Machine.Execute returns an error on unknown instruction

creature.go

@@ -28,6 +28,7 @@ const (
 	// ErrorIDTaken is returned by Register when a new function is
 	// registered with the same ID as another one.
 	ErrorIDTaken Error = iota
+	ErrorUnknownInstruction
 )
 
 // Error returns a textual description of the error.
@@ -35,6 +36,8 @@ func (err Error) Error() (description string) {
 	switch err {
 	case ErrorIDTaken:
 		description = "this ID is taken by another function"
+	case ErrorUnknownInstruction:
+		description = "machine encountered unknown instruction"
 	}
 
 	return
@@ -49,8 +52,9 @@ func (machine *Machine) Reset() {
 
 // Execute starts execution at the address specified by offset. The machine will
 // run until a HALT instruction is encountered, or the end of program memory is
-// reached.
-func (machine *Machine) Execute(offset int) {
+// reached. If an unknown instruction is encountered, the machine halts and
+// returns an error.
+func (machine *Machine) Execute(offset int) (err error) {
 	machine.counter = offset
 
 	for machine.counter < len(machine.Program) {
@@ -173,9 +177,15 @@ func (machine *Machine) Execute(offset int) {
 					return
 				}
 			}
+
+		default:
+			err = ErrorUnknownInstruction
+			return
 		}
 		machine.counter++
 	}
+
+	return
 }
 
 // Instruction returns the current instruction in program memory.