Add the ASTVisitor class (#451)

For compatibility with pre-0.11 plugins, this class lives on the same namespace as the orignal visitor helper and also exports a static method called 'build'.

lib/compiler/visitor.js

@@ -3,60 +3,103 @@
 const util = require( "../util" );
 const __slice = Array.prototype.slice;
 
-// Simple AST node visitor builder.
+// Abstract syntax tree visitor for PEG.js
-const visitor = {
+class ASTVisitor {
-    build( functions ) {
 
-        function visit( node ) {
+    // Will traverse the node, strictly assuming the visitor can handle the node type.
+    visit( node ) {
 
-            return functions[ node.type ].apply( null, arguments );
+        const args = __slice.call( arguments, 0 );
+
+        return this[ node.type ].apply( this, args );
 
     }
 
-        const visitNop = util.noop;
+    // Simple AST node visitor builder for PEG.js
+    static build( functions ) {
 
-        function visitExpression( node ) {
+        const visitor = new ASTVisitor();
+        util.extend( visitor, functions );
+        return visitor.visit.bind( visitor );
+
+    }
+
+}
+module.exports = ASTVisitor;
+
+// Helper's to create visitor's for use with the ASTVisitor class
+const on = ASTVisitor.for = {
+
+    // Visit a node that is defined as a property on another node
+    property( name ) {
+
+        return function visitProperty( node ) {
 
             const extraArgs = __slice.call( arguments, 1 );
+            const value = node[ name ];
 
-            visit.apply( null, [ node.expression ].concat( extraArgs ) );
+            if ( extraArgs.length )
+
+                this.visit.apply( this, [ value ].concat( extraArgs ) );
+
+            else
+
+                this.visit( value );
+
+        };
+
+    },
+
+    // Visit an array of nodes that are defined as a property on another node
+    children( name ) {
+
+        return function visitProperty( node ) {
+
+            const args = __slice.call( arguments, 0 );
+            const children = node[ name ];
+            const visitor = this;
+
+            const cb = args.length < 2
+                ? function withoutArgs( child ) {
+
+                    visitor.visit( child );
 
                 }
-
+                : function withArgs( child ) {
-        function visitChildren( children, extraArgs ) {
-
-            const args = [ void 0 ].concat( extraArgs );
-            const cb = extraArgs.length
-                ? function withArgs( child ) {
 
                     args[ 0 ] = child;
-                    visit.apply( null, args );
+                    visitor.visit.apply( visitor, args );
-
-                }
-                : function withoutArgs( child ) {
-
-                    visit( child );
 
                 };
 
             children.forEach( cb );
 
-        }
+        };
+
+    },
+
+};
+
+// Build the default ast visitor functions
+
+const visitNop = util.noop;
+const visitExpression = on.property( "expression" );
+
+const DEFAULT_FUNCTIONS = {
 
-        const DEFAULT_FUNCTIONS = {
     grammar( node ) {
 
         const extraArgs = __slice.call( arguments, 1 );
 
         if ( node.initializer ) {
 
-                    visit.apply( null, [ node.initializer ].concat( extraArgs ) );
+            this.visit.apply( this, [ node.initializer ].concat( extraArgs ) );
 
         }
 
         node.rules.forEach( rule => {
 
-                    visit.apply( null, [ rule ].concat( extraArgs ) );
+            this.visit.apply( this, [ rule ].concat( extraArgs ) );
 
         } );
 
@@ -65,9 +108,9 @@ const visitor = {
     initializer: visitNop,
     rule: visitExpression,
     named: visitExpression,
-            choice: util.createVisitor( "alternatives", visitChildren ),
+    choice: on.children( "alternatives" ),
     action: visitExpression,
-            sequence: util.createVisitor( "elements", visitChildren ),
+    sequence: on.children( "elements" ),
     labeled: visitExpression,
     text: visitExpression,
     simple_and: visitExpression,
@@ -81,14 +124,12 @@ const visitor = {
     rule_ref: visitNop,
     literal: visitNop,
     class: visitNop,
-            any: visitNop
+    any: visitNop,
-        };
 
-        util.extend( functions, DEFAULT_FUNCTIONS );
-
-        return visit;
-
-    }
 };
 
-module.exports = visitor;
+util.each( DEFAULT_FUNCTIONS, ( fn, name ) => {
+
+    ASTVisitor.prototype[ name ] = fn;
+
+} );