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

From:

Kaz Kylheku <kkylheku@gmail.com>

Newsgroups:

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

Date:

Fri, 15 May 2009 18:57:26 +0000 (UTC)

Message-ID:

<20090526224656.591@gmail.com>

["Followup-To:" header set to comp.lang.lisp.]
On 2009-05-15, Lew <lew@lewscanon.com> wrote:

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 );
}

This is cheating; you are redefining the object's type to suit the code.

Now valueObject must be a Multi<something> .

Can you make this gadget Just Work on the type Object?

Common Lisp:

  (defmethod get-value ((obj string-value))
    (get-string-value obj))

  (defmethod get-value ((obj spray-value))
    (get-spray-value obj))

Done; now we can juse use get-value to pull out the value of ither a
string-value or a spray-value.

class StringVersion implements Multi<String>
{
   public String getValue( String arg )
   {
     return getStringValue( arg );
   }
}

Even worse, you are wrapping objects now, yuck.

God for job security in tough economic times, though.