Flow scoping

[John Rose]

On Jan 4, 2019, at 6:07 AM, Tagir Valeev <amaembo at gmail.com> wrote:
> 
> For the record: I heavily support this. If then-branch cannot complete normally, then unwrapping the else-branch should preserve the program semantics. It works today, and it should work in future Java as well.

I agree also.  But it is uncomfortable that the binding of the flow-scoped
variable gets buried in a place that is harder to spot.

Here's a possible compromise:  Allow flow-scoped variables to leak
out the bottom of a statement, but only if they are predeclared before
the statement, in the parent block.  They would be predeclared blank.
Example:

preconditions();
if (mist() && shadow() && !(x instanceof String s))
  throw q;
else {
  manyLinesOfStuff();
  println(s);  // s obscured by mist and shadow
}

<==>

preconditions();
String s;
if (mist() && shadow() && !(x instanceof String s))
  throw q;
manyLinesOfStuff();
println(s);  // s obscured by mist and shadow

Since the original s is always DU and never DA, we could choose
to allow the flow-bound s to merge with it.  The binding would
then be discoverable as a dominating declaration, before the "if".

In this compromise, the dominating declaration would have to
be introduced before an entire if/else chain containing flow-scoped
bindings that are to be passed outside of the chain.  That seems
reasonable to me, as a compromise between conciseness and
ease of reading.

— John
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.java.net/pipermail/amber-spec-experts/attachments/20190108/175c2693/attachment-0001.html>