Assuming you are happy with the plan in the previous message, let's move
on to the ...
SPECIAL BONUS ROUND -- POSSIBLY LESS NULL-HOSTILE SWITCHES
(If you are not happy with the previous mail, please direct your
comments to that first; this is purely a maybe-add-on to the previous.
Also, remember that patterns in switch is not coming in the next round,
but the following one.)
In the past, when we tried to relax the null-hostility to switch, we got
some pushback along the lines of "Please don't make me grovel through
all the cases to determine if a switch is null-friendly or
null-hostile." Making the user do an O(n) analysis would be bad here,
but maybe an O(1) analysis is acceptable. (We could have an alternate
name for switch (`switch-nullable`), and this would address the problem
(at the cost of, arguably, creating a new problem), but I'd like
something a little more general.)
Let's start with `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 Object o -> System.out.println("Yay, non-null: " + o);
}
I don't think anyone would argue that it is all that confusing that this
switch can deal with null inputs; the proximity of `case null` to the
`switch` makes it pretty clear that we want to handle nulls. (We could
restrict this to the top of the switch, though that would be an
exception to the general rule about pattern domination, though an
acceptable one.)
OK, now what about at the other end of the switch? What if the last
pattern is total (say, an any pattern.) Is it also reasonable for that
to match null?
switch (o) {
case String s: ...
case _: ...
}
Is it reasonable for the last line to match null? After all, we're
saying "everything". (For now, please ignore any latent assumptions
that the last line means the same thing as "default".)
I realize that there may be mixed opinions here, but what I'm getting at
is: what if the reader only had to look at the first and last case to
determine how nulls were handled in a switch? Would the benefits of
uniform treatment of nulls in pattern matching (eliminating the
attendant refactoring anomalies) outweigh the fact that users have to
make a slight shift in their thinking about "switch is always
null-hostile"? (Note that there is no actual code compatibility issue;
this is all mental-model compatibility.)
The key idea here is to shift our orientation from "switch is null
hostile" to "matching the `default` clause of a switch is null-hostile"
-- and saying that non-total switches get an implicit default clause
(just as we insert an extra throwing default clause into total switch
expressions, just in case the classfiles change in an incompatible way
between compilation and runtime.)
So, if there is a `case null`, or the last clause is a total pattern, it
gets the null; otherwise, there is either an explicit or implicit
default clause, which is null-hostile.
The main costs are two:
- a `default` is not the same as `case _` or `case var x` or `case
Object x`;
- the mental model that switches are null-hostile needs to be shifted
to "switch defaults are null-hostile."
These are not trivial costs, but neither are the benefits. I don't
think its a slam-dunk either way; it's really a question of whether we
want to spend some of our "surprise the user" budget on making the
language more regular.
Note that whichever way we go here, has no effect on the semantics
outlined in the previous mail.
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.
