
+			t.Errorf("got %q want %q", got, want)
+		}
+	})
+
+	t.Run("values of a slice sorted", func(t *testing.T) {
+		got := Concatenate(slices.Values(slices.Sorted(slices.Values([]string{"c", "a", "b"}))))
+		want := "abc"
+		if got != want {
+			t.Errorf("got %q want %q", got, want)
+		}
+	})
+}

mocking.md

@@ -636,3 +636,45 @@ In this post about mocking we have only covered **Spies**, which are a kind of m
 > [Test Double is a generic term for any case where you replace a production object for testing purposes.](https://martinfowler.com/bliki/TestDouble.html)
 
 Under test doubles, there are various types like stubs, spies and indeed mocks! Check out [Martin Fowler's post](https://martinfowler.com/bliki/TestDouble.html) for more detail.
+
+## Bonus - Example of iterators from go 1.23
+
+In Go 1.23 [iterators were introduced](https://tip.golang.org/doc/go1.23). We can use iterators in various ways, in this instance we can make a `countdownFrom` iterator, which will return the numbers to countdown in reverse order.
+
+```go
+func Countdown(out io.Writer, sleeper Sleeper) {
+	for i := range countDownFrom(3) {
+		fmt.Fprintln(out, i)
+		sleeper.Sleep()
+	}
+
+	fmt.Fprint(out, finalWord)
+}
+```
+
+Before we get into how we write custom iterators, let's see how we use it. Rather than writing a fairly imperative looking loop to count down from a number, we can make this code look more expressive by `range`-ing over our custom `countdownFrom` iterator.
+
+To write an iterator, a function that can be used in a `range` loop, you need to write a function in a particular way. From the docs:
+
+    The “range” clause in a “for-range” loop now accepts iterator functions of the following types
+        func(func() bool)
+        func(func(K) bool)
+        func(func(K, V) bool)
+
+(The `K` and `V` stand for key and value types, respectively.)
+
+In our case, we don't have keys, just values. Go also provides a convenience type `iter.Seq[T]` which is a type alias for `func(func(T) bool)`.
+
+```go
+func countDownFrom(from int) iter.Seq[int] {
+	return func(yield func(int) bool) {
+		for i := from; i > 0; i-- {
+			if !yield(i) {
+				return
+			}
+		}
+	}
+}
+```
+
+This is a simple iterator, which will yield the numbers in reverse order - perfect for our usecase. 

mocking/v6/countdown_test.go

@@ -0,0 +1,81 @@
+package main
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+	"time"
+)
+
+func TestCountdown(t *testing.T) {
+
+	t.Run("prints 3 to Go!", func(t *testing.T) {
+		buffer := &bytes.Buffer{}
+		Countdown(buffer, &SpyCountdownOperations{})
+
+		got := buffer.String()
+		want := `3
+2
+1
+Go!`
+
+		if got != want {
+			t.Errorf("got %q want %q", got, want)
+		}
+	})
+
+	t.Run("sleep before every print", func(t *testing.T) {
+		spySleepPrinter := &SpyCountdownOperations{}
+		Countdown(spySleepPrinter, spySleepPrinter)
+
+		want := []string{
+			write,
+			sleep,
+			write,
+			sleep,
+			write,
+			sleep,
+			write,
+		}
+
+		if !reflect.DeepEqual(want, spySleepPrinter.Calls) {
+			t.Errorf("wanted calls %v got %v", want, spySleepPrinter.Calls)
+		}
+	})
+}
+
+func TestConfigurableSleeper(t *testing.T) {
+	sleepTime := 5 * time.Second
+
+	spyTime := &SpyTime{}
+	sleeper := ConfigurableSleeper{sleepTime, spyTime.Sleep}
+	sleeper.Sleep()
+
+	if spyTime.durationSlept != sleepTime {
+		t.Errorf("should have slept for %v but slept for %v", sleepTime, spyTime.durationSlept)
+	}
+}
+
+type SpyCountdownOperations struct {
+	Calls []string
+}
+
+func (s *SpyCountdownOperations) Sleep() {
+	s.Calls = append(s.Calls, sleep)
+}
+
+func (s *SpyCountdownOperations) Write(p []byte) (n int, err error) {
+	s.Calls = append(s.Calls, write)
+	return
+}
+
+const write = "write"
+const sleep = "sleep"
+
+type SpyTime struct {
+	durationSlept time.Duration
+}
+
+func (s *SpyTime) Sleep(duration time.Duration) {
+	s.durationSlept = duration
+}

mocking/v6/main.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"iter"
+	"os"
+	"time"
+)
+
+// Sleeper allows you to put delays.
+type Sleeper interface {
+	Sleep()
+}
+
+// ConfigurableSleeper is an implementation of Sleeper with a defined delay.
+type ConfigurableSleeper struct {
+	duration time.Duration
+	sleep    func(time.Duration)
+}
+
+// Sleep will pause execution for the defined Duration.
+func (c *ConfigurableSleeper) Sleep() {
+	c.sleep(c.duration)
+}
+
+const finalWord = "Go!"
+
+// Countdown prints a countdown from 3 to out with a delay between count provided by Sleeper.
+func Countdown(out io.Writer, sleeper Sleeper) {
+	for i := range countDownFrom(3) {
+		fmt.Fprintln(out, i)
+		sleeper.Sleep()
+	}
+
+	fmt.Fprint(out, finalWord)
+}
+
+func countDownFrom(from int) iter.Seq[int] {
+	return func(yield func(int) bool) {
+		for i := from; i > 0; i-- {
+			if !yield(i) {
+				return
+			}
+		}
+	}
+}
+
+func main() {
+	sleeper := &ConfigurableSleeper{1 * time.Second, time.Sleep}
+	Countdown(os.Stdout, sleeper)
+}