On Wednesday, 11 April 2018 at 16:16:33 UTC, Jonathan M Davis wrote:
If we actually end up with a language improvement that makes it so that contracts are compiled in based on the caller instead of the callee, then I'll start using contracts. Until then, I'm not generally going to bother.

My first proposal suggested allowing the contracts at the top of the function body. If you mixed that idea with what H.S. Teoh later proposed, the result would have looked like:

int fun(int a) {
   in(a >= 0);
   out(r; r > 0);

It's not much different from if you did:

int fun(int a) {
   assert(a >= 0);
   typeof(return) result;
   scope(success) assert(result > 0);
   ...use `result`...

For anyone who wants even more convenience in how to write contracts this idea would still possible to add, although it's probably not that important.

But that idea received criticism on principle, that contracts are part of the signature and not the body. I didn't much care about the criticism myself, because I just wanted the contracts to be as ergonomic as possible so that people would use them. They seemed like a feature of D whose syntax was not up to the same standards as the rest of D, whose syntax otherwise is a major selling point.

However, eventually I was convinced that the accepted proposal is better precisely because of the possibility of a future implementation where the caller checks rather than the callee. In this case the contracts *must* be in the signature, because the body could be missing altogether. So basically, I agree with H.S. Teoh. A future DIP which allows caller-side checking will be all about the implementation, rather than about the syntax, and may or may not face opposition precisely for that reason, I don't know. From the user's point of view it amounts to nothing more than being able to use contracts in more places, i.e. in precompiled code, and with better error messages that fault the caller instead of the callee. I don't feel technically qualified to write that DIP, but I'm glad that the current DIP is designed with that one in mind.

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