text/template: add an if func example

Updates #13880

Change-Id: I0fd3b1a32e485bc8f15238c86a8bbdc161f20fa5
GitHub-Last-Rev: d1e92b3124
GitHub-Pull-Request: golang/go#71922
Reviewed-on: https://go-review.googlesource.com/c/go/+/651916
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/text/template/examplefunc_test.go

@@ -96,3 +96,28 @@ func ExampleTemplate_funcs() {
 	// def
 	// def
 }
+
+// This example demonstrates how to use "if".
+func ExampleTemplate_if() {
+	type book struct {
+		Stars float32
+		Name  string
+	}
+
+	tpl, err := template.New("book").Parse(`{{ if (gt .Stars 4.0) }}"{{.Name }}" is a great book.{{ else }}"{{.Name}}" is not a great book.{{ end }}`)
+	if err != nil {
+		log.Fatalf("failed to parse template: %s", err)
+	}
+
+	b := &book{
+		Stars: 4.9,
+		Name:  "Good Night, Gopher",
+	}
+	err = tpl.Execute(os.Stdout, b)
+	if err != nil {
+		log.Fatalf("failed to execute template: %s", err)
+	}
+
+	// Output:
+	// "Good Night, Gopher" is a great book.
+}