If Statement with Declaration

[ArturG via Digitalmars-d]

On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry 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?

Hi, i refactored some templates[1] i had, so that you can provide 
your own predicate functions.

Variant 1 eager version:

auto when(alias pred, alias fun, T)(T type = T.init)

Variant 2 a lazy version, uses two templates:

auto when(alias pred, T)(T type = T.init)
auto call(alias fun, T)(T type)

check/modify the examples found here
[1] http://melpon.org/wandbox/permlink/g7V0gj4V7SGPjwqL

*OT is dpaste broken (cant seem to create new pastes) or is it 
just me?