DIP 1002 (TryElseExpression) added to the queue

[Nick Treleaven via Digitalmars-d]

On Wednesday, 28 September 2016 at 21:00:00 UTC, Steven 
Schveighoffer wrote:
>> Moreover, Idan's suggestions about scope sharing make sense to 
>> me and I
>> don't think his line of thinking would be compatible with 
>> doing it the
>> way you suggest.
>
> Declaring variables that you need in the right scopes is pretty 
> straightforward. Having scopes magically continue around other 
> separate scopes (catch scopes) doesn't look correct. I get why 
> it's desired, but it doesn't look clean at all.

Assuming for a minute that we want some form of DIP1002, we might 
be able to extend the `try` scope in an interesting way:

try {
     auto f = foo();
}
catch (Exception) {
     ...
}
finally (bool ok) { // true if no exception
     bar();
     __guard (ok) f.baz();
     f.baz(); // error: f not in scope
}

Here the guard statement is a runtime check of `ok`, but it has 
the compile-time effect of enabling the `try` scope to be 
extended. Obviously this is a bit extreme if it was the only use 
of the guard statement, but the guard statement could be useful 
in other situations too (e.g. non-null code checked at 
compile-time).

Note that finally(bool) is more flexible than finally/else as you 
can interleave code arbitrarily. __guard makes it clearer 
something special is happening rather than just implicitly 
extending `try` scope in the `else` clause. Also, 'else' is not 
clear enough IMO.