Implement errors.As

Author: waschik

error.go

@@ -202,3 +202,8 @@ func (err *Error) TypeName() string {
 	}
 	return reflect.TypeOf(err.Err).String()
 }
+
+// Return the wrapped error (implements api for As function).
+func (err *Error) Unwrap() error {
+	return err.Err
+}

error_1_13.go

@@ -6,6 +6,11 @@ import (
 	baseErrors "errors"
 )
 
+// find error in any wrapped error
+func As(err error, target interface{}) bool {
+	return baseErrors.As(err, target)
+}
+
 // Is detects whether the error is equal to a given error. Errors
 // are considered equal by this function if they are matched by errors.Is
 // or if their contained errors are matched through errors.Is

error_backward.go

@@ -2,6 +2,41 @@
 
 package errors
 
+import (
+	"reflect"
+)
+
+type unwrapper interface {
+	Unwrap() error
+}
+
+// As assigns error or any wrapped error to the value target points
+// to. If there is no value of the target type of target As returns
+// false.
+func As(err error, target interface{}) bool {
+	targetType := reflect.TypeOf(target)
+
+	for {
+		errType := reflect.TypeOf(err)
+
+		if errType == nil {
+			return false
+		}
+
+		if reflect.PtrTo(errType) == targetType {
+			reflect.ValueOf(target).Elem().Set(reflect.ValueOf(err))
+			return true
+		}
+
+		wrapped, ok := err.(unwrapper)
+		if ok {
+			err = wrapped.Unwrap()
+		} else {
+			return false
+		}
+	}
+}
+
 // Is detects whether the error is equal to a given error. Errors
 // are considered equal by this function if they are the same object,
 // or if they both contain the same error inside an errors.Error.

error_test.go

@@ -30,6 +30,29 @@ func BenchmarkStackFormat(b *testing.B) {
 	}
 }
 
+func TestAs(t *testing.T) {
+	var errStrIn errorString = "TestForFun"
+
+	var errStrOut errorString
+	if As(errStrIn, &errStrOut) {
+		if errStrOut != "TestForFun" {
+			t.Errorf("direct errStr value is not returned")
+		}
+	} else {
+		t.Errorf("direct errStr is not returned")
+	}
+
+	errStrOut = ""
+	err := Wrap(errStrIn, 0)
+	if As(err, &errStrOut) {
+		if errStrOut != "TestForFun" {
+			t.Errorf("wrapped errStr value is not returned")
+		}
+	} else {
+		t.Errorf("wrapped errStr is not returned")
+	}
+}
+
 func TestStackFormat(t *testing.T) {
 
 	defer func() {
@@ -316,3 +339,9 @@ func callersToFrames(callers []uintptr) []runtime.Frame {
 		}
 	}
 }
+
+type errorString string
+
+func (e errorString) Error() string {
+	return string(e)
+}