Andrzej Krzemienski wrote
2012/6/13 lcaminiti <lorcaminiti@>
suggestion. I read in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1962.html that you can only relax preconditions; that is, you mustn't make them stronger. The issue I see with this is that the "mustn't" is left entirely to the good will of the derived class designer and never checked by anyone. So
I don't think that is correct. In fact, there's no way the derived class designer can strength a base class precondition because the base class precondition is checked in logic-or. For dervied::f the subcontracted preconditions are:
derived::f::subcontracted_pre := base::f::pre or derived::f::pre
This is always true when base::f::pre is true and regardless of dervied::f::pre so the subcontracted precondition cannot be stronger (more demanding, false when the base is true) no matter what the derived class designer does with derived::f::pre. In fact, subcontracting allows the base class designer to always remain in control as the base class contract (precondition, postconditions, and class invariants) must always be satisfied by all derived classes no matter how you design the derived classes. (Of course, if you don't use the library macros do define the derived class then you're on your own, there's no way my library can enforce anything.)
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.
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: 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). Eiffel stresses this out also syntactically using different keywords for subcontracted preconditions `ensure else` (else = logic-or) and for subcontracted postconditions `require then` (then = logic-and). Eiffel will give you an error if you use `ensure` or `require` instead of `ensure else` or `require then` for a derived class function that is overriding a base class function.
base preconditions is such a semantic error made by the author of the derived class. Your framework makes the (successful) effort to work around this error (using the logic-or), but I believe it is not the right thing to do. The whole purpose of the concept of DbC (apart from documentation) is to detect any "logic" errors at run-time,report them, and ideally prevent the further execution of the program. The logic-or appears to me like doing exactly the opposite: hiding a logic error. Even if other frameworks in Eiffel or D do it this way, I would dare to say that they are doing it wrong.
but when it comes to preconditions the problem is that the precondition in overriding function is not relaxed but in fact more constrained. This is the bug that your library could help detect. Of course it is not
to assert in general that one predicate represents a relaxation of another, but what you could do is, when you check preconditions and find that
'natural' it is stressed that one should not derive one type from another, possible the
overriding precondition failed but the overridden precondition passed, signal this at run-time as design error. In other words:
if (neither base nor derived precondition holds) { report precondition violation } else { if (derived precondition fails but base precond holds) { report "precondition design" error } else { precondition is satisfied } }
Or perhaps your library already works like that?
This would be easy to implement because my library internally goes over nested if statements like the one above when evaluating subcontracted preconditions. However, IMO having yet another configuration macro to enable detecting this case and explaining this case to the user complicates the situation even more. Again, I think this design is simpler and sufficient for the user "if you want to deal with subcontracted preconditions then they are as from the substitution principle (and Eiffel), otherwise you can disable subcontracted preconditions all together".
You are probably right that since the relaxation of preconditions is likely not to be usefull at all, making too much effort to have it work this way or the other, is not the economically correct way to go. Assuming that the implementation of Boost.Contract (didn't see it yet) is as thorough as its documentation, I can only imagine how much effort you must have put into it, and still will have to.
It is an impressive piece of work.
OK, now I'm going to brag a bit about my library ;) The docs are like an after-thought compared to the implementation. There are 289 source files for 32,286 lines of code of which 80% is preprocessor meta-programming. I wouldn't be surprised if this is one of the most extensive (ab)use of the pp ever! The programming required to parse the syntax (using pp), handle subcontracting (using pp and later introspection), and handle the `=` symbol in the `old_var = CONTRACT_OLDOF expr` (using pp, TMP, type deduction, default function parameters with initialization, and more) were quite a trip. But after all of this (fun) stuff that taught me a lot of C++, the real question still stands: will anyone ever find this library useful?? Ciao. --Lorenzo -- View this message in context: http://boost.2283326.n4.nabble.com/contract-relaxing-preconditions-in-subcon... Sent from the Boost - Dev mailing list archive at Nabble.com.