[m-dev.] Re: Sort of array bug.

[Fergus Henderson]

On 06-Feb-2001, Ralph Becket <rbeck at microsoft.com> wrote:
> >From Fergus Henderson on 06/02/2001 15:14:10
> > arrays don't use unique modes.  This implies that the compiler will
> > order the first ("FAULTY") example just using ordinary left-to-right
> > ordering, as constrained by the ordinary in/out modes, but ignoring
> > uniqueness.  So 
> 
> Thanks!  I need a `--do-what-I-mean' flag on the compiler.
> 
> > > Assuming this isn't my bug (I've spent half a day getting this 
> > > far...) I reckon some kind of public warning is in order!
> > 
> > Good idea.  A comment at the top of library/array.m is probably
> > the right place to document it.  There should be a similar comment
> > for bitmap and hash_table.
> 
> This sound all right (at the top of array.m with references to it
> in bitmap.m and hash_table.m)?
> 
> % WARNING!
> %
> % Arrays are currently not unique objects - until this situation is
> % resolved it is up to the programmer to ensure that arrays are used
> % in such a way as to preserve correctness.  In the absence of mode
> % reordering, one should therefore assume that evaluation will take
> % place in left-to-right order.  For example, the following code will
> % probably not work as expected (f is a function, A an array, I an
> % index, and X an appropriate value):
> %
> %       Y = f(A ^ elem(I) := X, A ^ elem(I))
> %
> % The compiler is likely to compile this as
> %
> %       V0 = A ^ elem(I) := X,
> %       V1 = A ^ elem(I),
> %       Y  = f(V0, V1)
> %
> % and will be unaware that the first line should be ordered
> % *after* the second.  The safest thing to do is write things out
> % by hand in the form
> %
> %       A0I = A0 ^ elem(I),
> %       A1  = A0 ^ elem(I) := X,
> %       Y   = f(A1, A0I)

That looks great.

> Of course, this is another place where ordered conjunction would be
> handy :)

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
--------------------------------------------------------------------------
mercury-developers mailing list
Post messages to:       mercury-developers at cs.mu.oz.au
Administrative Queries: owner-mercury-developers at cs.mu.oz.au
Subscriptions:          mercury-developers-request at cs.mu.oz.au
--------------------------------------------------------------------------