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1   ////////////////////////////////////////////////////////////////////////////////
2   // MillScript: an Open Spice interpreter and batch website creation tool
3   // Copyright (C) 2001-2004 Open World Ltd
4   //
5   // This file is part of MillScript.
6   //
7   // MillScript is free software; you can redistribute it and/or modify it under
8   // the terms of the GNU General Public License as published by the Free
9   // Software Foundation; either version 2 of the License, or (at your option)
10  // any later version.
11  //
12  // MillScript is distributed in the hope that it will be useful, but WITHOUT
13  // ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
14  // FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
15  // more details.
16  //
17  // You should have received a copy of the GNU General Public License along with
18  // MillScript; if not, write to the Free Software Foundation, Inc., 59 Temple
19  // Place, Suite 330, Boston, MA  02111-1307  USA
20  ////////////////////////////////////////////////////////////////////////////////
21  package org.millscript.millscript.expr;
22  
23  import org.millscript.millscript.action.MemoAction;
24  import org.millscript.millscript.vm.CompilerState;
25  
26  /**
27   * This class implements a memo expression. A memo expression wraps a normal
28   * lambda expression, with the exception that the lambda expression must have
29   * one argument and return a single result. When the memo expression is applied
30   * the first time, the lambda expression is applied and it's result is
31   * remembered and returned. Subsequent applications of the memo expression with
32   * the same argument return the remembered value, rather than re-applying the
33   * function.
34   *
35   * @see org.millscript.millscript.syntax.DefineSyntax
36   * @see LambdaExpr
37   * @see MemoAction
38   */
39  public final class MemoExpr extends Expr< MemoAction > implements OneResult {
40  
41      /**
42       * The lambda expression, whose results will be remembered.
43       */
44      private final LambdaExpr lambda;
45  
46      /**
47       * Creates a new memo expression, for the specified lamba expression.
48       *
49       * @param   e   the lamba expression, whose results should be remembered
50       */
51      public MemoExpr( final LambdaExpr e ) {
52          lambda = e;
53      }
54  
55      /**
56       * @see org.millscript.millscript.expr.Expr#compileIt()
57       */
58      @Override
59      public MemoAction compileIt() {
60          return new MemoAction( lambda.compileIt() );
61      }
62  
63      /**
64       * @see org.millscript.millscript.expr.Expr#resolve(org.millscript.millscript.vm.CompilerState)
65       */
66      @Override
67      public void resolve( final CompilerState state ) {
68          this.lambda.resolve( state );
69      }
70  
71      /**
72       * @see org.millscript.millscript.expr.Expr#showComponents(int)
73       */
74      @Override
75      void showComponents( final int n ) {
76          this.lambda.show( n );
77      }
78  
79  }