fix: restore RPN stack on ParseAndEvaluate error in RPNHandler

RPNHandler.Handle silently swallowed ParseAndEvaluate errors on
multi-word input, leaving the persistent REPL stack in a corrupted
state. evaluate() modifies r.currentStack directly during token
processing, so partial evaluation on failure left orphan values.

Fix: save the stack before ParseAndEvaluate, restore it on error.
Applied to both the bare RPN path and the rpn/calc prefix path.

Added tests:
- TestRPNHandlerStackNotCorruptedOnError (bare multi-word input)
- TestRPNHandlerErrorReturnedNotSwallowed (error propagation)
- TestRPNHandlerStackNotCorruptedOnPrefixedError (rpn/calc prefix)

2 files changed, 82 insertions, 2 deletions

diff --git a/internal/repl/handlers.go b/internal/repl/handlers.go
index c5b28fc..fd4c1a6 100644
--- a/internal/repl/handlers.go
+++ b/internal/repl/handlers.go
@@ -152,8 +152,13 @@ func (h *RPNHandler) Handle(repl *REPL, input string) (output string, handled bo
 	if strings.HasPrefix(lowerInput, "rpn ") || strings.HasPrefix(lowerInput, "calc ") {
 		// Extract the expression after rpn/calc
 		rest := strings.TrimSpace(strings.TrimPrefix(input, strings.SplitN(input, " ", 2)[0]))
+
+		// Save stack state so a failed expression doesn't corrupt it
+		savedStack := repl.rpnState.rpnCalc.GetCurrentStack()
+
 		result, err := repl.rpnState.rpnCalc.ParseAndEvaluate(rest)
 		if err != nil {
+			repl.rpnState.rpnCalc.SetCurrentStack(savedStack)
 			return "", true, err
 		}
 		return result, true, nil
@@ -163,10 +168,16 @@ func (h *RPNHandler) Handle(repl *REPL, input string) (output string, handled bo
 	if repl.rpnState != nil {
 		// Check if input looks like RPN (contains spaces or is a single known operator)
 		if strings.Contains(input, " ") {
+			// Save stack state so a failed expression doesn't corrupt it
+			savedStack := repl.rpnState.rpnCalc.GetCurrentStack()
+
 			result, err := repl.rpnState.rpnCalc.ParseAndEvaluate(input)
-			if err == nil {
-				return result, true, nil
+			if err != nil {
+				// Restore the stack to its state before the failed expression
+				repl.rpnState.rpnCalc.SetCurrentStack(savedStack)
+				return "", true, err
 			}
+			return result, true, nil
 		}
 
 		// Try evaluating as a single operator on the current RPN stack
diff --git a/internal/repl/repl_test.go b/internal/repl/repl_test.go
index 89fb89a..f8e22a2 100644
--- a/internal/repl/repl_test.go
+++ b/internal/repl/repl_test.go
@@ -774,3 +774,72 @@ func TestREPLDefaultGetCommandDescription(t *testing.T) {
 		})
 	}
 }
+
+func TestRPNHandlerStackNotCorruptedOnError(t *testing.T) {
+	repl := createTestREPL()
+
+	// First, push some values onto the stack
+	defaultExecutor(repl, "1 2 3 ")
+
+	// Save the stack state before the failing expression
+	beforeStack := repl.rpnState.rpnCalc.GetCurrentStack()
+	if len(beforeStack) != 3 {
+		t.Fatalf("expected 3 values on stack before error, got %d", len(beforeStack))
+	}
+
+	// Now try a failing multi-word RPN expression
+	defaultExecutor(repl, "4 5 + invalidtoken")
+
+	// Verify the stack was restored to its state before the failed expression
+	afterStack := repl.rpnState.rpnCalc.GetCurrentStack()
+	if len(afterStack) != len(beforeStack) {
+		t.Errorf("stack corrupted by failed expression: had %d values, now has %d", len(beforeStack), len(afterStack))
+	}
+	for i := range beforeStack {
+		if afterStack[i].String() != beforeStack[i].String() {
+			t.Errorf("stack[%d] corrupted: was %q, now %q", i, beforeStack[i].String(), afterStack[i].String())
+		}
+	}
+}
+
+func TestRPNHandlerErrorReturnedNotSwallowed(t *testing.T) {
+	repl := createTestREPL()
+
+	// Test that ParseAndEvaluate errors on multi-word input are propagated
+	h := &RPNHandler{}
+	_, handled, err := h.Handle(repl, "3 4 + invalidtoken")
+
+	if !handled {
+		t.Error("multi-word RPN with error should be handled (not fall through)")
+	}
+	if err == nil {
+		t.Error("multi-word RPN with invalid token should return an error")
+	}
+}
+
+func TestRPNHandlerStackNotCorruptedOnPrefixedError(t *testing.T) {
+	repl := createTestREPL()
+
+	// Push values onto the stack
+	defaultExecutor(repl, "10 20 ")
+
+	// Save the stack state before the failing prefixed expression
+	beforeStack := repl.rpnState.rpnCalc.GetCurrentStack()
+	if len(beforeStack) != 2 {
+		t.Fatalf("expected 2 values on stack before error, got %d", len(beforeStack))
+	}
+
+	// Try a failing prefixed RPN expression
+	defaultExecutor(repl, "rpn 30 40 + invalidtoken")
+
+	// Verify the stack was restored
+	afterStack := repl.rpnState.rpnCalc.GetCurrentStack()
+	if len(afterStack) != len(beforeStack) {
+		t.Errorf("stack corrupted by failed rpn-prefixed expression: had %d values, now has %d", len(beforeStack), len(afterStack))
+	}
+	for i := range beforeStack {
+		if afterStack[i].String() != beforeStack[i].String() {
+			t.Errorf("stack[%d] corrupted: was %q, now %q", i, beforeStack[i].String(), afterStack[i].String())
+		}
+	}
+}