[m-rev.] Minor addition to string.m

[Fergus Henderson]

On 31-Jan-2003, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> Peter Moulder, Thursday, 30 January 2003:
> > 
> > In perl, chomp removes no more than one newline.  E.g. "foo\n\n" -> "foo\n".
> > If the name `chomp' is retained, then I think the previously-posted
> > behaviour/implementation should be used.
> 
> I considered this.  I'm not sure the difference would be significant
> since it's almost certain only to be used to snip off the "\n" after
> a call to read_line_as_string.  I will change it to snip off just the
> one if anybody feels strongly.

I agree with Peter Moulder on this one.  If we're going to reuse the
Perl name, then we should use the same semantics.

> > On a similar vein, can use
> > 
> >   ( if I < N then
> >   	( if P(S ^ unsafe_elem(I)) then ... else I )
> >   else
> >   	I
> >   ).
> 
> I've applied the s/elem/unsafe_elem/ suggestion.  I don't think adding
> the extra if-then-else makes the code any more readable.

I agree about the readability, but making the s/elem/unsafe_elem/ change
on its own is not acceptable, because Mercury's commutative optional
semantics does not guarantee that the code will work -- the range
check might get reordered to occur after the call to unsafe_elem.

(That is, presuming I understood what you meant.  Since you didn't post
the revised diff, it's difficult to be sure.)

> The existing
> formulation, I < N, P(S ^ unsafe_elem(I)) works fine with the current
> compiler,

Are you sure?  With every possible combination of compiler options?
Have you tested them all?

Even if it does happen to work, it would still be a latent bug,
just waiting to be triggered by the next compiler optimization
that someone adds.

> but the correct solution is for us to add an explicit ordered
> conjunction which would forbid the reordering of its conjuncts for any
> reason.

I agree that adding a sequential conjunction operator would be a good
extension.  However, that doesn't justify writing broken code in the
mean time.

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