Document copied from here. Authored by Brian Goetz.
Time to check in on where things are in the bigger picture of patterns as class members. Note: while this document may have illustrative examples, you should not take that as a definitive statement of syntax, and Remi will not be commenting on the syntax at this time.
We've already dipped our toes in the water with record patterns. A record pattern looks like:
case R(p1, p2, ... pn):
where R
is a record type and p1..pn
are nested patterns that are
matched to
its components. Because records are defined by their state description,
we can
automatically derive record patterns "for free", just as we derive record
constructors, accessors, etc.
There are many other classes that would benefit from being deconstructible with patterns. To that end, we will generalize record patterns to deconstruction patterns, where any class can declare an explicit deconstruction pattern and participate in pattern matching like records do.
Deconstruction patterns are not the end of the user-declared pattern story. Just as some classes prefer to expose static factories rather than constructors, they will be able to expose corresponding static patterns. And there is also a role for "instance patterns" and "pattern objects" as well.
Looking only at record and deconstruction patterns, it might be tempting to
think that patterns are "just" methods with multiple return. But this
would be
extrapolating from a special case. Pattern matching is intrinsically
conditional; the extraction of values from a target is conditioned on
whether
the target matches the pattern. For the patterns we've seen so far --
type
patterns and record patterns -- matching can be determined entirely by
types.
But more sophisticated patterns can also depend on other aspects of object
state. For example, a pattern corresponding to the static factory
Optional::of
requires not only that the match candidate be of type
Optional
,
but that the match candidate is an Optional
that actually holds a value.
Similarly, a pattern corresponding a regular expression requires the match
candidate to not only be a String
, but to match the regular expression.
A key capability of objects is aggregation; the combination of component values into a higher-level composite that incorporates those components. Java facilitates a variety of idioms for aggregation, including constructors, factories, builders, etc. The dual of aggregation is destructuring or decomposition, which takes an aggregate and attempts to recover its "ingredients". However, Java's support for destructuring has historically been far more ad-hoc, largely limited to "write some getters". Pattern matching seeks to put destructuring on the same firm foundation as aggregation.
Deconstruction patterns (such as record patterns) are the dual of construction. If we construct an object:
Object o = new Point(x, y);
we can deconstruct it with a deconstruction pattern:
if (o instanceof Point(var x, var y)) { ... }
Intuitively, this pattern match asks "could this object have come from invoking the constructor
new Point(x, y)
for somex
andy
, and if so, tell me what they are."
While not all patterns exist in direct correspondence to another constructor or method, this intuition that a pattern reconstructs the ingredients to an aggregation operation is central to the design; we'll explore the limitations of this intuition in greater detail later.
Before turning to how patterns fit into the object model, let's look at some of the potential use cases for patterns in APIs.
Deconstruction patterns are the dual of constructors; where a
constructor takes
N arguments and aggregates them into an object, a deconstruction pattern
takes
an aggregate and decomposes it into its components. Constructors are
unusual in
that they are instance behavior (they have an implicit this
argument),
but are
not inherited; deconstruction patterns are the same. For deconstruction
patterns (but not for all instance patterns), the match candidate is
always the
receiver. Tentatively, we've decided that deconstruction patterns are
always
unconditional; that a deconstruction pattern for class Foo
should
match any
instance of Foo
. At the use site, deconstruction patterns use the
same syntax
as record patterns:
case Point(int x, int y):
Just as constructors can be overloaded, so can deconstruction patterns.
However,
the reasons we might overload deconstruction patterns are slightly different
than for constructors, and so it may well be the case that we end up
with fewer
overloads of deconstruction patterns than we do of constructors.
Constructors
often form telescoping sets, both for reasons of syntactic convenience
at the
use site (fewer arguments to specify) and to avoid brittleness (clients
can let
the class implementation pick the defaults rather than hard-coding
them.) This
motivation is less pronounced for deconstruction patterns (unwanted
bindings can
be ignored with _
), so it is quite possible that authors will choose
to have
one deconstruction pattern overload per telescoping constructor set,
rather
than one per constructor.
There is no requirement for deconstruction patterns to expose the exact same API as constructors, but we expect this will be common, at least for classes for which the construction process is effectively an aggregation operation on the constructor arguments.
Not all classes want to expose their constructors; sometimes classes prefer to expose static factories instead. In this case, the class should be able to expose corresponding static patterns as well.
