Implement elsif do

include/elna/boot/ast.h

@@ -617,6 +617,7 @@ namespace boot
         conditional_statements *m_body;
 
     public:
+        std::vector<conditional_statements *> branches;
         while_statement(const struct position position, conditional_statements *body);
         virtual void accept(parser_visitor *visitor) override;
 

include/elna/boot/symbol.h

@@ -29,7 +29,7 @@ namespace boot
     /**
      * Symbol table.
      */
-    template<typename T>
+    template<typename T, T nothing>
     class symbol_table
     {
     public:
@@ -73,7 +73,7 @@ namespace boot
         }
 
         /**
-         * Looks for symbol in the table by name. Returns nullptr if the symbol
+         * Looks for symbol in the table by name. Returns nothing if the symbol
          * can not be found.
          *
          * \param name Symbol name.
@@ -91,7 +91,7 @@ namespace boot
             {
                 return this->outer_scope->lookup(name);
             }
-            return nullptr;
+            return nothing;
         }
 
         /**
@@ -104,7 +104,14 @@ namespace boot
          */
         bool enter(const std::string& name, symbol_ptr entry)
         {
-            return entries.insert({ name, entry }).second;
+            if (lookup(name) == nothing)
+            {
+                return entries.insert({ name, entry }).second;
+            }
+            else 
+            {
+                return nothing;
+            }
         }
 
         /**

include/elna/gcc/elna-generic.h

@@ -37,7 +37,7 @@ namespace gcc
     class generic_visitor final : public boot::empty_visitor
     {
         tree current_expression{ NULL_TREE };
-        std::shared_ptr<boot::symbol_table<tree>> symbol_map;
+        std::shared_ptr<symbol_table> symbol_map;
 
         tree build_label_decl(const char *name, location_t loc);
         tree build_type(boot::top_type& type);
@@ -61,7 +61,7 @@ namespace gcc
                 tree symbol, const std::vector<boot::expression *>& arguments);
 
     public:
-        generic_visitor(std::shared_ptr<boot::symbol_table<tree>> symbol_table);
+        generic_visitor(std::shared_ptr<symbol_table> symbol_table);
 
         void visit(boot::program *program) override;
         void visit(boot::procedure_definition *definition) override;

include/elna/gcc/elna-tree.h

@@ -33,6 +33,8 @@ namespace elna
 {
 namespace gcc
 {
+    using symbol_table = boot::symbol_table<tree, NULL_TREE>;
+
     bool is_pointer_type(tree type);
     bool is_integral_type(tree type);
     bool is_numeric_type(tree type);