Next up for patterns: type patterns in switch

[Brian Goetz]

One thing this left open was the actual syntax of guards.  (We know its 
snowing in hell now, because I am actually encouraging a syntax 
conversation.)

Patterns in `instanceof` do not need guards, because `instanceof` is an 
expression, and expressions conjoin with `&&`:

     if (x instanceof Foo f && f.bar() == 3) { ... }

We explored the approach of making the boolean guard expression part of 
the pattern -- where the above would parse as `x instanceof P`, where P 
is `Foo f && f.bar() == 3`. Since pattern matching is conditional, we 
could think of boolean expressions as patterns that are independent of 
their target. But, this approach didn't pan out due to various ambiguities.

The most obvious ambiguity was the obvious interpretation of constant 
patterns; that `0` could be the literal zero or a pattern that matches 
zero.  (I have since proposed we try to avoid constant patterns 
entirely.)  Under this interpretation, not only was there room for 
confusion ("Is foo(0) an invocation of foo, or a pattern?"), but there 
were puzzlers like:

     switch (b) {
         case true && false: ...
         case false && true: ...
     }

It would not be clear whether this would be two patterns conjoined 
together (neither of which would match anything) or the constant pattern 
`true && false`.  (There are similar ambiguities with deconstruction 
patterns with no bindings.)

Overall, all of this was enough to sour me on trying to press && into 
service here.  Which leaves some relatively mundane syntax options:

     case P when e
     case P where e
     case P if e
     case P only-if e
     case P where (e)

and I guess weirder things like

     case P &&& e
     case P : e
     etc

I particularly don't like `if` since it makes it even harder to tell 
where the case ends and the consequent begins.  Also, `e` should not 
contain a switch expression, since no one wants to try to parse:

     case Foo f where switch (f.bar()) {
         case Bar b -> 3;
     } > 3 -> blah();

in their heads.  (We already excluded switch expressions from candidates 
for constant expressions in 12 for similar reasons.)

I mostly think this is a matter of picking a (contextual) keyword.  (I 
kind of like that `only-if` could be an actual keyword.)



(Looking ahead, if we ever want to reenable support for merging, we 
might have patterns like:

     case Foo(int x), Bar(int x) where x > 0:

and we'd have to accept that the guard applies to _all_ the patterns.)


