text/template: add an if func example

Updates #13880

src/text/template/examplefunc_test.go

@@ -52,3 +52,28 @@ Output 2: {{printf "%q" . | title}}
 	// Output 1: "The Go Programming Language"
 	// Output 2: "The Go Programming Language"
 }
+
+// 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.
+}