Want to make Exceptions instances of Show? about cello HOT 2 CLOSED

Yeah this is a tricky issue. You're analysis about the cause is correct. The reason we can't declare Exceptions as actual instances of types is this:

Through some tricks we've managed to let the user declare Type objects such as "Int" at compile time. But it is still impossible for a user to declare instances of this object type at compile time because Types such as "Int" are declared as "extern" (or "global" in Cello). For example this will not compile.

var x = $(Int, 5);

int main(int argc, char** argv) {
  show(x);
}

This is why we declare Exceptions using Singleton, which makes them Type objects like Int. This is done because it would be very annoying for users to have to declare their exception types at the start of the program code and delete them at the end (and we couldn't do it for builtin exceptions anyway).

One hack might be this: rather than use Singleton use a different macro (say Exception) which similarly constructs an empty type, but leaves in an extra flag specifying that this type is an exception type and that upon show to as_str it should also print out any active exception message.

from cello.

Although there are various ways to do this I think the correct way at the end of the day is to just let users get the exception message manually using Exception_Message().

There are a number of alternative workarounds but all of them would essentially be tricking the users where it is better if they gain a real understanding of what is going on as is said above. So I'm closing this issue for now.

from cello.