On 6/24/2020 10:44 AM, Brian Goetz wrote:
> There are a lot of directions we could take next for pattern 
> matching.  The one that builds most on what we've already done, and 
> offers significant incremental expressiveness, is extending the type 
> patterns we already have to a new context: switch.  (There is still 
> plenty of work to do on deconstruction patterns, pattern assignment, 
> etc, but these require more design work.)
>
> Here's an overview of where I think we are here.
>
> [JEP 305][jep305] introduced the first phase of [pattern 
> matching][patternmatch]
> into the Java language.  It was deliberately limited, focusing on only 
> one kind
> of pattern (type test patterns) and one linguistic context (`instanceof`).
> Having introduced the concept to Java developers, we can now extend 
> both the
> kinds of patterns and the linguistic context where patterns are used.
>
> ## Patterns in switch
>
> The obvious next context in which to introduce pattern matching is 
> `switch`;  a
> switch using patterns as `case` labels can replace `if .. else if` 
> chains with
> a more direct way of expressing a multi-way conditional.
>
> Unfortunately, `switch` is one of the most complex, irregular 
> constructs we have
> in Java, so we must teach it some new tricks while avoiding some 
> existing traps.
> Such tricks and traps may include:
>
> **Typing.**  Currently, the operand of a `switch` may only be one of the
> integral primitive types, the box type of an integral primitive, 
> `String`, or an
> `enum` type.  (Further, if the `switch` operand is an `enum` type, the 
> `case`
> labels must be _unqualified_ enum constant names.)  Clearly we can 
> relax this
> restriction to allow other types, and constrain the case labels to only be
> patterns that are applicable to that type, but it may leave a seam of 
> "legacy"
> vs "pattern" switch, especially if we do not adopt bare constant 
> literals as
> the denotation of constant patterns.  (We have confronted this issue 
> before with
> expression switch, and concluded that it was better to rehabilitate 
> the `switch`
> we have rather than create a new construct, and we will make the same 
> choice
> here, but the cost of this is often a visible seam.)
>
> **Parsing.**  The grammar currently specifies that the operand of a 
> `case` label
> is a `CaseConstant`, which casts a wide syntactic net, later narrowed with
> post-checks after attribution.  This means that, since parsing is done 
> before we
> know the type of the operand, we must be watchful for ambiguities between
> patterns and expressions (and possibly refine the production for 
> `case` labels.)
>
> **Nullity.**  The `switch` construct is currently hostile to `null`, 
> but some
> patterns do match `null`, and it may be desirable if nulls can be handled
> within a suitably crafted `switch`.
>
> **Exhaustiveness.**  For switches over the permitted subtypes of 
> sealed types,
> we will want to be able to do exhaustiveness analysis -- including for 
> nested
> patterns (i.e., if `Shape`  is `Circle` or `Rect`, then `Box(Circle 
> c)` and
> `Box(Rect r)` are exhaustive on `Box<Shape>`.)
>
> **Fallthrough.**  Fallthrough is everyone's least favorite feature of 
> `switch`,
> but it exists for a reason.  (The mistake was making fallthrough the 
> default
> behavior, but that ship has sailed.)  In the absence of an OR pattern
> combinator, one might find fallthrough in switch useful in conjunction 
> with
> patterns:
>
> ```
> case Box(int x):
> case Bag(int x):
>     // use x
> ```
>
> However, it is likely that we will, at least initially, disallow 
> falling out
> of, or into, a case label with binding variables.
>
> #### Translation
>
> Switches on primitives and their wrapper types are translated using the
> `tableswitch` or `lookupswitch` bytecodes; switches on strings and 
> enums are
> lowered in the compiler to switches involving hash codes (for strings) or
> ordinals (for enums.)
>
> For switches on patterns, we would need a new strategy, one likely 
> built on
> `invokedynamic`, where we lower the cases to a densely numbered `int` 
> switch,
> and then invoke a classifier function with the operand which tells us 
> the first
> case number it matches.  So a switch like:
>
> ```
> switch (o) {
>     case P: A
>     case Q: B
> }
> ```
>
> is lowered to:
>
> ```
> int target = indy[BSM=PatternSwitch, args=[P,Q]](o)
> switch (target) {
>     case 0: A
>     case 1: B
> }
> ```
>
> A symbolic description of the patterns is provided as the bootstrap 
> argument
> list, which builds a decision tree based on analysis of the patterns 
> and their
> target types.
>
> #### Guards
>
> No matter how rich our patterns are, it is often the case that we will 
> want
> to provide additional filtering on the results of a pattern:
>
> ```
> if (shape instanceof Cylinder c && c.color() == RED) { ... }
> ```
>
> Because we use `instanceof` as part of a boolean expression, it is easy to
> narrow the results by conjoining additional checks with `&&`.  But in 
> a `case`
> label, we do not necessarily have this opportunity.  Worse, the 
> semantics of
> `switch` mean that once a `case` label is selected, there is no way to say