For a class like Optional
, which exposes factories Optional::of
and
Optional::empty
, the object state incorporates not only the factory
arguments,
but which factory was chosen. Accordingly, it makes sense to
deconstruct the
object in the same way:
switch (optional) {
case Optional.of(var payload): ...
case Optional.empty(): ...
}
Such patterns are necessarily conditional, asking the Pattern Question:
"could
this Optional
have come from the Optional::of
factory, and if so,
with what
argument?" Static patterns, like static methods, lack a receiver, so
this
is
not defined in the body of a static pattern. However, we will need a way to
denote the match candidate, so its state can be examined by the pattern
body.
Another feature of static methods is that they can be used to put a
factory for
a class C
in another class, whether one in the same maintenance
domain (such
as the Collections
) or in some other package. This feature is shared by
static patterns.
Another application for static patterns is the dual of static methods for
conversions. For a static method like Integer::toString
, which
converts an
int
to its String
representation, a corresponding static pattern
Integer::toString
can ask the Pattern Question: "could this String
have come
from converting an integer to String
, and if so, what integer".
Some groups of query methods in existing APIs are patterns in disguise. The
class java.lang.Class
has a pair of instance methods, Class::isArray
and
Class::getComponentType
, that work together to determine if the Class
describes an array type, and if so, provide its component type. This
question
is much better framed as a single pattern:
case Class.arrayClass(var componentType):
The two existing methods are made more complicated by their relationship
to each
other; Class::getComponentType
has a precondition (the Class
must
describe
an array type) and therefore has to specify and implement what to do if the
precondition fails, and the relationship between the methods is captured
only in
documentation. By combining them into a single pattern, it become
impossible to
misuse (because of the inherent conditionality of patterns) and easier to
understand (because it can all be documented in one place.)
This hypothetical Class::arrayClass
pattern also has a sensible dual as a
factory method:
static<T> Class<T[]> arrayClass(Class<T> componentType)
which produces the array Class
for the array type whose component type is
provided. An API need not provide both directions of a conversion, but if it
does, the two generally strengthen each other. This method/pattern pair
could
be either static or instance members, depending on API design choice.
Another form of "conversion" method / pattern pair, even though both
types are
the same, is "power of two". A powerOfTwo
method takes an exponent and
returns the resulting power of two; a powerOfTwo
pattern asks if its match
candidate is a power of two, and if so, binds the base-two logarithm.
As Project Valhalla gives us the ability to declare new numeric types, we will want to be able to convert these new types to other numeric types. For unconditional conversions (such as widening half-float to float), an ordinary method will suffice:
float widen(HalfFloat f);
But the reverse is unlikely to be unconditional; narrowing conversions
can fail
if the value cannot be represented in the narrower type. This is better
represented as a pattern which asks the Pattern Question: "could this
float
have come from widening a HalfFloat
, and if so, tell me what
HalfFloat
that
is." A widening conversion (or boxing conversion) is best represented by a
pair of members, an ordinary method for the unconditional direction, and a
pattern for the conditional direction.
Some operations, such as matching a string to a regular expression with
capture
groups, are pattern matches in disguise. We should be able to take a
regular
expression R and match against it with instanceof
or switch
, binding
capture
groups (using varargs patterns) if it matches.
We currently have three kinds of executable class members: constructors, static methods, and instance methods. (Actually constructors are not members, but we will leave this pedantic detail aside for now.) As the above examples show, each of these can be amenable to a dual member which asks the Pattern Question about it.
Patterns are dual to constructors and methods in two ways: structurally and semantically. Structurally, patterns invert the relationship between inputs and outputs: a method takes N arguments as input and produces a single result, and the corresponding pattern takes a candidate result (the "match candidate") and conditionally produces N bindings. Semantically, patterns ask the Pattern Question: could this result have originated by some invocation of the dual operation.
One way to frame patterns in the object model is as inverse constructors and inverse methods. For purposes of this document, I will use an illustrative syntax that directly evokes this duality (but remember, we're not discussing syntax now):
class Point {
final int x, y;
// Constructor
Point(int x, int y) { ... }
// Deconstruction pattern
inverse Point(int x, int y) { ... }
}
class Optional<T> {
// Static factories
static<T> Optional<T> of(T t) { ... }
static<T> Optional<T> empty() { ... }
// Static patterns
static<T> inverse Optional<T> of(T t) { ... }
static<T> inverse Optional<T> empty() { ... }
}
Point
has a constructor an an inverse constructor (deconstruction
pattern) for
the external representation (int x, int y)
; in an inverse constructor, the
binding list appears where the parameter list does in the constructor.
