GoComparableType原理深入解析

package reflect
type Type interface {
	// Comparable reports whether values of this type are comparable.
	Comparable() bool
}

type rtype struct {
	// function for comparing objects of this type
	// (ptr to object A, ptr to object B) -> ==?
	equal func(unsafe.Pointer, unsafe.Pointer) bool
}
func (t *rtype) Comparable() bool {
	return t.equal != nil
}

// emptyInterface is the header for an interface{} value.
type emptyInterface struct {
	typ  *rtype
	Word unsafe.Pointer
}

func efaceeq(t *_type, x, y unsafe.Pointer) bool {
	if t == nil {
		return true
	}
	eq := t.equal
	if eq == nil {
		panic(errorString("comparing uncomparable type " + t.string()))
	}
	if isDirectIface(t) { // t.kind == kindDirectIface
		// Direct interface types are ptr, chan, map, func, and single-element structs/arrays thereof.
		// Maps and funcs are not comparable, so they can't reach here.
		// Ptrs, chans, and single-element items can be compared directly using ==.
		return x == y
	}
	return eq(x, y)
}

type CustomError struct {
	Metadata map[string]string
	Message string
}
func (c CustomError) Error() string {
		return c.Message
}
var (
	ErrorA = CustomError{Message:"A", Matadata: map[string]string{"Reason":""}}
	ErrorB = CustomError{Message:"B"}
)
func DoSomething() error {
	return ErrorA
}

err:=DoSomething()
if errors.Is(err, ErrorA) {
	// handle err
}

func Is(err, target error) bool {
	if target == nil {
		return err == target
	}
	isComparable := reflect.TypeOf(target).Comparable()
	for {
		if isComparable && err == target {
			return true
		}
		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
			return true
		}
		if err = errors.Unwrap(err); err == nil {
			return false
		}
	}
}

var (
	ErrorA = &CustomError{Message:"A", Matadata: map[string]string{"Reason":""}}
	ErrorB = &CustomError{Message:"B"}
)

func returnsError() error {
	var p *MyError = nil
	if bad() {
		p = ErrBad
	}
	return p // Will always return a non-nil error.
}

func returnsError() error {
	if bad() {
		return ErrBad
	}
	return nil
}

func (c *valueCtx) Value(key interface{}) interface{} {
	if c.key == key {
		return c.val
	}
	return c.Context.Value(key)
}

ctx = Context.Background()
ctx = context.WithValue(ctx, "key", "123")
ctx = context.WithValue(ctx, "key", "456")
ctx.Value("key") // 456

var key = byte(0)
ctx = context.WithValue(ctx, &key, "123")
ctx.Value(&key)

type key struct {}
ctx = context.WithValue(ctx, key{}, "123")
ctx.Value(key{})