content: /* Alloy Analyzer 4 -- Copyright (c) 2006-2009, Felix Chang
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files
   * (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify,
   * merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
   * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
   * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF
   * OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
   */
  
  package edu.mit.csail.sdg.ast;
  
  import edu.mit.csail.sdg.alloy4.Err;
  import edu.mit.csail.sdg.ast.Sig.Field;
  import edu.mit.csail.sdg.parser.Macro;
  
  /**
   * This abstract class implements a Query visitor that walks over an Expr and
   * its subnodes. <br>
   * As soon as one of the node returns a nonnull value,the nonnull value will be
   * propagated to be the output of this visitor.
   * <p>
   * This default implementation will return null on all the leaf Expr nodes and
   * thus the final answer will be null. <br>
   * To implement a particular query, you need to extend this class.
   */
  
  public abstract class VisitQuery<T> extends VisitReturn<T> {
  
      /** Constructs a VisitQuery object. */
      public VisitQuery() {}
  
      /**
       * Visits an ExprBinary node (A OP B) by calling accept() on A then B.
       */
      @Override
      public T visit(ExprBinary x) throws Err {
          T ans = x.left.accept(this);
          if (ans == null)
              ans = x.right.accept(this);
          return ans;
      }
  
      /**
       * Visits an ExprList node F[X1,X2,X3..] by calling accept() on X1, X2, X3...
       */
      @Override
      public T visit(ExprList x) throws Err {
          for (Expr y : x.args) {
              T ans = y.accept(this);
              if (ans != null)
                  return ans;
          }
          return null;
      }
  
      /**
       * Visits an ExprCall node F[X1,X2,X3..] by calling accept() on X1, X2, X3...
       */
      @Override
      public T visit(ExprCall x) throws Err {
          for (Expr y : x.args) {
              T ans = y.accept(this);
              if (ans != null)
                  return ans;
          }
          return null;
      }
  
      /**
       * Visits an ExprConstant node (this default implementation simply returns null)
       */
      @Override
      public T visit(ExprConstant x) throws Err {
          return null;
      }
  
      /**
       * Visits an ExprITE node (C => X else Y) by calling accept() on C, X, then Y.
       */
      @Override
      public T visit(ExprITE x) throws Err {
          T ans = x.cond.accept(this);
          if (ans == null)
              ans = x.left.accept(this);
          if (ans == null)
              ans = x.right.accept(this);
          return ans;
      }
  
      /**
       * Visits an ExprLet node (let a=x | y) by calling accept() on "a", "x", then
       * "y".
       */
      @Override
      public T visit(ExprLet x) throws Err {
          T ans = x.var.accept(this);
          if (ans == null)
              ans = x.expr.accept(this);
          if (ans == null)
              ans = x.sub.accept(this);
          return ans;
      }
  
      /**
       * Visits an ExprQt node (all a,b,c:X1, d,e,f:X2... | F) by calling accept() on
       * a,b,c,X1,d,e,f,X2... then on F.
       */
      @Override
      public T visit(ExprQt x) throws Err {
          for (Decl d : x.decls) {
              for (ExprHasName v : d.names) {
                  T ans = v.accept(this);
                  if (ans != null)
                      return ans;
              }
              T ans = d.expr.accept(this);
              if (ans != null)
                  return ans;
          }
          return x.sub.accept(this);
      }
  
      /**
       * Visits an ExprUnary node (OP X) by calling accept() on X.
       */
      @Override
      public T visit(ExprUnary x) throws Err {
          return x.sub.accept(this);
      }
  
      /**
       * Visits a ExprVar node (this default implementation simply returns null)
       */
      @Override
      public T visit(ExprVar x) throws Err {
          return null;
      }
  
      /**
       * Visits a Sig node (this default implementation simply returns null)
       */
      @Override
      public T visit(Sig x) throws Err {
          return null;
      }
  
      @Override
      public T visit(Func x) throws Err {
          return null;
      }
  
      @Override
      public T visit(Assert x) throws Err {
          return null;
      }
  
      @Override
      public T visit(Macro x) throws Err {
          return null;
      }
  
      /**
       * Visits a Field node (this default implementation simply returns null)
       */
      @Override
      public T visit(Field x) throws Err {
          return null;
      }
  }
  