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<font size="4"><font face="monospace">We're preparing a third
preview of type patterns in switch. Normally we would release
after a second preview, but (a) we're about to get record
patterns, which may disclose additional issues with switch, so
best to keep it open for at least another round, and (b) we're
proposing some nontrivial changes which deserve another
preview. <br>
<br>
Here's where we are on these. <br>
<br>
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<blockquote type="cite" cite="mid:06062579-A7DD-499C-839D-B77A319E38ED@oracle.com">
<pre class="moz-quote-pre" wrap="">1. Treatment of total patterns in switch / instanceof</pre>
</blockquote>
<br>
Quite honestly, in hindsight, I don't know why we didn't see this
sooner; the incremental evolution proposed here is more principled
than where we were in the previous round; now the construct
(instanceof, switch, etc) *always* gets first crack at enforcing its
nullity (and exception) opinions, and *then* delegates to the
matching semantics of the pattern if it decides to do so. This
fully separates pattern semantics from conditional construct
semantics, rather than complecting them (which in turn deprived
users of seeing the model more clearly.) In hindsight, this is a
no-brainer (which is why we preview things.) We'll be addressing
this in the 3rd preview. <br>
<br>
<blockquote type="cite" cite="mid:06062579-A7DD-499C-839D-B77A319E38ED@oracle.com">
<pre class="moz-quote-pre" wrap="">2. Positioning of guards</pre>
</blockquote>
<br>
Making guards part of switch also feels like a better factoring than
making them part of patterns; it simplifies patterns and totality,
and puts switch on a more equal footing with our other conditional
constructs. We did go back and forth a few times on this, but
having given this a few weeks to settle, I'm pretty convinced we'd
regret going the other way. <br>
<br>
There were two sub-points here: (a) is the guard part of the pattern
or part of switch, and (b) the syntax. There was general agreement
on (a), but some had preference for && on (b). I spent some
more time thinking about this choice, and have come down firmly on
the `when` side of the house as a result for a number of reasons. <br>
<br>
- Possibility for ambiguity. If switching over booleans (which we
will surely eventually be forced into), locutions like `case false
&& false` will be very confusing; it's pure puzzler
territory. <br>
- && has a stronger precedence than keyword-based operators
like `instanceof`'; we want guards to be weakest here.<br>
- Using && will confuse users about whether it is part of
the expression, or part of the switch statement. If we're deciding
it is part of the switch, this should be clear, and a `when` clause
makes that clear. <br>
- There are future constructs that may take patterns, and may (or
may not) want to express guard-like behavior, such as `let`
statements (e.g., let .. when .. else.) Expressing guards here with
&& is even less evocative of "guard condition" than it is
with switches.<br>
- Users coming from other languages will find `case...when` quite
clear. <br>
- We've talked about "targetless" switches as a possible future
feature, which express multi-way conditionals:<br>
<br>
switch { <br>
case when (today() == TUESDAY): ...<br>
case when (location() == GREENLAND): ...<br>
...<br>
}<br>
<br>
This would look quite silly with &&. Similarly, one could
mix guards with a targeted switch:<br>
<br>
switch (x) { <br>
case Time t: ...<br>
case Place p: ...<br>
default when (today() == TUESDAY): ... tuesday-specific
default<br>
default: ... regular default ... <br>
<br>
Expressing guards that are the whole condition with `when` is much
more natural than with &&. <br>
<br>
tl;dr: inventing a `when` modifier on switch now will save us from
having to invent something else in the future; choosing &&
will not. <br>
<br>
We can continue to discuss the bikeshed at low volume (at least
until we start repeating ourselves), but we need to address both of
these points in the 3rd preview. <br>
<br>
<blockquote type="cite" cite="mid:06062579-A7DD-499C-839D-B77A319E38ED@oracle.com">
<pre class="moz-quote-pre" wrap="">3. Type refinements for GADTs</pre>
</blockquote>
<br>
I've been working through the details here, and there are a number
of additional touch points where GADTs can provide type refinement,
not just on the RHS of a case, such as totality and inference. I'll
be pulling all these together to try to get a total picture here.
It's not a blocker for the 3rd preview, it can be a future
refinement. <br>
<br>
<blockquote type="cite" cite="mid:06062579-A7DD-499C-839D-B77A319E38ED@oracle.com">
<pre class="moz-quote-pre" wrap="">4. Diamond for type patterns (and record patterns)
</pre>
</blockquote>
This seems desirable, but there are details to work out. It's not a
blocker for the 3rd preview, it can be a future refinement.
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