Unwrap tests and readme

README.md

@@ -59,7 +59,7 @@ Or:
 err := tracerr.Errorf("some error %d", num)
 ```
 
-### Add stack trace to existing error
+### Add Stack Trace to Existing Error
 
 > If `err` is `nil` then it still be `nil` with no stack trace added.
 
@@ -142,3 +142,17 @@ Or if `err` is of type `*tracerr.Error`:
 ```go
 frames := err.StackTrace()
 ```
+
+### Get Original Error
+
+> Unwrapped error will be `nil` if `err` is `nil` and will be the same error if `err` is not an instance of `*tracerr.Error`.
+
+```go
+err = tracerr.Unwrap(err)
+```
+
+Or if `err` is of type `*tracerr.Error`:
+
+```go
+err = err.Unwrap()
+```

error.go

@@ -40,6 +40,18 @@ func Wrap(err error) *Error {
 	return trace(err, 2)
 }
 
+// Unwrap returns the original error.
+func Unwrap(err error) error {
+	if err == nil {
+		return nil
+	}
+	e, ok := err.(*Error)
+	if !ok {
+		return err
+	}
+	return e.Unwrap()
+}
+
 // Error returns error message.
 func (e *Error) Error() string {
 	if e == nil {

error_test.go

@@ -206,3 +206,38 @@ func TestStackTraceNotInstance(t *testing.T) {
 		)
 	}
 }
+
+type UnwrapTestCase struct {
+	Error error
+	Wrap bool
+}
+
+func TestUnwrap(t *testing.T) {
+	cases := []UnwrapTestCase{
+		{
+			Error: nil,
+		},
+		{
+			Error: fmt.Errorf("some error #%d", 9),
+			Wrap: false,
+		},
+		{
+			Error: fmt.Errorf("some error #%d", 9),
+			Wrap: true,
+		},
+	}
+
+	for i, c := range cases {
+		err := c.Error
+		if c.Wrap {
+			err = tracerr.Wrap(err)
+		}
+		unwrappedError := tracerr.Unwrap(err)
+		if unwrappedError != c.Error {
+			t.Errorf(
+				"tracerr.Unwrap(cases[%#v].Error) = %#v; want %#v",
+				i, unwrappedError, c.Error,
+			)
+		}
+	}
+}