RE: [Sablevm-developer] Continuations (Part 1)

[Brent F. <bre...@xp...>]

> In fact, can you write for me a simple example that requires
> continuation?
> 
Here goes:  

I took this example from 
http://www.nightmare.com/schintro-v14/schintro_127.html
which I need to read through -- it seems to have lots of useful
information.
-------------------------------------------
(define some-flag #t)

(define (my-abortable-proc escape-proc)
(display "in my-abortable-proc")
(if some-flag
      (escape-proc "ABORTED"))
(display "still in my-abortable-proc")
  "NOT ABORTED")

(define (my-resumable-proc)
  (do-something)
  (display (call-with-current-continuation my-abortable-proc))
  (do-some-more))

(define (do-something)
  (display "Doing something."))

(define (do-some-more)
  (display "Doing some more."))

(my-resumable-proc)
-------------------------------------------------

The result of this is:

Doing something.
in my-abortable-proc
still in my-abortable-proc
ABORTED
Doing some more.

The "ABORTED" shows that the call-with-current-continuation
was used to jump back to its calling point.

A neat Scheme-on-java implementation (SILK)
http://www.cs.brandeis.edu/silk/silkweb/doc/resources/HOWTO.html#class
allows you to compile Scheme to Java.  Running this test.scm
through that yields this:

------------------------------------------------------------------
// no package name;
  /**
    * this file is automatically generated by the silk->javac compiler
Compiler.scm. 
    * Modify at your own risk!
    */
  import silk.*;
  import java.lang.reflect.*;
  import java.util.*;
  
  
  public class test extends silk.Procedure implements silk.Function,
Runnable {  
      public int whichcode=0;  // corresponds to a numbering of the toplevel
procedures of the program
      public int whichtype=0;  // 0 = user defined procedure, 1 = java
literal
      public static final int USER_DEF=0, JAVA_LIT=1;
      public Pair frame;
      public test() { super();}
      public test(int t, int n, Pair f) {
          whichtype = t;
          whichcode = n;
          frame = f;
      }
  
      private Boolean addImport(String s) {
         silk.Import.addImport(s);
         return Boolean.TRUE;
      }
  
      public void run() {
        this.invoke(null);
      } 
  
      public Object apply(Object[] args) {
        return invoke((Pair)args[0]);
      }
  
      public Object apply(Pair args) {
        return invoke(args);
      }
  
      public Object invoke(Pair args)
      {
        return LCO.eval(invoke1(args));
      }
  
    static Object tmp;
  
  
    public static void load() { 
       new test().init();
    }

    public static void load(String shellArgs[]) {
       silk.Symbol.intern("shellArgs").setGlobalValue(shellArgs);
       load();
    }

    public static void main(String shellArgs[]) {
       Symbol main = silk.Symbol.intern("main");
       load(shellArgs);
       if (main.isDefined())
          ((silk.Procedure) main.getGlobalValue()).apply(new
Pair(shellArgs,Pair.EMPTY));
     }

      public Object invoke1(Pair args) {
      if (whichtype == USER_DEF) {
       switch (whichcode) {
          case 0: return(_L0(args)); 
          case 1: return(_L1(args)); 
          case 2: return(_L2(args)); 
          case 3: return(_L3(args)); 

         default: System.exit(0); break;
       }}
      else {
       switch (whichcode) {
 
         default: System.exit(0); break;
       }}



      return null;
      }


  
   public void init() {
     Pair Args = null;
     Symbol.intern("this").setGlobalValue(this);
     Class _p = Primitive.class; // this loads the primitives
Symbol.intern("some-flag").setGlobalValue(_C0);

Symbol.intern("my-abortable-proc").setGlobalValue(new test(USER_DEF, 0, new
Pair( Args, this.frame)));

Symbol.intern("my-resumable-proc").setGlobalValue(new test(USER_DEF, 1, new
Pair( Args, this.frame)));

Symbol.intern("do-something").setGlobalValue(new test(USER_DEF, 2, new Pair(
Args, this.frame)));

Symbol.intern("do-some-more").setGlobalValue(new test(USER_DEF, 3, new Pair(
Args, this.frame)));

((silk.Procedure)
((Symbol)my_45_resumable_45_proc).getGlobalValue()).apply(Pair.EMPTY);

  
   }

 // definitions of global variables 
public static Object some_45_flag = Symbol.intern("some-flag");

public static Object my_45_abortable_45_proc =
Symbol.intern("my-abortable-proc");

public static Object my_45_resumable_45_proc =
Symbol.intern("my-resumable-proc");


 // definitions of Scheme variables defined externally
   static Object display= Symbol.intern("display");

   static Object do_45_something= Symbol.intern("do-something");

   static Object call_45_with_45_current_45_continuation=
Symbol.intern("call-with-current-continuation");

   static Object do_45_some_45_more= Symbol.intern("do-some-more");


 // definitions of quoted terms 
   static Object _C0=Boolean.FALSE;

   static Object _C1="in my-abortable-proc";

   static Object _C2="ABORTED";

   static Object _C3=Boolean.TRUE;

   static Object _C4="still in my-abortable-proc";

   static Object _C5="NOT ABORTED";

   static Object _C6="Doing something.";

   static Object _C7="Doing some more.";


 // definitions of embedded lambdas 
 Object _L0(Pair Args){
   Object tmp; 
 tmp = ((silk.Procedure) ((Symbol)display).getGlobalValue()).apply(new
Pair(_C1,
    Pair.EMPTY));

tmp = 
  (Boolean.FALSE.equals(((Symbol)some_45_flag).getGlobalValue())
  ?
  _C3
  :
  ((silk.Procedure) (( Pair) Args).getEltNover2(1)).apply(new Pair(_C2,
    Pair.EMPTY))
  );

tmp = ((silk.Procedure) ((Symbol)display).getGlobalValue()).apply(new
Pair(_C4,
    Pair.EMPTY));

tmp = _C5;


   return tmp; 
}

 Object _L1(Pair Args){
   Object tmp; 
 tmp = ((silk.Procedure)
((Symbol)do_45_something).getGlobalValue()).apply(Pair.EMPTY);

tmp = ((silk.Procedure) ((Symbol)display).getGlobalValue()).apply(new
Pair(((silk.Procedure)
((Symbol)call_45_with_45_current_45_continuation).getGlobalValue()).apply(ne
w Pair(((Symbol)my_45_abortable_45_proc).getGlobalValue(),
    Pair.EMPTY)),
    Pair.EMPTY));

tmp = new LCO(((Symbol)do_45_some_45_more).getGlobalValue(),Pair.EMPTY);


   return tmp; 
}

 Object _L2(Pair Args){
   Object tmp; 
 tmp = new LCO(((Symbol)display).getGlobalValue(),new Pair(_C6,
    Pair.EMPTY));


   return tmp; 
}

 Object _L3(Pair Args){
   Object tmp; 
 tmp = new LCO(((Symbol)display).getGlobalValue(),new Pair(_C7,
    Pair.EMPTY));


   return tmp; 
}

-------------------------------

This class obviously relies on the Silk interpreter code.  I
tried creating a stand-alone version (in a *.class file) under
Kawa, but couldn't get it to work right under Windows (damn
pathnames!).

Thanks,

-Brent