zlacker

"Print 1" is trivial according to this: https://en.wikipedia.org/wiki/Rice%27s_theorem.

But day to day programs are not trivial... as for your example, just switch it with this code: `print(gcd(user_input--,137))`... now it's quite more hard to "just ban some final states"

>>yonixw+(OP)
Indeed, but panic is easier because in some ways checking if a program can panic is akin to checking if the program links the panic function.

And that's pretty easy to statically analyze.

>>yonixw+(OP)
That's not what "trivial property" means w.r.t. Rice's Thm.

The point is that you can produce a perfectly working analysis method that is either sound or complete but not both. "Nowhere in the entire program does the call 'panic()' appear is a perfectly workable analysis - it just has false positives.

>>yonixw+(OP)
Turing machines have a set of states and a transition function that governs how it moves between states. The transition function is a bunch of mappings like this:

    (input state, input symbol) --> (output state, output symbol, move left/right)

This is all static, so you can look at the transition table to see all the possible output symbols. If no transition has output symbol 1, then it never outputs 1. It doesn't matter how big the Turing machine is or what input it gets, it won't do it. This is basically trivial, but it's still a type of very simple static analysis that you can do. Similarly, if you don't have any states that halt, the machine will never halt.

This is like just not linking panic() into the program: it isn't going to be able to call it, no matter what else is in there.