Add C++17-style if-init blocks

cheatsheet.md

@@ -278,6 +278,18 @@ while (some_condition()) { /* do something */ }
 for (x : [1,2,3]) { print(i); }
 ```
 
+## Conditionals
+
+```
+if (expression) { }
+```
+
+```
+// C++17-style init-if blocks
+// Value of 'statement' is scoped for entire `if` block
+if (statement; expression) { }
+```
+
 ## Built in Types
 
 ```

include/chaiscript/language/chaiscript_eval.hpp

@@ -823,6 +823,29 @@ namespace chaiscript
 
     };
 
+    template<typename T>
+    struct If_Init_AST_Node final : AST_Node_Impl<T> {
+        If_Init_AST_Node(std::string t_ast_node_text, Parse_Location t_loc, std::vector<AST_Node_Impl_Ptr<T>> t_children) :
+          AST_Node_Impl<T>(std::move(t_ast_node_text), AST_Node_Type::If, std::move(t_loc), std::move(t_children)) 
+        { 
+          assert(this->children.size() == 3 || this->children.size() == 4);
+        }
+
+        Boxed_Value eval_internal(const chaiscript::detail::Dispatch_State &t_ss) const override {
+          chaiscript::eval::detail::Scope_Push_Pop spp(t_ss);
+          this->children[0]->eval(t_ss);
+          if (this->get_bool_condition(this->children[1]->eval(t_ss))) {
+            return this->children[2]->eval(t_ss);
+          } else {
+            if (this->children.size() == 4) {
+              return this->children[3]->eval(t_ss);
+            } else {
+              return void_var();
+            }
+          }
+        }
+    };
+
     template<typename T>
     struct If_AST_Node final : AST_Node_Impl<T> {
         If_AST_Node(std::string t_ast_node_text, Parse_Location t_loc, std::vector<AST_Node_Impl_Ptr<T>> t_children) :

include/chaiscript/language/chaiscript_parser.hpp

@@ -1557,7 +1557,13 @@ namespace chaiscript
             throw exception::eval_error("Incomplete 'if' expression", File_Position(m_position.line, m_position.col), *m_filename);
           }
 
-          if (!(Operator() && Char(')'))) {
+          if (!Equation()) {
+            throw exception::eval_error("Incomplete 'if' expression", File_Position(m_position.line, m_position.col), *m_filename);
+          }
+
+          const bool is_if_init = Eol() && Equation();
+
+          if (!Char(')')) {
             throw exception::eval_error("Incomplete 'if' expression", File_Position(m_position.line, m_position.col), *m_filename);
           }
 
@@ -1585,7 +1591,11 @@ namespace chaiscript
             }
           }
 
-          build_match<eval::If_AST_Node<Tracer>>(prev_stack_top);
+          if (!is_if_init) {
+            build_match<eval::If_AST_Node<Tracer>>(prev_stack_top);
+          } else {
+            build_match<eval::If_Init_AST_Node<Tracer>>(prev_stack_top);
+          }
         }
 
         return retval;

unittests/init_if.chai

@@ -0,0 +1,7 @@
+if (var x = 2; x == 3) {
+  assert_true(false);
+} else if (var y = 3; y == 6) {
+  assert_true(false);
+} else { 
+  assert_equal(5, y + x);
+}