unwind without throw

example/parser.cpp

@@ -0,0 +1,127 @@
+
+//          Copyright Oliver Kowalke 2016.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#include <cstdio>
+#include <cstdlib>
+#include <exception>
+#include <functional>
+#include <iostream>
+#include <memory>
+#include <sstream>
+
+#include <boost/context/fiber_context.hpp>
+
+namespace ctx = boost::context;
+
+/*
+ * grammar:
+ *   P ---> E '\0'
+ *   E ---> T {('+'|'-') T}
+ *   T ---> S {('*'|'/') S}
+ *   S ---> digit | '(' E ')'
+ */
+class Parser{
+   char next;
+   std::istream& is;
+   std::function<void(char)> cb;
+
+   char pull(){
+        return std::char_traits<char>::to_char_type(is.get());
+   }
+
+   void scan(){
+       do{
+           next=pull();
+       }
+       while(isspace(next));
+   }
+
+public:
+   Parser(std::istream& is_,std::function<void(char)> cb_) :
+      next(), is(is_), cb(cb_)
+    {}
+
+   void run() {
+      scan();
+      E();
+   }
+
+private:
+   void E(){
+      T();
+      while (next=='+'||next=='-'){
+         cb(next);
+         scan();
+         T();
+      }
+   }
+
+   void T(){
+      S();
+      while (next=='*'||next=='/'){
+         cb(next);
+         scan();
+         S();
+      }
+   }
+
+   void S(){
+      if (isdigit(next)){
+         cb(next);
+         scan();
+      }
+      else if(next=='('){
+         cb(next);
+         scan();
+         E();
+         if (next==')'){
+             cb(next);
+             scan();
+         }else{
+             throw std::runtime_error("parsing failed");
+         }
+      }
+      else{
+         throw std::runtime_error("parsing failed");
+      }
+   }
+};
+
+int main() {
+    try {
+        std::istringstream is("1+1");
+        // user-code pulls parsed data from parser
+        // invert control flow
+        char c;
+        bool done = false;
+        // execute parser in new execution context
+        ctx::fiber_context source{[&is,&c,&done](ctx::fiber_context && sink){
+            // create parser with callback function
+            Parser p( is,
+                      [&sink,&c](char c_){
+                            // resume main execution context
+                            c = c_;
+                            sink = std::move( sink).resume();
+                    });
+            // start recursive parsing
+            p.run();
+            // signal termination
+            done = true;
+            // resume main execution context
+            return std::move(sink);
+        }};
+        source = std::move( source).resume();
+        while(!done){
+            printf("Parsed: %c\n",c);
+            source = std::move( source).resume();
+        }
+        std::cout << "main: done" << std::endl;
+        return EXIT_SUCCESS;
+    } catch (std::exception const& e) {
+        std::cerr << "exception: " << e.what() << std::endl;
+    }
+    return EXIT_FAILURE;
+}