Is there a kind of static print in D?

Question

We do have static branching and traits along with a contract programming features on the other. Contracts are great because of the assert feature of delivering the proper message. And static checking doesn't bring any overhead to a compiled code. So naturally it would be very helpful to have a feature for giving proper error messages in a compile time.

For instance, when one tries to get a cross product of a vector and a differential 1-form with my function, with asserts I can specifically tell him about the issue in terms of domain, not about how the compiler sees it in terms of structures and arrays. Or I can just narrow down function arguments types statically but then the user would have to deal with compiler messages, not mine.

So, the question is: is there some way to print custom messages while compile time?

Answer 1

Yes, you can emit custom messages using several ways:

1. pragma(msg, message) will evaluate the message expression and print the result during compilation.
2. static assert(expression, message) will cause compilation to be aborted when expression, which is evaluated at compile-time, is false. This is the D equivalent of the C #error preprocessor directive.
3. During CTFE (Compile-Time Function Execution), uncaught exceptions and assert failures will act as a compile-time error, and emit the attached message as well.
4. The deprecated keyword can now have an explanation message.
5. There is currently no way to print an evaluated expression during CTFE interpretation, although ctfeWriteln is proposed as a writeln variant which works during CTFE interpretation.