Implement css error function for parser

Print context from actual parsed source as ruby sass does!

debugger.hpp

@@ -298,7 +298,7 @@ inline void debug_ast(AST_Node* node, string ind = "", Env* env = 0)
     cerr << ind << "Map " << expression << " [Hashed]" << endl;
   } else if (dynamic_cast<List*>(node)) {
     List* expression = dynamic_cast<List*>(node);
-    cerr << ind << "List " << expression <<
+    cerr << ind << "List " << expression << " (" << expression->length() << ") " <<
       (expression->separator() == Sass::List::Separator::COMMA ? "Comma " : "Space ") <<
       " [delayed: " << expression->is_delayed() << "] " <<
       " [interpolant: " << expression->is_interpolant() << "] " <<

parser.cpp

@@ -2187,4 +2187,39 @@ namespace Sass {
     throw Sass_Error(Sass_Error::syntax, ParserState(path, pos.line ? pos : before_token, Offset(0, 0)), msg);
   }
 
+  // print a css parsing error with actual context information from parsed source
+  void Parser::css_error(const string& msg, const string& prefix, const string& middle)
+  {
+    int max_len = 14;
+    const char* pos = peek < optional_spaces >();
+    bool ellipsis_left = false;
+    const char* pos_left(pos);
+    while (*pos_left && pos_left >= source) {
+      if (pos - pos_left > max_len) {
+        ellipsis_left = true;
+        break;
+      }
+      if (*pos_left == '\r') break;
+      if (*pos_left == '\n') break;
+      -- pos_left;
+    }
+    bool ellipsis_right = false;
+    const char* pos_right(pos);
+    while (*pos_right && pos_right <= end) {
+      if (pos_right - pos > max_len) {
+        ellipsis_right = true;
+        break;
+      }
+      if (*pos_right == '\r') break;
+      if (*pos_right == '\n') break;
+      ++ pos_right;
+    }
+    string left(pos_left, pos);
+    string right(pos, pos_right);
+    if (ellipsis_left) left = ellipsis + left;
+    if (ellipsis_right) right = right + ellipsis;
+    // now pass new message to the more generic error function
+    error(msg + prefix + quote(left) + middle + quote(right), pstate);
+  }
+
 }

parser.hpp

@@ -212,6 +212,11 @@ namespace Sass {
 #endif
 
     void error(string msg, Position pos);
+    // generate message with given and expected sample
+    // text before and in the middle are configurable
+    void css_error(const string& msg,
+                   const string& prefix = " after ",
+                   const string& middle = ", was: ");
     void read_bom();
 
     Block* parse();