Re: [Jscheme-user] string-set!

[Timothy J. H. <ti...@cs...>]

On Monday, January 5, 2004, at 03:19 PM, Ken Anderson wrote:

> Is this a good idea?
>
> I've checked in a version of string-set! to allow for better use of 
> slib.
> The definition is:
>
> (define (string-set! s i v)
>   (.hash$# s 0)
>   (vector-set! (.value$# s) i v))
Is this going to create a problem with interned strings?
I seem to remember that we were interning literal strings by default in 
Jscheme.
>
> So when a character is set the hashcode is set to 0 so it will be 
> recomputed when needed.  So as long as you stop calling string-set! on 
> a string before putting it in a hashtable you'll be OK.  Using 
> string-set! on a string that in a hashtable will of course be a bad 
> thing to do.

I wasn't sure what you were doing until I went into Jscheme and played 
around a little, as follows:

 > (define s "abcdef")
"abcdef"
 > (.hash$# s)
0
 > (.value$# s)
[C@dc9766
 > (load "elf/basic.scm")
#t
 > (describe s)
abcdef
  is an instance of java.lang.String

   // from java.lang.String
   value: [C@dc9766
   offset: 0
   count: 6
   hash: 0
()
 >

So, you are messing around with the undocumented implementation of 
String, so as to make strings mutable!
This is clever, but violates all sorts of information hiding rules and 
fundamental precepts about object-oriented programming.

Very cool!

Sounds good!

Lets use it!


>
> I've put this in jsint/primproc.scm because it is part of r4rs, but 
> maybe it should
> get located with slib/  i can't decide.
>
> k

But maybe put it in the slib code since we only really need strings to 
be mutable inside slib and there may be
unforseen consequences (e.g. it might not work on all JDKs now and in 
the future...)

Eventually, we might need to make it uglier by having it check which 
Java version it is using before mucking around with
the internal undocumented representation....

---Tim---



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