I think this is quite a hard problem to solve. A Python wrapper of some some monolithic C library can check & reject any input which doesn't meet the spec, and

I think this is quite a hard problem to solve. A Python wrapper of some some monolithic C library can check & reject any input which doesn't meet the spec, and tell you why. But a Julia library typically wants to be generic, you should (for instance) be able to pass it weird numbers that contain units/error bars/gradient info, that the library designer knew nothing about, and have these propagate through. When this works it's great, but when it fails, the failure point tends to be 10 layers down, in internal workings you've never heard of. That's part of why error messages are inscrutable.