Added readme and test for nexted Is()

errors/README.md

@@ -175,3 +175,9 @@ This function can be used also for logging to determine the caller from the runt
 ## func Stack()
 
 This function is similar to Caller() but reports an array of callers, not only one.
+
+## func Is()
+
+You can compare a message with errors.Is() to some error specification.
+It will look at the error or any cause to match the spec.
+The spec is the error message.
\ No newline at end of file

errors/errors_test.go

@@ -109,6 +109,23 @@ func TestIs(t *testing.T) {
 			t.Logf("n=%d worked", n)
 		}
 	}
+
+	//same but err is wrapped deeper
+	for n := 0; n <= 4; n++ {
+		if err := UserExists(n); err != nil {
+			switch {
+			case errors.Is(err, errNotFound):
+				t.Logf("u=%d failed with NOT FOUND", n)
+			case errors.Is(err, errDisabled):
+				t.Logf("u=%d failed because disabled", n)
+			default:
+				t.Logf("u=%d failed for unknown cause: %+v", n, err)
+			}
+		} else {
+			t.Logf("u=%d worked", n)
+		}
+	}
+
 }
 
 var (
@@ -130,3 +147,10 @@ func ReadDb(x int) error {
 		return errors.Errorf("invalid x=%d", x)
 	}
 }
+
+func UserExists(u int) error {
+	if err := ReadDb(u); err != nil {
+		return errors.Wrapf(err, "cannot read user %d", u)
+	}
+	return nil
+}