Daniel Le Berre wrote:
Oleg Gusakov a écrit :
The tree could be walked in two directions, and these approaches are
equivalent, except for b and c optionality:
Representation #1: P2
a1 -> b1 or b2 or b3
a1 -> c1 or c2
b1 + b2 + b3 <= 1 # this one implicates that b is optional
c1 + c2 <= 1 # this one implicates that c is optional
Representation #2: Mercury
b1 -> a1
b2 -> a1
b3 -> a1
c1 -> a1
c2 -> a1
b1 + b2 + b3 = 1
c1 + c2 + c3 = 1
In the second one it's easier for me to keep the tabs on the generated
system. Timewise both are solved in 10s of millis.
Oleg,
I agree that you can encode either "provides" or "depends" semantic
using constraints.
However, you cannot ask the same question:
Representation "depends":
Append the package you want to install (a1=1).
Ask for a solution consistent with the constraints (or the best one).
(What do I need to install to be able to install a1).
Representation "provides":
Append the packages you have.
Ask for all the packages that can be safely installed according to the
constraints.
(what can I install if I have all those packages available in my
repositories)
If you append a1=1 in your representation, the Xx -> a1 constraints are
all satisfied, so are useless to discriminate a possible solution.
You find the right solution because of the equalities that force one bi
and one cj to be satisfied.
Just compare the two approaches on the same example with
additional d1, d2, d3 and e1, e2, e3, e4 in your repository.
I suspect that you will anyway throw them away because they will not be
part of the dirty tree.
So I have a simpler representation for you:
Representation #3: Mercury new proposal
b1 + b2 + b3 = 1
c1 + c2 = 1
Together with the objective function, it will find the expected answer :)
It would in this particular case. But any of the dependencies could be
declared as optional, then the fun begins ! So Xx -> a1 are essential if
Xx is declared optional. Plus - and it is critical - in the dirty tree a
node is a variable, so optional are not only explicitly declared as
such, but any node can potentially be eliminated.
At first I tried #3 approach and almost got it "working", but luckily,
the first real world try killed it.
Thanks for raising "depends" vs. "provides" semantics question, I have
not seen it that way, will have to add it to my mental model :)
Thanks,
Oleg
Daniel
