Re: Compiling procedures questions

[Per Bothner <per at bothner dot com>]

Crystal, Mayer wrote:

> I'm relatively new to kawa (and scheme for that matter) and have a question
> about compiling procedures into bytecode and using them in a java program.
> The scenario I am coding is as follows:

You've done a good job of figuring out APIs that aren't realy documented
or designed for end-users, so it's understandable you got some things
wrong.

> -  I have created three procedures (which extend Procedure0) and they all
> return Boolean.TRUE or Boolean.FALSE.
> -  I have written a rule of the following form:  (or proc1? (or proc2?
> proc3?))
> -  I compile the rule into a procedure using the following code:
>
>     /**
>      * A helper method to compile a String rule into a Procedure.
>      *
>      * @param rule The string representation of the rule
>      */
>     private static final Procedure compileRule( String rule ) throws
> Exception {
>             Scheme scheme = new Scheme( Scheme.builtin() );

Leave out the 'Scheme.builtin()':
              Scheme scheme = new Scheme();

>             scheme.eval( "(load \" + MyProcs.class.getName() + ".class\")"
> );

Loading a class file by name isn't really recommended.  The meaning of
that is hard to nail down.  On the other hand require is better:
   (require <MyProcs>)

>             InPort port = new InPort( new CharArrayInPort( rule ) );
>             SourceMessages messages = new SourceMessages();
>             Compilation comp = scheme.parseFile( port, true, messages );
>
>             Class cl = ModuleExp.evalToClass( comp );
>
>             if ( messages.getErrorCount() > 0 ) {
>                 StringBuffer sb = new StringBuffer();
>                 sb.append( "Failed compiling: " );
>                 sb.append( messages.toString( messages.getErrorCount() ) );
>                 throw new Exception( sb.toString() );
>             }
>
>             Procedure p = null;
>             try {
>                 p = (Procedure) cl.newInstance();
>             }
>             catch ( Exception e ) {
>                 throw new Exception( "Could not create new instance of rule:
> " + rule );
>             }
>
>             return p;
>     }

The 'p' which is an instance of 'cl' is a ModuleBody.  It's not a
normal procedure, but the top-level forms of the module.

> - The compilation succeeds and is cached.
> - I evaluate the rule using the following code:
>
> 	return ((Boolean)rule.apply0()).booleanValue();

Not sure where 'rule' is coming from.  The only one in your
program is the parameter to compileRule, which is a String.

> The evaluation throws a ClassCastException reporting that I am attempting to
> cast my proc1 class to a Boolean.  Did I miss something in the compilation
> step?  How do a compile the procedure (and any references it may have to
> other procedures) such that after compilation I can call an apply() method
> and retrieve the final boolean result?

One machanism is:

(Boolean) (((Procedure) scheme.eval("rule")).apply0())

--
	--Per Bothner
per@bothner.com   http://per.bothner.com/