Fix issue #18

++ and -- operators are now forbidden in function definitions. This is the correct behavior for mathematical functions

• concerning function arguments, it was already not supported but resulted in a crash
• with regard to outer variables, the behavior is changed since now these cannot be modified anymore while calling the function. (observe these external parameters can still be modified outside of the function definition itself)

src/language/ast/ASTNodeExpressionBuilder.cpp

@@ -25,6 +25,19 @@
 #include <language/node_processor/WhileProcessor.hpp>
 #include <language/utils/ParseError.hpp>
 
+void
+ASTNodeExpressionBuilder::_checkIsPureFunction(const ASTNode& node) const
+{
+  if (node.is_type<language::unary_plusplus>() or node.is_type<language::unary_minusminus>() or
+      node.is_type<language::post_plusplus>() or node.is_type<language::post_minusminus>()) {
+    throw ParseError("invalid function definition. Function data must be constant!", node.begin());
+  }
+
+  for (auto&& child : node.children) {
+    this->_checkIsPureFunction(*child);
+  }
+}
+
 void
 ASTNodeExpressionBuilder::_buildExpression(ASTNode& n)
 {
@@ -42,6 +55,8 @@ ASTNodeExpressionBuilder::_buildExpression(ASTNode& n)
   } else if (n.is_type<language::tuple_expression>()) {
     n.m_node_processor = std::make_unique<TupleToVectorProcessor<ASTNodeExpressionListProcessor>>(n);
   } else if (n.is_type<language::function_definition>()) {
+    this->_checkIsPureFunction(n);
+
     n.m_node_processor = std::make_unique<FakeProcessor>();
 
   } else if (n.is_type<language::function_evaluation>()) {