> "oops, forget it, keep trying from the next label".
>
> It is common in languages with pattern matching to support some form 
> of "guard"
> expression, which is a boolean expression that conditions whether the case
> matches, such as:
>
> ```
> case Point(var x, var y)
>     __where x == y: ...
> ```
>
> Bindings from the pattern would have to be available in guard expressions.
>
> Syntactic options (and hazards) for guards abound; users would 
> probably find it
> natural to reuse `&&` to attach guards to patterns; C# has chosen 
> `when` for
> introducing guards; we could use `case P if (e)`, etc. Whatever we do 
> here,
> there is a readability risk,  as the more complex guards are, the 
> harder it is
> to tell where the case label ends and the "body" begins.  (And worse 
> if we allow
> switch expressions inside guards.)
>
> An alternate to guards is to allow an imperative `continue` statement in
> `switch`, which would mean "keep trying to match from the next 
> label."  Given
> the existing semantics of `continue`, this is a natural extension, but 
> since
> `continue` does not currently have meaning for switch, some work would 
> have to
> be done to disambiguate continue statements in switches enclosed in 
> loops.  The
> imperative version is strictly more expressive than most reasonable 
> forms of the
> declarative version, but users are likely to prefer the declarative 
> version.
>
> ## Nulls
>
> Almost no language design exercise is complete without some degree of 
> wrestling
> with `null`.  As we define more complex patterns than simple type 
> patterns, and
> extend constructs such as `switch` (which have existing opinions about 
> nullity)
> to support patterns, we need to have a clear understanding of which 
> patterns
> are nullable, and separate the nullity behaviors of patterns from the 
> nullity
> behaviors of those constructs which use patterns.
>
> ## Nullity and patterns
>
> This topic has a number of easily-tangled concerns:
>
>  - **Construct nullability.**  Constructs to which we want to add pattern
>    awareness (`instanceof`, `switch`) already have their own opinion about
>    nulls.  Currently, `instanceof` always says false when presented with a
>    `null`, and `switch` always NPEs.  We may, or may not, wish to 
> refine these
>    rules in some cases.
>  - **Pattern nullability.**  Some patterns clearly would never match 
> `null`
>    (such as deconstruction patterns), whereas others (an "any" 
> pattern, and
>    surely the `null` constant pattern) might make sense to match null.
>  - **Refactoring friendliness.**  There are a number of cases that we 
> would like
>    to freely refactor back and forth, such as certain chains of `if 
> ... else if`
>    with switches.
>  - **Nesting vs top-level.**  The "obvious" thing to do at the top 
> level of a
>    construct is not always the "obvious" thing to do in a nested 
> construct.
>  - **Totality vs partiality.**  When a pattern is partial on the 
> operand type
>    (e.g., `case String` when the operand of switch is `Object`), it is 
> almost
>    never the case we want to match null (except in the case of the `null`
>    constant pattern), whereas when a pattern is total on the operand 
> type (e.g.,
>    `case Object` in the same example), it is more justifiable to match 
> null.
>  - **Inference.**  It would be nice if a `var` pattern were simply 
> inference for
>    a type pattern, rather than some possibly-non-denotable union.
>
> As a starting example, consider:
>
> ```
> record Box(Object o) { }
>
> Box box = ...
> switch (box) {
>     case Box(Chocolate c):
>     case Box(Frog f):
>     case Box(var o):
> }
> ```
>
> It would be highly confusing and error-prone for either of the first two
> patterns to match `Box(null)` -- given that `Chocolate` and `Frog` 
> have no type
> relation, it should be perfectly safe to reorder the two. But, because 
> the last
> pattern seems so obviously total on boxes, it is quite likely that 
> what the
> author wants is to match all remaining boxes, including those that 
> contain null.
> (Further, it would be terrible if there were _no_ way to say "Match 
> any `Box`,
> even if it contains `null`.  (While one might initially think this 
> could be
> repaired with OR patterns, imagine that `Box` had _n_ components -- 
> we'd need to
> OR together _2^n_ patterns, with complex merging, to express all the 
> possible
> combinations of nullity.))
>
> Scala and C# took the approach of saying that "var" patterns are not 
> just type
> inference, they are "any" patterns -- so `Box(Object o)` matches boxes
> containing a non-null payload, where `Box(var o)` matches all boxes.  This
> means, unfortunately, that `var` is not mere type inference -- which 
> complicates
> the role of `var` in the language considerably.  Users should not have 
> to choose
> between the semantics they want and being explicit about types; these 
> should be
> orthogonal choices.  The above `switch` should be equivalent to:
>
> ```
> Box box = ...
> switch (box) {
>     case Box(Chocolate c):
>     case Box(Frog f):
>     case Box(Object o):
> }
> ```
>
> and the choice to use `Object` or `var` should be solely one of 
