[OT] OT: Null checks.

[Max Samukha]

On Tuesday, 6 May 2025 at 23:51:05 UTC, Walter Bright wrote:

> 2. if in invalid state, shut down immediately

There's one case where D blatantly violates this principle. 
Consider a system with allowed state transitions A->B, B->C, 
C->B, B->A:

```
enum State
{
     A, B, C
}

State state;

void AtoB()
{
     assert(state == State.A);
     state = State.B;
}

void BtoC()
{
     assert(state == State.B);
     state = State.C;
}

void CtoB()
{
     assert(state == State.C);
     throw new Exception("CtoB failed"); // or an Error, doesn't 
matter
     state = State.B;
}

void BtoA()
{
     assert(state == state.B);
     state = State.A;
}

void main()
{
     AtoB();
     scope(exit)
         BtoA(); // relies on the downstream handler having 
completed successfully.

     BtoC();
     scope(exit)
         CtoB(); // this handler runs, even though the program is 
in an invalid state.
}
```