Promise


1   package kawa.lang;
2   import gnu.mapping.*;
3   import gnu.text.Printable;
4   import gnu.lists.Consumer;
5   
6   /** Implement Scheme "promises".
7    * @author Per Bothner
8    */
9   
10  public class Promise implements Printable
11  {
12    Procedure thunk;
13  
14    /** The result - or null if it is not ready. */
15    Object   result;
16  
17    /** Create a new Promise that will evaluate thunk when forced. */
18    public Promise (Procedure thunk)
19    {
20      this.thunk = thunk;
21    }
22  
23    public Object   force () throws Throwable  
24    {
25      if (result == null)
26        {
27      Object   x = thunk.apply0 ();
28      if (result == null)
29        result = x;
30        }
31      return result;
32    }
33  
34    public void print (Consumer out)
35    {
36      if (result == null)
37        out.write("#<promise - not forced yet>");
38      else
39        {
40      out.write("#<promise - forced to a ");
41      out.write(result.getClass().getName());
42      out.write ('>');
43        }
44    }
45  }
46

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