Added more tests

src/errors/CompileErrors.go

@@ -2,5 +2,7 @@ package errors
 
 var (
 	InvalidStatement   = &Base{"Invalid statement"}
+	InvalidExpression  = &Base{"Invalid expression"}
 	MissingAssignValue = &Base{"Missing assignment value"}
+	NotImplemented     = &Base{"Not implemented"}
 )

src/errors/UnknownIdentifier.go

@@ -0,0 +1,18 @@
+package errors
+
+import "fmt"
+
+// UnknownIdentifier represents unknown variables.
+type UnknownIdentifier struct {
+	Name        string
+	CorrectName string
+}
+
+// Error generates the string representation.
+func (err *UnknownIdentifier) Error() string {
+	if err.CorrectName != "" {
+		return fmt.Sprintf("Unknown variable '%s', did you mean '%s'?", err.Name, err.CorrectName)
+	}
+
+	return fmt.Sprintf("Unknown variable '%s'", err.Name)
+}