Parallel visitor supports editing

Author: leebyron

src/language/__tests__/visitor.js

@@ -851,6 +851,128 @@ describe('Visitor', () => {
       ]);
     });
 
+    it('allows for editing on enter', () => {
+      var visited = [];
+
+      var ast = parse('{ a, b, c { a, b, c } }', { noLocation: true });
+      var editedAst = visit(ast, visitInParallel([
+        {
+          enter(node) {
+            if (node.kind === 'Field' && node.name.value === 'b') {
+              return null;
+            }
+          }
+        },
+        {
+          enter(node) {
+            visited.push([ 'enter', node.kind, node.value ]);
+          },
+          leave(node) {
+            visited.push([ 'leave', node.kind, node.value ]);
+          }
+        },
+      ]));
+
+      expect(ast).to.deep.equal(
+        parse('{ a, b, c { a, b, c } }', { noLocation: true })
+      );
+
+      expect(editedAst).to.deep.equal(
+        parse('{ a,    c { a,    c } }', { noLocation: true })
+      );
+
+      expect(visited).to.deep.equal([
+        [ 'enter', 'Document', undefined ],
+        [ 'enter', 'OperationDefinition', undefined ],
+        [ 'enter', 'SelectionSet', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'a' ],
+        [ 'leave', 'Name', 'a' ],
+        [ 'leave', 'Field', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'c' ],
+        [ 'leave', 'Name', 'c' ],
+        [ 'enter', 'SelectionSet', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'a' ],
+        [ 'leave', 'Name', 'a' ],
+        [ 'leave', 'Field', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'c' ],
+        [ 'leave', 'Name', 'c' ],
+        [ 'leave', 'Field', undefined ],
+        [ 'leave', 'SelectionSet', undefined ],
+        [ 'leave', 'Field', undefined ],
+        [ 'leave', 'SelectionSet', undefined ],
+        [ 'leave', 'OperationDefinition', undefined ],
+        [ 'leave', 'Document', undefined ]
+      ]);
+    });
+
+    it('allows for editing on leave', () => {
+      var visited = [];
+
+      var ast = parse('{ a, b, c { a, b, c } }', { noLocation: true });
+      var editedAst = visit(ast, visitInParallel([
+        {
+          leave(node) {
+            if (node.kind === 'Field' && node.name.value === 'b') {
+              return null;
+            }
+          }
+        },
+        {
+          enter(node) {
+            visited.push([ 'enter', node.kind, node.value ]);
+          },
+          leave(node) {
+            visited.push([ 'leave', node.kind, node.value ]);
+          }
+        },
+      ]));
+
+      expect(ast).to.deep.equal(
+        parse('{ a, b, c { a, b, c } }', { noLocation: true })
+      );
+
+      expect(editedAst).to.deep.equal(
+        parse('{ a,    c { a,    c } }', { noLocation: true })
+      );
+
+      expect(visited).to.deep.equal([
+        [ 'enter', 'Document', undefined ],
+        [ 'enter', 'OperationDefinition', undefined ],
+        [ 'enter', 'SelectionSet', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'a' ],
+        [ 'leave', 'Name', 'a' ],
+        [ 'leave', 'Field', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'b' ],
+        [ 'leave', 'Name', 'b' ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'c' ],
+        [ 'leave', 'Name', 'c' ],
+        [ 'enter', 'SelectionSet', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'a' ],
+        [ 'leave', 'Name', 'a' ],
+        [ 'leave', 'Field', undefined ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'b' ],
+        [ 'leave', 'Name', 'b' ],
+        [ 'enter', 'Field', undefined ],
+        [ 'enter', 'Name', 'c' ],
+        [ 'leave', 'Name', 'c' ],
+        [ 'leave', 'Field', undefined ],
+        [ 'leave', 'SelectionSet', undefined ],
+        [ 'leave', 'Field', undefined ],
+        [ 'leave', 'SelectionSet', undefined ],
+        [ 'leave', 'OperationDefinition', undefined ],
+        [ 'leave', 'Document', undefined ]
+      ]);
+    });
+
   });
 
 });

src/language/visitor.js

@@ -268,8 +268,7 @@ function isNode(maybeNode) {
  * Creates a new visitor instance which delegates to many visitors to run in
  * parallel. Each visitor will be visited for each node before moving on.
  *
- * Visitors must not directly modify the AST nodes and only returning false to
- * skip sub-branches or BREAK to exit early is supported.
+ * If a prior visitor edits a node, no following visitors will see that node.
  */
 export function visitInParallel(visitors) {
   const skipping = new Array(visitors.length);
@@ -285,6 +284,8 @@ export function visitInParallel(visitors) {
               skipping[i] = node;
             } else if (result === BREAK) {
               skipping[i] = BREAK;
+            } else if (result !== undefined) {
+              return result;
             }
           }
         }
@@ -298,6 +299,8 @@ export function visitInParallel(visitors) {
             const result = fn.apply(visitors[i], arguments);
             if (result === BREAK) {
               skipping[i] = BREAK;
+            } else if (result !== undefined && result !== false) {
+              return result;
             }
           }
         } else if (skipping[i] === node) {