[Jscheme-user] Re: JScheme questions

[Ken A. <kan...@bb...>]

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At 05:09 PM 7/18/2004 -0400, Alan Donovan wrote:

>Hi Ken, I've had a little play around with JScheme this weekend and I
>love it!  Especially for building GUIs, interactive programming is
>obviously the Right Way; it avoids the long library start-up times
>since you never stop.  There's also so much less "crap" to be typed in
>to implement a simple event handler.  It reminds me of interacting
>with 'wish' (Tcl/Tk).

Yes! I'm glad you like it. I find it a lot of fun!

>I have a couple of questions though:
>
>1) How does "eq?" and "eqv?" relate to ".equals"?  "eq?" seems to use
>   object identity and "eqv?" seems to implement 'equivalence', but
>   only for certain datatypes (e.g. Integer but not String).  What's
>   going on here?
> 
>   For example:
>
>   (eq? (Integer. 1) (Integer. 1))   =   #f   ;; reference equality
>
>   (eqv? (Integer. 1) (Integer. 1))   =   #t  ;; interpreter knows
>                                              ;; equivalence relation of
>                                              ;; Integers (?)
>
>   (eqv? (String. "foo") (String. "foo")) = #f  ;; interp doesn't know
>                                                ;; about Strings
>
>   (.equals (String. "foo") (String. "foo")) = #t   ;; use Java equivalence 
>   

http://www.math.grin.edu/courses/Scheme/r5rs-html/r5rs_48.html

(eq? x y) checks that x and y are the same object.  However, JScheme caches small Integers and Characters, so they may sometimes be eq?:

> (eq? 10 10)
#t
> (eq? 34141 34141)
#f

Also, JScheme uses Java semantics and interns string constants:
> (eq? "foo" "foo")
#t
> (eq? "foo" (String. "foo"))
#f

(eqv? x y) is  x and y are of the same atomic class and they are .equals?
This is actully different than the current implementation.  Currently,

> (eqv? 3L 3)
#t
> (eqv? 3.0 3)
#t
which i think should both return #f.

Tim, i'd like to propose this as a new definition of eqv?
(define (atomicClass? c)
  (or (eq? c Boolean.class)
      (eq? c Byte.class)
      (eq? c Short.class)
      (eq? c Integer.class)
      (eq? c Long.class)
      (eq? c Float.class)
      (eq? c Double.class)
      (eq? c Symbol.class)))

(define (eqv? x y)
  (or (eqv? x y)
      (let ((c (.getClass x)))
        (and (atomicClass? c)
             (eq? c (.getClass y))
             (.equals x y)))))

>   Also, I don't understand how this interacts with 'case', which
>   according to R4RS, uses the semantics of the 'eqv?' relation.
>   Therefore:
>
>       (case (String. "foo")
>         ((String. "foo")     #t)
>        (else                 #f))
> 
>   should yield the same as the eqv? expression above (#f) but does
>   not: it returns #t, which is surprising.  Does 'case' use .equals?

You're using case incorrectly.  Your code expands into:
(let ((temp var (String. "foo")))
  (cond ((member temp var '(String. "foo")) #t)
        (else #f)))

>   I suspect that many Java programs don't implement equals() very
>   well, in particular, some classes throw an exception if the RHS
>   operand cannot be casted as expected -- i.e. equals is implemented
>   as a partial function in the RHS.  I can see that this might be a
>   reason to avoid eagerly calling .equals.
>
>2) Do you have an Emacs scheme mode that correctly handles the
>   character literals #'(' etc?  My standard scheme mode gets rather
>   confused.

I use src/emacs/scheme-init.el, but it doesn't handle #'x' either.  This might be worth looking into.

>3) Do you have any JLib library + example code for building tree
>   widgets?  I've never much liked the Swing API, it's super-complex
>   when most of the time you just want to make something simple.

src/jsintc/jlib/demo/ClassBrowser.scm

>4) Is there any online documentation of all the available library
>   procedures?

http://jscheme.sourceforge.net/jscheme/doc/R4RSprimitives.html

You can also do apropos. and describing a function with print its definition:

car: {jsint.Primitive car[1]}
set-car!: {jsint.Primitive set-car![2]}
#f
> (describe describe)
Closure named describe
(lambda (x)
  (if (eq? x '#null)
    (display (!{} '"" x '" is null
"))
    (describe-object x)))
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