Re: the benefits of immutability

["Joe Marshall" <address@hidden>]

Hmmm,  a few points off:

  1.  When I said `the square root function and the ToString function'
       I intended that those be the `standard' Math.sqrt and Object.toString
       operators.  Although your `toString' and `sqrt' functions do perform
       the appropriate operations, they are essentially arbitrary objects
with
       the appropriate name, not the functions I asked for.

  2.  The returned object isn't the same kind of thing as Object.ToString,
       or Math.sqrt.

  3.  Consider these lines:
       return new Double(Math.sqrt(((Double)a).doubleValue()));}},
       return ((Fn)((Object[])a)[1]).ap(((Fn)((Object[])a)[1]).ap(b));
       System.out.println( ((Fn)(compose.ap(new
Object[]{sqrt,toString}))).ap(new Double(10)) );

       Too many parenthesis.


> Guess I didn't get the extra credit. ;)

Programming in Java is its own reward.



----- Original Message ----- 
From: <vkarvone@mappi.helsinki.fi>
To: "Joe Marshall" <jrm@ccs.neu.edu>
Cc: "Vadim Nasardinov" <el-vadimo@comcast.net>; "Perry E. Metzger"
<perry@piermont.com>; <ll1-discuss@ai.mit.edu>
Sent: Tuesday, August 19, 2003 12:27
Subject: Re: the benefits of immutability


> Quoting Joe Marshall <jrm@ccs.neu.edu>:
> > [...] write the following in Java:
> >
> > (defun compose (f g)
> >     (lambda (x) (funcall f (funcall g x))))
> >
> > and use it to compose the square root function and the
> > ToString function to create a function that takes numbers
> > and produces the printed representation of their square root.
>
> public class FunctionalJava {
>   public static class Fn {
>     public Object ap(Object a) {
>       return a;
>     }
>   }
>
>   public static final Fn
>     sqrt = new Fn() {
>         public Object ap(Object a) {
>           return new Double(Math.sqrt(((Double)a).doubleValue()));}},
>     toString = new Fn() {
>         public Object ap(Object a) {
>           return a.toString();}},
>     compose = new Fn() {
>         public Object ap(final Object a) {
>           return new Fn() {
>               public Object ap(Object b) {
>                 return
((Fn)((Object[])a)[1]).ap(((Fn)((Object[])a)[1]).ap(b));
>               }
>             };
>         }
>       };
>
>   public static void main(String argv[]) {
>     System.out.println( ((Fn)(compose.ap(new
> Object[]{sqrt,toString}))).ap(new Double(10)) );
>   }
> }
>
> > Extra credit if you can do it in 240 characters.  (three 80 character
> > lines)
>

>
> Regards,
>   Vesa Karvonen
>