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>()) {

src/language/ast/ASTNodeExpressionBuilder.hpp

@@ -5,7 +5,9 @@
 
 class ASTNodeExpressionBuilder
 {
+ private:
   void _buildExpression(ASTNode& n);
+  void _checkIsPureFunction(const ASTNode& n) const;
 
  public:
   ASTNodeExpressionBuilder(ASTNode& n);