Add FailureMessage.Err method and AssertionError type (#141)

CHANGELOG.md

@@ -7,6 +7,11 @@ and this library adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/fluentassert/verify/compare/v1.1.0...HEAD)
 
+### Added
+
+- Add `FailureMessage.Err` method together with `AssertionError` type
+  to represent assertion results as `error` type.
+
 ## [1.1.0](https://github.com/fluentassert/verify/releases/tag/v1.1.0) - 2024-02-06
 
 This release adds length assertions.

assertion_error.go

@@ -0,0 +1,12 @@
+package verify
+
+// AssertionError is an error type used to represent failure messages from assertions.
+// It is compatible with the error interface and can be used in instances where an error shall be returned instead of failing a test.
+type AssertionError struct {
+	Message FailureMessage
+}
+
+// Error returns the failure message as a string. It makes AssertionError compatible with the error interface.
+func (err *AssertionError) Error() string {
+	return string(err.Message)
+}

failmsg.go

@@ -82,3 +82,12 @@ func (msg FailureMessage) Prefix(s string) FailureMessage {
 	}
 	return FailureMessage(s) + msg
 }
+
+// Err returns the failure message as an error type, or nil if the message is empty.
+func (msg FailureMessage) Err() *AssertionError {
+	if msg == "" {
+		return nil
+	}
+
+	return &AssertionError{Message: msg}
+}

failmsg_test.go

@@ -137,6 +137,18 @@ func TestFailureMessage(t *testing.T) {
 			assertEqual(t, got, "[fail] errored")
 		})
 	})
+
+	t.Run("AsError", func(t *testing.T) {
+		t.Run("With Message", func(t *testing.T) {
+			got := verify.FailureMessage("failed").Err()
+			assertEqual(t, got.Error(), "failed")
+		})
+
+		t.Run("Empty", func(t *testing.T) {
+			got := verify.FailureMessage("").Err()
+			assertEqual(t, got, nil)
+		})
+	})
 }
 
 type errorMock struct {