doc: add The Laws of Reflection article

Originally published on The Go Programming Language Blog, September 6, 2011.

http://blog.golang.org/2011/09/laws-of-reflection.html

Update #2547

R=golang-dev, r, adg
CC=golang-dev
https://golang.org/cl/5689054

1 files changed, 112 insertions, 0 deletions

diff --git a/doc/progs/interface2.go b/doc/progs/interface2.go
new file mode 100644
index 0000000000..e2716cf16d
--- /dev/null
+++ b/doc/progs/interface2.go
@@ -0,0 +1,112 @@
+package main
+
+import (
+	"fmt"
+	"reflect"
+)
+
+func main() {
+	var x float64 = 3.4
+	fmt.Println("type:", reflect.TypeOf(x))
+	// STOP OMIT
+	// TODO(proppy): test output OMIT
+}
+
+// STOP main OMIT
+
+func f1() {
+	// START f1 OMIT
+	var x float64 = 3.4
+	v := reflect.ValueOf(x)
+	fmt.Println("type:", v.Type())
+	fmt.Println("kind is float64:", v.Kind() == reflect.Float64)
+	fmt.Println("value:", v.Float())
+	// STOP OMIT
+}
+
+func f2() {
+	// START f2 OMIT
+	var x uint8 = 'x'
+	v := reflect.ValueOf(x)
+	fmt.Println("type:", v.Type())                            // uint8.
+	fmt.Println("kind is uint8: ", v.Kind() == reflect.Uint8) // true.
+	x = uint8(v.Uint())                                       // v.Uint returns a uint64.
+	// STOP OMIT
+}
+
+func f3() {
+	// START f3 OMIT
+	type MyInt int
+	var x MyInt = 7
+	v := reflect.ValueOf(x)
+	// START f3b OMIT
+	y := v.Interface().(float64) // y will have type float64.
+	fmt.Println(y)
+	// START f3c OMIT
+	fmt.Println(v.Interface())
+	// START f3d OMIT
+	fmt.Printf("value is %7.1e\n", v.Interface())
+	// STOP OMIT
+}
+
+func f4() {
+	// START f4 OMIT
+	var x float64 = 3.4
+	v := reflect.ValueOf(x)
+	v.SetFloat(7.1) // Error: will panic.
+	// STOP OMIT
+}
+
+func f5() {
+	// START f5 OMIT
+	var x float64 = 3.4
+	v := reflect.ValueOf(x)
+	fmt.Println("settability of v:", v.CanSet())
+	// STOP OMIT
+}
+
+func f6() {
+	// START f6 OMIT
+	var x float64 = 3.4
+	v := reflect.ValueOf(x)
+	// START f6b OMIT
+	v.SetFloat(7.1)
+	// STOP OMIT
+}
+
+func f7() {
+	// START f7 OMIT
+	var x float64 = 3.4
+	p := reflect.ValueOf(&x) // Note: take the address of x.
+	fmt.Println("type of p:", p.Type())
+	fmt.Println("settability of p:", p.CanSet())
+	// START f7b OMIT
+	v := p.Elem()
+	fmt.Println("settability of v:", v.CanSet())
+	// START f7c OMIT
+	v.SetFloat(7.1)
+	fmt.Println(v.Interface())
+	fmt.Println(x)
+	// STOP OMIT
+}
+
+func f8() {
+	// START f8 OMIT
+	type T struct {
+		A int
+		B string
+	}
+	t := T{23, "skidoo"}
+	s := reflect.ValueOf(&t).Elem()
+	typeOfT := s.Type()
+	for i := 0; i < s.NumField(); i++ {
+		f := s.Field(i)
+		fmt.Printf("%d: %s %s = %v\n", i,
+			typeOfT.Field(i).Name, f.Type(), f.Interface())
+	}
+	// START f8b OMIT
+	s.Field(0).SetInt(77)
+	s.Field(1).SetString("Sunset Strip")
+	fmt.Println("t is now", t)
+	// STOP OMIT
+}