I think part of the confusion stems from ignoring the fact that the
case X of Y then.... end
statement DECLARES the variable Y
your procedure Pair can thus be rewritten like:
proc{Pair X Y}
case X of Z then {Browse 'in pair'#X#Z} end
end
This gives exactly the same results. The case always succeeds because
it is equivalent to:
case X of _
If you want to re-use the Y variable in the case statement, you have
to do it like this:
proc{Pair X Y}
case X of !Y then {Browse 'in pair'#X#Y} end
end
But the ! also causes the case statement to wait until Y becomes bound.
cheers,
Fred.
On 26 Sep 2005, at 23:49, Russ Abbott wrote:
If you replace
case X of Y then {Browse 'in pair'#X#Y} end
with simple unification
X = Y
to get
local A B
proc {Pair X Y}
X = Y
end
in
{Pair a A}
{Browse 1#A#B}
{Pair A b}
{Browse 2#A#B}
{Pair A B}
{Browse 3#A#B}
A = z
{Browse 4#A#B}
end
the output is
'in pair'#a#a
1#a#_
followed by a tell a = b exception. This is what I would expect
from straight unification.
But then I tried:
local A B
proc {Pair X Y}
{Browse 'before case'#X#Y}
case X of Y then {Browse 'after case'#X#Y} end
end
in
{Pair A b}
{Browse 1#A#B}
{Pair a A}
{Browse 2#A#B}
{Pair A B}
{Browse 3#A#B}
A = z
{Browse 4#A#B}
end
and got
'before case'#z#b
'after case'#z#z
1#z#_
'before case'#a#z
'after case'#a#a
2#z#_
'before case'#z#_
'after case'#z#z
3#z#_
4#z#_
How did Y change from b to z between the first line and the
second? Similar questions arise for the rest of the execution. How
should this be understood? Or is there a bug somewhere in the
implementation ?
-- Russ
_____________________________________________
Russ Abbott
Professor, Computer Science
California State University, Los Angeles
o Check out my blog at http://russabbott.blogspot.com/
On 9/26/05, Russ Abbott <[EMAIL PROTECTED]> wrote:
Here's an extension of the program below, which I admit has me
completely confused.
local A B
proc {Pair X Y}
case X of Y then {Browse 'in pair'#X#Y} end
end
in
{Pair a A}
{Browse 1#A#B}
{Pair A b}
{Browse 2#A#B}
{Pair A B}
{Browse 3#A#B}
A = z
{Browse 4#A#B}
end
The output is
'in pair'#a#a
1#z#_
'in pair'#z#z
2#z#_
'in pair'#z#z
3#z#_
4#z#_
On 9/26/05, Russ Abbott <[EMAIL PROTECTED] > wrote:
Well, I'll admit to being a bit mind twisted. (Actually I like that
state.) I learned prolog about 2 decades ago and have not kept up
with newer developments. I think in terms of Edinburgh prolog. I
use setof/3, etc. (But I don't do asserts.) But I'm still not
convinced that the way case is defined is the best approach.
For example, on p 80 in CTM we see that
local <pattern> = <expression> in ... end
does full unification even though it is not recommended to use it
that way.
In
local X in
case X of 3 then {Browse X} end
case 3 of X then {Browse X} end
end
the first of these suspends while the second succeeds. And in
local A
proc {Pair X Y}
case X of Y then {Browse X#Y} end
end
in
{Pair 3 A}
{Pair A 3}
end
both succeed although they give different results: 3#3 and _#_
respectively! (Why didn't the second call not suspend, and since
it didn't suspend, why didn't it bind Y to 3?)
I still think the semantics of case should be re-analyzed.
-- Russ
_____________________________________________
Russ Abbott
Professor, Computer Science
California State University, Los Angeles
o Check out my blog at http://russabbott.blogspot.com/
On 9/26/05, Raphael Collet <[EMAIL PROTECTED] > wrote:
Russ Abbott wrote:
> One solution would be to have two versions of the case statement
(case
> and caseunify), one that suspends if the pattern aribiter is not
> completely bound and one that does full unification.
No, this is definitely not a good idea. Prolog does pattern
matching by
*speculative* unification! It works only because Prolog is
*sequential*
and search is built-in.
Oz is concurrent, and search is encapsulated. The latter makes
that we
don't need a "cut" statement in Oz. Unification is more the
functional
style one.
> Not only would
> that offer a nice feature, it would solve the problem that new-
comers
> tend to get confused about this. If both language features were
> presented together in the documentation, there would be no
confusion.
Most newcomers are not from the Prolog world, and many find the "case"
statement quite natural. Prolog programmers are sometimes a bit
mind-twisted, and need some rehabilitation to normality ;-) I am sure
you will quickly get used to Oz.
Cheers,
raph
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Fred Spiessens
Reseacher Software Security
Université catholique de Louvain
Louvain-la-Neuve Belgium
[EMAIL PROTECTED]
http://www.info.ucl.ac.be/people/fsp/fred.html
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