Add some useful debug information to some exceptions (#475)

* Add some useful debug information to some exceptions

* Add guard for visitor functions preventing from cryptic errors due to incomplete visitors

* Add guard for js generator for preventing from cryptic errors due to incarrect stack manipulations

lib/compiler/passes/generate-js.js

@@ -547,7 +547,10 @@ function generateJS( ast, options ) {
             "",
             "        // istanbul ignore next",
             "        default:",
-            "          throw new Error(\"Invalid opcode: \" + bc[ip] + \".\");",
+            "          throw new Error(",
+            "            \"Rule #\" + index + \"" + ( options.trace ? " ('\" + peg$ruleNames[ index ] + \"')" : "" ) + ", position \" + ip + \": \"",
+            "            + \"Invalid opcode \" + bc[ip] + \".\"",
+            "          );",
             "      }",
             "    }",
             "",
@@ -574,6 +577,9 @@ function generateJS( ast, options ) {
 
         function s( i ) {
 
+            // istanbul ignore next
+            if ( i < 0 ) throw new Error( "Rule '" + rule.name + "': Var stack underflow: attempt to use var at index " + i );
+
             return "s" + i;
 
         } // |stack[i]| of the abstract machine
@@ -629,6 +635,7 @@ function generateJS( ast, options ) {
 
             function compileCondition( cond, argCount ) {
 
+                const pos = ip;
                 const baseLength = argCount + 3;
                 const thenLength = bc[ ip + baseLength - 2 ];
                 const elseLength = bc[ ip + baseLength - 1 ];
@@ -651,7 +658,9 @@ function generateJS( ast, options ) {
                     if ( thenSp !== elseSp ) {
 
                         throw new Error(
-                            "Branches of a condition must move the stack pointer in the same way."
+                            "Rule '" + rule.name + "', position " + pos + ": "
+                            + "Branches of a condition can't move the stack pointer differently "
+                            + "(before: " + baseSp + ", after then: " + thenSp + ", after else: " + elseSp + ")."
                         );
 
                     }
@@ -672,6 +681,7 @@ function generateJS( ast, options ) {
 
             function compileLoop( cond ) {
 
+                const pos = ip;
                 const baseLength = 2;
                 const bodyLength = bc[ ip + baseLength - 1 ];
                 const baseSp = stack.sp;
@@ -685,7 +695,11 @@ function generateJS( ast, options ) {
                 // istanbul ignore if
                 if ( bodySp !== baseSp ) {
 
-                    throw new Error( "Body of a loop can't move the stack pointer." );
+                    throw new Error(
+                        "Rule '" + rule.name + "', position " + pos + ": "
+                        + "Body of a loop can't move the stack pointer "
+                        + "(before: " + baseSp + ", after: " + bodySp + ")."
+                    );
 
                 }
 
@@ -907,7 +921,10 @@ function generateJS( ast, options ) {
 
                     // istanbul ignore next
                     default:
-                        throw new Error( "Invalid opcode: " + bc[ ip ] + "." );
+                        throw new Error(
+                            "Rule '" + rule.name + "', position " + ip + ": "
+                            + "Invalid opcode " + bc[ ip ] + "."
+                        );
 
                 }
 

lib/compiler/visitor.js

@@ -9,9 +9,17 @@ class ASTVisitor {
     // Will traverse the node, strictly assuming the visitor can handle the node type.
     visit( node ) {
 
+        // istanbul ignore next
+        if ( ! node ) throw new Error( "Visitor function called with no or `null` node" );
+
+        const func = this[ node.type ];
+
+        // istanbul ignore next
+        if ( ! func ) throw new Error( "Visitor function for node type '" + node.type + " not defined" );
+
         const args = __slice.call( arguments, 0 );
 
-        return this[ node.type ].apply( this, args );
+        return func.apply( this, args );
 
     }