2012/6/14 lcaminiti <lorcaminiti@gmail.com>
IMO, a real good subcontracting example is:
http://contractpp.svn.sourceforge.net/viewvc/contractpp/releases/contractpp_...
This is a bit off topic, but I think there is something wrong with the postconditions of unique_identifiers::add(). If you require that the added indentifier must not be in the collection yet, why should you check in the postcondition if it was there before the addition: it is obvious it wasn't.
I'll clarify this in the docs, but there's nothing wrong with guarding a postcondition with its precondition. On the contrary, a function promises to satisfy its postconditions only when its preconditions were satisfied before body execution. Because subcontracted preconditions are checked in logic-or, overridden postconditions can be checked even if the related overridden preconditions were false (because the overriding preconditions are true and subcontracted preconditions are checked in logic-or). So guarding a base function postconditions with its preconditions allows the base class designer to program the fact that "the base function postcondition are ensured to be true only when the base function precondition are true". That gives the derived class designed more flexibility because he/she doesn't have to program the derived function to satisfy the base function postcondition under cases that satisfy the derived preconditions but not the base preconditions.
base::f::pre := (pre1) base::f::post := (pre1 ? post1 : true)
derived::f::pre := (pre1) or (pre2) // automatic `or` from subcontracting derived::f::post := (pre1 ? post1 : true) and (post2) // automatic `and` from subcontracting
1) base::f::pre must always satisfy post1 because it is valid to call it only when pre1 (preconditions) is true in which case post1 must be true too (postconditions).
2) However, derived::f does not have to always satisfy post1. If derived::f is called with pre1 false and pre2 true the call is valid because the subcontracted preconditions are:
(pre1) or (pre2) == (false) or (true) == true)
The subcontracted postconditions are (note that post1 disappears):
(pre1 ? post1 : true) and (post2) == (false ? post1 : true) and (post2) == (true) and (post2) == post2
When pre1 is false, dervied::f does not to satisfy the overridden postcondition post1, just post2 is ensured!
Note that this is possible only because the base class designer guarded post1 with pre1 so the base class designer is always in control and he/she can freely chose if to program guarded postconditions that can be "disabled" by derived classes so to grant derived class designers more flexibility. By default, that flexibility is not there in subcontracting that evaluates postconditions in logic-and, unless the base class designers intentionally guards its postconditions.
I thought this was a nice trick from [Micthell02] that shows how subcontracting preconditions might actually be of some use (but the logic it's tricky to follow and it might be confusing so if programmers don't need this they can disable it with the configuration macros and as required by N1962). Note that N1962 authors mentioned on this ML that they looked at extensive Eiffel code and talked with Eiffel programmers only to find out that no one actually uses subcontracted preconditions in practice. So all this might be of little practical interest.
Ok, I get it now. Thanks for taking the time to explain that. Now I also see what you meant by "confusing" the other day.
Keeping in mind the substitution principle: http://en.wikipedia.org/wiki/Liskov_substitution_principle If derived::f overrides base::f then derived::f must be used wherever base::f is used. Where base::f can be called, derived::f can be called. So where base::f preconditions are true, also derived::f (subcontracted) preconditions are true. That is ensured by subcontracting using the logic-or:
derived::f::subcontracted_pre := base::f::pre or dervied::f::pre
This is always true where base::f::pre is true so it always OK to call dervied::f where it was OK to call base::f (as the substitution principle says).
I think we interpret the "substitution principle" somewhat differently (tell me, if I am wrong). My interpretation is, that it is the author of
AFAIU, the substitution principle means that the Contract Programming methodology (and it's implementation in my library) should ensure the following:
I am trying to identify where precisely we disagree. I am not sure if this is just a matter of opinion. I do not think that "Contract Programming methodology [...] should ensure" anything. I have read the link you sent, and this is what I understand: A programmer John may (or may not) choose to write his program using the "Contract Programming methodology". If he does so, he is bound to obey some rules, one of such rules being that "subcontracted" preconditions cannot be strengthened. -- John must make sue that this happens. The role of the framework as yours -- as I see it (but I might be wrong) -- is to help _verify_ if John did his job right. That is, the job of your framework is somewhat "negative": you should be looking for mistakes and pointing them out; be a pain for the programmer (hence the default behavior of terminating the program). If John makes a mistake and over-constrains the precondition in the overridden method and you apply the logic-or, his mistake will never be revealed.
From: http://en.wikipedia.org/wiki/Liskov_substitution_principle
Liskov and Jeannette Wing formulated the principle succinctly in a 1994 paper as follows:
"Let q(x) be a property provable about objects x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T."
In the same paper, Liskov and Wing detailed their notion of behavioral subtyping in an extension of Hoare logic, which bears a certain resemblance with Bertrand Meyer's Design by Contract in that it considers the interaction of subtyping with pre- and postconditions. ... These are detailed in a terminology resembling that of design by contract methodology, leading to some restrictions on how contracts can interact with inheritance: 1. Preconditions cannot be strengthened in a subtype. 2. Postconditions cannot be weakened in a subtype. 3. Invariants of the supertype must be preserved in a subtype.
1. is implemented by checking subcontracted preconditions in logic-or with each other. 2. and 3. by checking subcontracted postconditions and class invaraints in logic-and.
the derived class (and the relaxed preconditions) that is responsible for making sure that certain semantic constraints are met -- not your framework. Overriding the preconditions so that they are not relaxation of
I think it is the Contract Programming responsibility to check and enforce the subcontracting semantic, and not the derived class designer. Note that when you program subcontracted preconditions, postconditions, and postconditions, you are not just programming assertions for the derived class in isolation from the base classes (just like when you implement a derived class you don't do that without considering the implications of inheriting from the base classes -- e.g., inherited members, etc).
Agreed that when constraining derived class John has to consider the constraints of the base class, but still having considered this he has to state his constraints correctly. Well, let me show by example what I mean: void Base::fun( int i ) precondition{ i > 0; } void Derived::fun( int i ) precondition{ i == 0; } The two conditions are completely unrelated, and to me, it is John's logic error to have put conditions like that. I can imagine now circumstances (e,g, some postconditions, or invariants, or whatever context) where such definition could be considered correct (am I wrong?). But if logic-or is guaranteed, you can treat the subcontracted precondition as valid one, which in contrived way says that (i >= 0). But it feels wrong to allow John saying his intentions this way. The framework should (this is how I see it) force John to state the precondition i >= 0 explicitly rather than the condition being "collected" from the inherited preconditions. But this is just one point of view, I guess. Regards, &rzej