> whether the
> manifest types are deemed to improve or impair readability.
>
> #### Construct and pattern nullability
>
> Currently, `instanceof` always says `false` on `null`, and `switch` always
> throws on `null`.  Whatever null opinions a construct has, these are 
> applied
> before we even test any patterns.
>
> We can formalize the intuition outlined above as: type patterns that 
> are _total_
> on their target operand (`var x`, and `T t` on an operand of type `U`, 
> where `U
> <: T`) match null, and non-total type patterns do not. (Another way to say
> this is: a `var` pattern is the "any" pattern, and a type pattern that 
> is  total
> on its operand type is also an "any" pattern.)  Additionally, the `null`
> constant pattern matches null.  These are the _only_ nullable patterns.
>
> In our `Box` example, this means that the last case (whether written 
> as `Box(var
> o)` or `Box(Object o)`) matches all boxes, including those containing null
> (because the nested pattern is total on the nested operand), but the 
> first two
> cases do not.
>
> If we retain the current absolute hostility of `switch` to nulls, we can't
> trivially refactor from
>
> ```
> switch (o) {
>     case Box(Chocolate c):
>     case Box(Frog f):
>     case Box(var o):
> }
> ```
> to
>
> ```
> switch (o) {
>     case Box(var contents):
>         switch (contents) {
>             case Chocolate c:
>             case Frog f:
>             case Object o:
>         }
>     }
> }
> ```
>
> because the inner `switch(contents)` would NPE before we tried to 
> match any of
> the patterns it contains.  Instead, the user would explicitly have to 
> do an `if
> (contents == null)` test, and, if the intent was to handle `null` in 
> the same
> way as the `Object o` case, some duplication of code would be needed.  
> We can
> address this sharp corner by slightly relaxing the null-hostility of 
> `switch`,
> as described below.
>
> A similar sharp corner is the decomposition of a nested pattern `P(Q)` 
> into
> `P(alpha) & alpha instanceof Q`; while this is intended to be a 
> universally
> valid transformation, if P's 1st component might be null and Q is 
> total,  this
> transformation would not be valid because of the existing (mild) 
> null-hostility
> of `instanceof`.  Again, we may be able to address this by adjusting 
> the rules
> surrounding `instanceof` slightly.
>
> ## Generalizing switch
>
> The refactoring example above motivates why we might want to relax the
> null-handling behavior of `switch`.  On the other hand, the one thing the
> current behavior has going for it is that at least the current 
> behavior is easy
> to reason about; it always throws when confronted with a `null`.  Any 
> relaxed
> behavior would be more complex; some switches would still have to 
> throw (for
> compatibility with existing semantics), and some (which can't be expressed
> today) would accept nulls.  This is a tricky balance to achieve, but I 
> think we
> have a found a good one.
>
> A starting point is that we don't want to require readers to do an _O(n)_
> analysis of each of the `case` labels just to determine whether a 
> given switch
> accepts `null` or not; this should be an _O(1)_ analysis.  (We do not 
> want to
> introduce a new flavor of `switch`, such as `switch-nullable`; this 
> might seem
> to fix the proximate problem but would surely create others. As we've 
> done with
> expression switch and patterns, we'd rather rehabilitate `switch` than 
> create
> an almost-but-not-quite-the-same variant.)
>
> Let's start with the null pattern, which we'll spell for sake of 
> exposition
> `case null`.  What if you were allowed to say `case null` in a switch, 
> and the
> switch would do the obvious thing?
>
> ```
> switch (o) {
>     case null -> System.out.println("Ugh, null");
>     case String s -> System.out.println("Yay, non-null: " + s);
> }
> ```
>
> Given that the `case null` appears so close to the `switch`, it does 
> not seem
> confusing that this switch would match `null`; the existence of `case 
> null` at
> the top of the switch makes it pretty clear that this is intended 
> behavior.  (We
> could further restrict the null pattern to being the first pattern in 
> a switch,
> to make this clearer.)
>
> Now, let's look at the other end of the switch -- the last case.  What 
> if the
> last pattern is a total pattern?  (Note that if any `case` has a total 
> pattern,
> it _must_ be the last one, otherwise the cases after that would be 
> dead, which
> would be an error.)  Is it also reasonable for that to match null?  
> After all,
> we're saying "everything":
>
> ```
> switch (o) {
>     case String s: ...
>     case Object o: ...
> }
> ```
>
> Under this interpretation, the switch-refactoring anomaly above goes away.
>
> The direction we're going here is that if we can localize the 
> null-acceptance of
> switches in the first (is it `case null`?) and last (is it total?) 
> cases, then
> the incremental complexity of allowing _some_ switches to accept null 
> might be
> outweighed by the incremental benefit of treating `null` more 
> uniformly (and
> thus eliminating the refactoring anomalies.)  Note also that there is 
