Michael G Schwern wrote: > How exactly does one "weaken" a precondition?
Say I define a mathematical mod() function in my parent number class. precondition: $a > 0, $b > 0, $b is int mod( $a, $b ) Then in my subclass, I want to make it work in a wider variety of contexts. I change the definition to: precondition: $b is int, $b != 0 mod( $a, $b ) It now properly supports modulo arithmetic for negative numbers, because the requirements for running the function have been weakened. It is still completely substitutable anywhere that the parent implementation would work, but if code is dealing with my subclass it is allowed to work with the weakened precondition. Since your proposal was based upon the idea of having every parent condition apply to child implementations, the possibility for strengthened postconditions falls out of the proposal nicely. But since preconditions work in the opposite direction, they don't quite gel with that particular proposal. Mike Lambert
