But then why did you add your contract system to D? You implemented your contract system in the "early 90's", and D was released in 2001, so that's near a decade of "nobody wanted it". So then why add them as a core language feature of a new programming language if no one wanted it? Why is it still a core language feature? And why object to C++ finally adding contracts. I just don't get what you're even arguing here.
It's a great question! I simply had faith that it was a good idea.
The reason I started D in the first place is the C++ community was uninterested in any of my ideas for improvement. Ironically, C++ would up adopting a lot of them one by one! Such as contracts!
Contracts did find an enthusiastic constituency in the D community.
Contracts are a good idea, but I find the implementation of them to be clunky. I'd much rather contracts be part of the type system than as function signatures. Using the example in your earlier link, instead of defining day's 1..31 range within the Date-struct invariant, you'd instead declare a "day" type that's an int whose value cannot exceed 31 or be less than 1. This would be checked and enforced anytime a variable of the type is [re]assigned, set as a field, or passed-in as a parameter.
