How to test exceptions

[polidobj]

I'm thinking of writing test classes in Fusion to test my code. But how could we test that a function that throws exceptions throws one when it is expected to throw? The first way that comes to mind is being able to catch a thrown exception.

[pfusik]

I will add try/catch. This should be rather straightforward in most target languages, except for C, OpenCL (which don't have exceptions) and JavaScript/TypeScript (which cannot catch by type).

For now, you can test in one of the target languages, e.g. call Fusion code from C# and catch the exception in C# (either directly or with one of the unit test frameworks).

[msteinbeck]

Regarding exception handling in C, what is your opinion on transpiling exceptions to setjmp/longjmp? Functions that declare throws Exception could specify an additional argument, e.g., jmp_buf fut_jmp_buf, which is then used by throw. The exception message could be stored in a global (thread-local) variable. Typically, when using setjmp/longjmp, memory management is tricky because dynamically allocated memory is not released automatically. However, Fusion seems to already handle dynamically allocated memory when handling exceptions. This way, a function calling another function that declares throws Exception could create an "intermediate" jmp_buf instance that is passed to the called function and if an exception is thrown, clean up locally allocated memory before jumping to the caller via fut_jmp_buf. It is even imaginable to track the stack trace this way.

[pfusik]

Thanks for your comment!

I'm not happy with how the exceptions are currently transpiled to C:

• many return types (int, bool, enum, nullable references) are not allowed for functions throwing exceptions
• the exception type and the message are lost

I'm not a big fan of setjmp/longjmp - it's awkward to use. Especially the memory management on the user side with such a non-trivial control flow.

I consider:

• thread-local variables
• returning structures that contain both the result type and the exception info
• returning the exception as the return value while returning the actual value via a pointer parameter
• setting up exception handler functions (via a parameter or thread-local variable) - this user-provided handler could do a longjmp if the user opts to