Fixing C's Biggest Mistake

[Sebastiaan Koppe]

On Saturday, 31 December 2022 at 06:34:38 UTC, Walter Bright 
wrote:
> NonNull pointers:
>
>   int* p = ...;
>   nonnull int* np = isPtrNull(p) ? fatalError("it's null!") : p;
>   *np = 3; // guaranteed not to fail!
>
> Null pointers:
>
>   int* p = ...;
>   *p = 3;  // seg fault!
>
> Which is better? Both cause the program to quit on a null 
> pointer.

In a larger program the first one allows the programmer to do the 
check once and rely on it for the remainder of the program.

Essentially it leverages the type system to make invalid state 
unrepresentable. This simplifies subsequent code.

It is very much similar to representing a phonenumber using 
either a string or a dedicated phonenumber type. The way you 
construct an instance of the phonenumber type is through a check, 
and any function accepting it can rely on it. In contrast, if one 
uses strings to pass around phonenumbers, you will need so many 
checks everywhere you likely forget one.

>
> Having a hardware check is perfectly valid for checking things.
>

Not all targets have said check though.