> no actual
> code compatibility issue; this is all mental-model compatibility.
>
> So far, we're suggesting:
>
>  - A switch with a constant `null` case  will accept nulls;
>  - If present, a constant `null` case must go first;
>  - A switch with a total (any) case matches also accepts nulls;
>  - If present, a total (any) case must go last.
>
> #### Relocating the problem
>
> It might be more helpful to view these changes as not changing the 
> behavior of
> `switch`, but of the `default` case of `switch`.  We can equally well 
> interpret
> the current behavior as:
>
>  - `switch` always accepts `null`, but matching the `default` case of 
> a `switch`
>    throws `NullPointerException`;
>  - any `switch` without a `default` case has an implicit do-nothing 
> `default`
>    case.
>
> If we adopt this change of perspective, then `default`, not `switch`, 
> is in
> control of the null rejection behavior -- and we can view these changes as
> adjusting the behavior of `default`.  So we can recast the proposed 
> changes as:
>
>   - Switches accept null;
>   - A constant `null` case will match nulls, and must go first;
>   - A total switch (a switch with a total `case`) cannot have a 
> `default` case;
>   - A non-total switch without a `default` case gets an implicit 
> do-nothing
>     `default` case;
>   - Matching the (implicit or explicit) default case with a `null` operand
>     always throws NPE.
>
> The main casualty here is that the `default` case does not mean the same
> thing as `case var x` or `case Object o`.  We can't deprecate 
> `default`, but
> for pattern switches, it becomes much less useful.
>
> #### What about method (declared) patterns?
>
> So far, we've declared all patterns, except the `null` constant 
> pattern and the
> total (any) pattern, to not match `null`.  What about patterns that are
> explicitly declared in code?  It turns out we can rule out these matching
> `null` fairly easily.
>
> We can divide declared patterns into three kinds: deconstruction 
> patterns (dual
> to constructors), static patterns (dual to static methods), and instance
> patterns (dual to instance methods.)  For both deconstruction and instance
> patterns, the match target becomes the receiver; method bodies are never
> expected to deal with the case where `this == null`.
>
> For static patterns, it is conceivable that they could match `null`, 
> but this
> would put a fairly serious burden on writers of static patterns to 
> check for
> `null` -- which they would invariably forget, and many more NPEs would 
> ensue.
> (Think about writing the pattern for `Optional.of(T t)` -- it would be
> overwhelmingly likely we'd forget to check the target for nullity.)  
> SO there
> is a strong argument to simply say "declared patterns never match 
> null", to
> not put writers of such patterns in this situation.
>
> So, only the top and bottom patterns in a switch could match null; if 
> the top
> pattern is not `case null`, and the bottom pattern is not total, then 
> the switch
> throws NPE on null, otherwise it accepts null.
>
> #### Adjusting instanceof
>
> The remaining anomaly we had was about unrolling nested patterns when 
> the inner
> pattern is total.  We can plug this by simply outlawing total patterns in
> `instanceof`.
>
> This may seem like a cheap trick, but it makes sense on its own.  If the
> following statement was allowed:
>
> ```
> if (e instanceof var x) { X }
> ```
>
> it would simply be confusing; on the one hand, it looks like it should 
> always
> match, but on the other, `instanceof` is historically null-hostile.  
> And, if the
> pattern always matches, then the `if` statement is silly; it should be 
> replaced
> with:
>
> ```
> var x = e;
> X
> ```
>
> since there's nothing conditional about it.  So by banning "any" 
> patterns on the
> RHS of `instanceof`, we both avoid a confusion about what is going to 
> happen,
> and we prevent the unrolling anomaly.
>
> For reasons of compatibility, we will have to continue to allow
>
> ```
> if (e instanceof Object) { ... }
> ```
>
> which succeeds on all non-null operands.
>
> We've been a little sloppy with the terminology of "any" vs "total"; 
> note that
> in
>
> ```
> Point p;
> if (p instanceof Point(var x, var y)) { }
> ```
>
> the pattern `Point(var x, var y)` is total on `Point`, but not an 
> "any" pattern
> -- it still doesn't match on p == null.
>
> On the theory that an "any" pattern in `instanceof` is silly, we may 
> also want
> to ban other "silly" patterns in `instanceof`, such as constant 
> patterns, since
> all of the following have simpler forms:
>
> ```
> if (x instanceof null) { ... }
> if (x instanceof "") { ... }
> if (i instanceof 3) { ... }
> ```
>
> In the first round (type patterns in `instanceof`), we mostly didn't 
> confront
> this issue, saying that `instanceof T t` matched in all the cases where
> `instanceof T` would match.  But given that the solution for `switch` 
> relies
> on "any" patterns matching null, we may wish to adjust the behavior of
> `instanceof` before it exits preview.
>
>
> [jep305]: https://openjdk.java.net/jeps/305
> [patternmatch]: pattern-match.html
>

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