Re: macros (was: Seeking computer-programming job (Sunnyvale, CA))

From:

tar@sevak.isi.edu (Thomas A. Russ)

Newsgroups:

comp.lang.lisp,comp.lang.java.programmer

Date:

15 May 2009 09:34:29 -0700

Message-ID:

<ymiy6synx4a.fsf@blackcat.isi.edu>

Lew <lew@lewscanon.com> writes:

Stefan Ram wrote:

?? The same coded in Java without macros /is/ more verbose,
?? but also more readable to someone who knows Java:

public static String getMultiValue( Object valueObject )
{ String result = null;
?? if( valueObject == null )result = null;

?? else if( valueObject instanceof StringValue )
?? { StringValue stringValue =( StringValue )valueObject;
?? ?? result = getStringValue( stringValue ); }

?? else if( valueObject instanceof SprayValue )
?? { SprayValue setValue =( SprayValue )valueObject;
?? ?? result = getSetValue( setValue ); }

?? return result; }

An experienced Java programmer will see this series of 'instanceof'
operators and look for a way to code the thing polymorphically.

interface Multi<T>
{
   public String getValue( T arg );
}
class StringVersion implements Multi<String>
{
   public String getValue( String arg )
   {
     return getStringValue( arg );
   }
}
class SprayVersion implements Multi<Spray>
{
   public String getValue( Spray arg )
   {
    return getSprayValue( arg );
   }
}

etc.

So, then you either need to carefully wrap everything you want to handle
in a new object just in order to do the dispatch on it for this one
method.

(BTW, this is caused by the inability to add methods to a Java class
once it is defined. Another nice, flexible benefit of CLOS).

--
Thomas A. Russ, USC/Information Sciences Institute