Closing the loop, this raises the question of "what about instanceof and
total patterns"? I posit that the following locutions are silly:
if (e instanceof var x) { ... } // always true
if (e instanceof _) { ... } // always true
and probably should be banned. If we did, though, what about:
if (e instanceof Object o) { ... } // always true
if (e instanceof Object) { ... } // always true, but currently allowed
I would think we would ban the former as well, but we have to keep the
latter around for compatibility. (Which is partially why I discouraged
calling the latter an "anonymous pattern" in the spec, and instead
proposed to treat it as a different flavor of `instanceof`.)
SO, proposed: disallow "any" patterns (_, var x, or total T x) in
instanceof. Instanceof is for partial patterns.
Note that
Point p;
if (p instanceof Point(var x, var y)) { }
is total, but we would't want to disallow it, as this pattern could
still fail if p == null.
We might want to go a little further, and ban constant patterns in
instanceof too, since all of the following have simpler forms:
if (x instanceof null) { ... }
if (x instanceof "") { ... }
if (i instanceof 3) { ... }
Or not -- I suspect not.
On 1/8/2020 3:27 PM, Brian Goetz wrote:
In the past, we've gone around a few times on nullability and pattern
matching. Back when we were enamored of `T?` types over in Valhalla
land, we tentatively landed on using `T?` also for nullable type
patterns. But the bloom came off that rose pretty quickly, and
Valhalla is moving away from it, and that makes it far less attractive
in this context.
There are a number of tangled concerns that we've tried a few times to
unknot:
- Construct nullability. Constructs to which we want to add pattern
awareness (instanceof, switch) already have their own opinion about
nulls. Instanceof always says false when presented with a null, and
switch always NPEs.
- Pattern nullability. Some patterns clearly would never match null
(deconstruction patterns), whereas others (an "any" pattern, and
surely the `null` constant pattern, if there was one) might make sense
to match null.
- Nesting vs top-level. Most of the time, we don't want to match
null at the top level, but frequently in a nested position we do. This
conflicts with...
- 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 (well, except for 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.
- Refactoring friendliness. There are a number of cases that we
would like to freely refactor back and forth (e.g., if-instanceof
chain vs pattern switch). In particular, refactoring a switch on
nested patterns to a nested switch (case Foo(T t), case Foo(U u) to a
nested switch on T and U) is problematic under some of the
interpretations of nested patterns.
- Inference. It would be nice if a `var` pattern were simply
inference for a type pattern, rather than some possibly-non-denotable
union. (Both Scala and C# treat these differently, which means you
have to choose between type inference and the desired semantics; I
don't want to put users in the position of making this choice.)
Let's try (again) to untangle these. A compelling example is this one:
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 (ok, maybe they both implement `Edible`), it should
be perfectly safe to reorder the two. But, because the last pattern
is 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 super-bad if there were _no_way to
say "Match any Box, even if it contains null. While one might 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 path 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. I find this choice to be both questionable (the story that
`var` is just inference is nice) and also that it puts users in the
position of having to choose between the semantics they want and being
explicit about types. I see the expedience of it, but I do not think
this is the right answer for Java.
In the previous round, we posited that there were _type
patterns_(denoted `T t`) and _nullable type patterns_(denoted `T? t`),
which had the advantage that you could be explicit about what you
wanted (nulls or not), and which was sort of banking on Valhalla
plunking for the `T? ` notation. But without that, only having `T?`
in patterns, and no where else, will stick out like a sore thumb.
There are many ways to denote "T or null", of course:
- Union types: `case (T|Null) t`
- OR patterns: `case (T t) | (Null t)`, or `case (T t) | (null t)`
(the former is a union with a null TYPE pattern, the latter with a
null CONSTANT pattern)
- Merging/fallthrough: `case T t, Null t`
- Some way to spell "nullable T": `case T? t`, `case nullable T t`,
`case T|null t`
But, I don't see any of these as being all that attractive in the Box
case, when the most likely outcome is that the user wants the last
case to match all boxes.
Here's a scheme that I think is workable, which we hovered near
sometime in the past, and which I want to go back to. We'll start with
the observation that `instanceof` and `switch` are currently hostile
to nulls (instanceof says false, switch throws, and probably in the
future, let/bind will do so also.)
- We accept that some constructs may have legacy hostility to nulls
(but, see below for a possible relaxation);
- There are no "nullable type patterns", just type patterns;
- Type patterns that are _total_ on their target operand (`case T` on
an operand of type `U`, where `U <: T`) match null, and non-total type
patterns do not.
- Var patterns can be considered "just type inference" and will mean
the same thing as a type pattern for the inferred type.
In this world, the patterns that match null (if the construct allows
it through) are `case null` and the total patterns -- which could be
written `var x` (and maybe `_`, or maybe not), or `Object x`, or even
a narrower type if the operand type is narrower.
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.
An objection raised against this scheme earlier is that readers will
have to look at the declaration site of the pattern to know whether
the nested pattern is total. This is a valid concern (to be traded off
against the other valid concerns), but this does not seem so bad in
practice to me -- it will be common to use var or other broad type, in
which case it will be obvious.)
One problem with this interpretation is that 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, because switch is
null-hostile. 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 bottom case, some duplication of code would be
needed. This is irritating, but I don't think it is disqualifying --
it is in the same category of null irritants that we have throughout
the language.
Similarly, we lose the pleasing decomposition that the nested pattern
`P(Q)` is the same pattern as `P(alpha) & alpha instanceof Q` when P's
1st component might be null and the pattern Q is total -- because of
the existing null-hostility of `instanceof`. (This is not unlike the
complaint that Optional doesn't follow the monad law, with a similar
consequence -- and a similar justification.)
So, summary:
- the null constant pattern matches null;
- "any" patterns match null;
- A total type pattern is an "any" pattern;
- var is just type inference;
- no other patterns match null;
- existing constructs retain their existing null behaviors.
I'll follow up with a separate message about switch null-hostility.
