https://d.puremagic.com/issues/show_bug.cgi?id=6856
fckingspmfr...@gmail.com changed:
What |Removed |Added
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CC| |fckingspmfr...@gmail.com
--- Comment #33 from fckingspmfr...@gmail.com 2013-11-30 02:42:07 PST ---
A weaker precondition B compared to the stronger precondition A is in the
mathematical and logical sense an implication:
A -> B
That means firstly, whenever the precondition A is met then the weaker
condition
B _must_ be met, too. Also logically this means:
not B -> not A.
As B has the weaker condition, every time B is not met, A _must_ not be met
too. This is because B meets _at least_ all cases that A meets. This is the
definition of a weaker condition.
Also:
interface A {
void foo()
in {
// strong precondition
}
}
class B : A {
override void foo()
in {
// weaker precondition
}
body {
}
}
A a = new B();
a.foo(); // The precondition of A must be checked.
B b = new B();
b.foo(); // The precondition of B must be checked.
Also in the above example's call a.foo(), we must use the static type A to
determine the precondition. If we would take the precondition of type B in
consideration, we would check a too weak precondition. this violates the Liskov
substitution principle!
In the call b.foo() we must only check the precondition of type B as this is
the weaker one. If it succeeds all is fine. If it fails then the precondition
of A would fail too, as B covers at least all cases in which of A does not fail
(see the implications at the top).
Also I recommend to force every derived precondition to enforce the use of a
keyword like "weaken" to explicitly show that this must be a weaker
precondition:
class B : A {
override void foo()
weaken in {
// weaker precondition
}
body {
}
}
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