Re: 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>)
    {
        // ...
    }
}