Add ErrorObject.Is method (#25)

* Add ErrorObject.Is method

* Fix ErrorObject.Is logic

* Improve test coverage for ErrorObject.Is

Author: goodevilgenius

error.go

@@ -112,6 +112,33 @@ func (e ErrorObject) Error() string {
 	return res.String()
 }
 
+// Is checks if this error matches the supplied error.
+// If err is not an ErrorObject, it always returns false.
+// It returns true if Code() and Message() are the same.
+// If err.Params() is non-nil, we also check that all the params match.
+// This way, we can check an error against a sentinel error, such as ErrLengthTooLong.
+func (e ErrorObject) Is(err error) bool {
+	eo, ok := err.(ErrorObject)
+	if !ok {
+		return false
+	}
+
+	if e.message != eo.message || e.code != eo.code {
+		return false
+	}
+
+	if len(eo.params) == 0 {
+		return true
+	}
+
+	for k, v := range e.params {
+		if eo.params[k] != v {
+			return false
+		}
+	}
+	return true
+}
+
 // Error returns the error string of Errors.
 func (es Errors) Error() string {
 	if len(es) == 0 {

error_test.go

@@ -128,6 +128,31 @@ func TestErrorObject_Params(t *testing.T) {
 	assert.Equal(t, err.Params(), p)
 }
 
+func TestErrorObject_Is(t *testing.T) {
+	err := ErrLengthOutOfRange.(ErrorObject)
+	err = err.SetParams(map[string]interface{}{"mix": 3, "max": 6}).(ErrorObject)
+
+	assert.True(t, err.Is(ErrLengthOutOfRange))
+	assert.True(t, errors.Is(err, ErrLengthOutOfRange))
+
+	err2 := ErrLengthOutOfRange.(ErrorObject)
+	err2 = err2.SetParams(map[string]interface{}{"min": 3, "max": 9}).(ErrorObject)
+
+	assert.False(t, err2.Is(err))
+	assert.False(t, errors.Is(err2, err))
+
+	assert.False(t, err.Is(errors.New(err.message)))
+
+	err3 := ErrorObject{code: err.code, message: "hello"}
+	assert.False(t, err.Is(err3))
+
+	err4 := ErrorObject{code: "test_error", message: err.message}
+	assert.False(t, err.Is(err4))
+
+	err5 := ErrorObject{code: err.code, message: err.message, params: err.params}
+	assert.True(t, err.Is(err5))
+}
+
 func TestErrorObject_AddParam2(t *testing.T) {
 	p := map[string]interface{}{"key": "val"}
 	err := NewError("code", "A").(ErrorObject)