[m-dev.] small bug fix to string.m

[Fergus Henderson]

Christopher Rodd SPEIRS, you wrote:
> 
> Fergus, could you please review this ...
> 	When looking at string__index, the comments stated that:
> %	`Char' is the (`Index' + 1)-th character of `String'.
> %	Fails if `Index' is out of range (negative, or greater than or
> %	equal to the length of `String').
>  but the code did not check if index was negative.

Nope, it did.

>  :- pragma(c_code, string__index(Str::in, Index::in, Ch::out), "
> -	if ((Word) Index >= strlen(Str)) {
> +	if ((Word) Index >= strlen(Str) || (Word) Index < 0) {

`Word' is an unsigned type.  The test `(Word) Index < 0' can
never succeed.

A better change would be to change the code to cast to `(Unsigned)'
rather than to `(Word)'.  That would make the code a little bit
clearer.  You might also want to add a comment.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3         |     -- the last words of T. S. Garp.