Try blocks are trying

[FeepingCreature]

On Monday, 11 October 2021 at 11:05:47 UTC, bauss wrote:
> On Monday, 11 October 2021 at 09:27:30 UTC, FeepingCreature
>> `export try` is the only feasible approach I can see offhand.
>
> Using alias this, opAssign and a boolean value you can actually 
> do something like this:
>
> ```d
> public struct BindOnce(T)
> {
>     private bool _isSet;
>     private T _value;
>
>     alias _value this;
>
>     void opAssign(T rhs)
>     {
>         if (_isSet)
>         {
>             throw new Error("The value has already been set.");
>         }
>
>         _isSet = true;
>         _value = rhs;
>     }
>
>     string toString() { return to!string(_value); } // Required 
> for writeln
> }
> ```
>
> Example Usage:
>
> ```
> int getNumber(int input)
> {
>     if (input == 0) throw new Exception("This function doesn't 
> allow multiplications of zero.");
>     return input * 2;
> }
>
> void main()
> {
>     BindOnce!int n;
>     try
>     {
>         n = getNumber(20);
>         //n = getNumber(40); // Uncommenting this line will 
> throw the error "The value has already been set."
>
>         writeln(n);
>     }
>     catch (Exception e)
>     {
>         writeln(e);
>         writeln(n);
>     }
> }
> ```
>
> Of course this assumes you're able to allow runtime errors and 
> that you don't blindly catches them and ignores them either 
> (which you shouldn't as Error shouldn't be recoverable!)

I almost don't want to say it.

Cough immutable cough...

That's why this really needs language support.

(Also I'd just use `Nullable` or `Rebindable` irl.)