Newsgroups: comp.lang.scheme
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From: dorai@tone.rice.edu (Dorai Sitaram)
Subject: Re: Explode/Implode Query
Message-ID: <1991Apr22.215944.19625@rice.edu>
Sender: news@rice.edu (News)
Organization: Rice University, Houston
References: <1991Apr22.180734.21661@cunixf.cc.columbia.edu> <raja.672349553@copper>
Date: Mon, 22 Apr 91 21:59:44 GMT

In article <raja.672349553@copper> raja@copper.ucs.indiana.edu (Raja Sooriamurthi) writes:
>But you can define these function yourself in any R3.99 scheme as
>below:
>
>; (explode 'apple) => (a p p l e)
>; (implode '(a p p l e)) => apple
>; (eq? 'apple (implode (explode 'apple))) => #t
>
> [perfectly reasonable Scheme "interpretations" of explode and implode deleted]

You have "typed" explode and implode to go between symbols and
single-char symbols.  However, the way I remember it, the following
were quite legal.

(explode 'apple2) => (a p p l e 2)      ;;; not (... |2|)
(implode '(a p p l e 2)) => apple2
(explode 42) => (4 2)			;;; not (|4| |2|) 
(implode '(4 2)) => 42			;;; not |42| 

Indeed, I think I remember seeing explode even take on lists!!  E.g.,

(explode '(apple 2)) => (|(| a p p l e | | 2 |)|)

An unsatisfactory first attempt at getting something like this in Ch*z
-- or a Scheme with stringports -- would be

(define digit-or-single-char-symbol
  (let ([zero (char->integer #\0)])
    (lambda (c)
      (if (and (char>=? c #\0) (char<=? c #\9))
	  (- (char->integer c) zero)
	  (string->symbol (format "~a" c))))))

(define explode
  (lambda (x)
    (map digit-or-single-char-symbol
	 (string->list (format "~a" x)))))

(define implode
  (lambda (s)
    (let ([p (open-input-string
	      (apply string-append
		     (map (lambda (c) (format "~a" c)) s)))])
      (begin0 (read p)
	      (close-input-port p)))))

This is unsatisfactory, since you'll have to do some more work to
get the second half of the following working:

(explode '|apple 2|) => (a p p l e | | 2)
(implode '(a p p l e | | 2)) => |apple 2|	;;; not apple

--d