Re: [Caml-list] Re: immutable strings (Re: Array 4 MB size limit)

[j h woodyatt <jhw@conjury.org>]

On May 24, 2006, at 10:56 PM, Martin Jambon wrote:
>
> So I'd really love to see actual examples where using immutable  
> strings would be such an improvement over mutable strings.
> If the problem is just to ensure that string data won't be changed  
> by the user of a library, then it is trivial using module  
> signatures and String.copy for the conversions.

I have no dog in this fight, but I can imagine a pragmatic approach  
that might satisfy some of these concerns without introducing much in  
the way of extra runtime complexity costs.

Change the way strings are boxed so that there is an additional tag  
for immutable strings as opposed to the normal mutable ones.  In all  
respects, allow string objects with either tag to be treated the same  
by functions that do not modify the content of the string.  When the  
"safe" variants of the string mutation functions are called on a  
string object with the immutable tag, throw a runtime exception.   
Finally, provide a function that changes the tag of a string from  
mutable to immutable, i.e. like a special blessed case of  
Obj.set_tag.  Not from immutable to mutable, though-- that should be  
disallowed.  If the -unsafe is not set, then allow a new compiler  
option -constliteral (or something) that would make all string  
literals immutable.  You'd need to use String.copy to get a mutable  
string with contents copied from an immutable one.

Of course, I can't offer a convincing reason to believe this would  
bring an improvement over what we have today.  My proposal would not  
give the programmer any assurance that contents of string values  
passed as parameters to library functions are not modified by the  
library, i.e. the "unsafe" functions would still work.  It *might*  
give a pragmatic benefit of surfacing unintentional programming  
errors earlier than they might otherwise be found, but I'm not  
prepared to argue in defense of that.

If you did immutable strings this way, it would be nice for the sake  
of consistency to have a comparable feature for arrays.


—
j h woodyatt <jhw@conjury.org>