If Statement with Declaration

[Jonathan Marler via Digitalmars-d]

On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote:
> So I was thinking of a way of extending if statements that have 
> declarations. The following being as example of the current use 
> of if statements with declarations:
>
>     if(int* weDontPollute = someFunc())
>     {
>          // use weDontPollute
>     }
>
> That's great and all, but it only works by checking if the 
> variable evaluates to true or false. Which is fine for a 
> pointer but otherwise useless for anything else, like integers 
> where zero is usually valid input (index for array). So 
> currently the only way to do something like this in the 
> language, that i've found, is to use a for statement.
>
>     for(int i = someFunc(); i >= 0;)
>     {
>         // use i
>
>         break;
>     }
>
> Not that ideal to use a for statement. It makes it hard to read 
> and if the break condition isn't there it might very well be an 
> infinite loop. So I was thinking of some sort of syntax like 
> this:
>
>     if(int i = someFunc(); i >= 0)
>     {
>         // use i
>     }
> Thoughts on this sort of feature?


Achieving the same semantics as your example today would look 
like this

{
     int i = someFunc();
     if(i >= 0)
     {
         // use i
     }
}

The disadvantage being that it creates an extra level of nesting. 
I can see how this extra scope can discourage people from 
properly scoping their variables and this feature would eliminate 
the need for the extra scope.

Also note that this example shows how this could be implemented 
using "syntax lowering".

if( <declaration1> ; <desclaration2> ; ... ; <condition)
{
   //
}

Becomes:

{
     <declaration1>;
     <declaration2>;
     ...
     if(<condition>)
     {
         // ...
     }
}