Optional<T>
has static factories and corresponding patterns for
empty
and
of
. As with inverse constructors, the binding list of a pattern
appears in
the position that the parameters appear in a method declaration;
additionally,
the match candidate type appears in the position that the return value
appears
in a method declaration. In both cases, the declaration site and use
site of
the pattern uses the same syntax.
In the body of an inverse constructor or method, we need to be able to talk
about the match candidate. In this model, the match candidate has a type
determined by the declaration (for an inverse constructor, the class; for an
inverse method, the type specified in the "return position" of the inverse
method declaration), and there is a predefined context variable (e.g.,
that
)
that refers to the match candidate. For inverse constructors, the receiver
(this
) is aliased to the match candidate (that
), but not necessarily
so for
inverse methods.
We've seen examples of constructors, static methods, and instance
methods that
have sensible inverses, but not all methods do. For example, methods that
operate primarily by side effects (such as mutative methods like setters or
List::add
) are not suitable candidates for inverses. Similarly, pure
functions that "co-mingle" their arguments (such as arithmetic
operators) are
also not suitable candidates for inverses, because the ingredients to the
operation typically can't be recovered from the result (i.e., 4
could
be the
result of plus(2, 2)
or plus(1, 3)
).
Intuitively, the methods that are invertible are the ones that are
aggregative. The constructor of a (well-behaved) record is
aggregative, since
all the information passed to the constructor is preserved in the result.
Factories like Optional::of
are similarly aggregative, as are non-lossy
conversions such as widening or boxing conversions.
Ideally, an aggregation operation and its corresponding inverse form an embedding projection pair between the aggregate and a component space. Intuitively, an embedding-projection pair is an algebraic structure defined by a pair of functions between two sets such that composing in one direction (embed-then-project) is an identity, and composing in the other direction (project-then-embed) is a well-behaved approximation.
Conversion methods are a frequent candidate for inversion. We already have
// Integer.java
static String toString(int i) { ... }
to which the obvious inverse is
static inverse String toString(int i) { ... }
and we can inspect a string to see if it is the string representation of an integer with
if (s instanceof Integer.toString(int i)) { ... }
This composes nicely with deconstruction patterns; if we have a
Box<String>
and want to ask whether the contained string is really the string
representation
of an integer, we can ask:
case Box(Integer.toString(int i)):
which conveniently looks just like the composition of constructors or
factories
used to create such an instance (new Box(Integer.toString(3))
).
When it comes to user-definable numeric conversions, the most likely strategy involves combining related operators in a single witness object. For example, numeric conversion might be modeled as:
interface NumericConversion<FROM, TO> {
TO convert(FROM from);
inverse TO convert(FROM from);
}
which reflects the fact that conversion is total in one direction (widening, boxing) and conditional in the other (narrowing, unboxing.)
Regular expressions are a form of ad-hoc pattern; a given string might match a given regex, or not, and if it does, it might product multiple bindings (the capture groups.) It would be nice to be able to express regular expression matches as ordinary pattern matches.
Conveniently, we already have an object representation of regular
expressions --
java.util.Pattern
. Which is an ideal place to put an instance pattern:
// varargs pattern
public inverse String match(String... groups) {
Matcher m = matcher(that); // *that* is the match candidate
if (m.matches()) // receiver for matcher() is the Pattern
__yield IntStream.range(1, m.groupCount())
.map(Matcher::group)
.toArray(String[]::new);
}
And now, we want to express "does string s match any of these regular expressions":
static final Pattern As = Pattern.compile("([aA]*)");
static final Pattern Bs = Pattern.compile("([bB]*)");
static final Pattern Cs = Pattern.compile("([cC]*)");
...
switch (aString) {
case As.match(String as) -> ...
case Bs.match(String bs) -> ...
case Cs.match(String cs) -> ...
...
}
Essentially, j.u.r.Pattern
becomes a pattern object, where the state
of the
object is used to determine whether or not it matches any given input.
(There
is nothing stopping a class from having multiple patterns, just as it
can have
multiple methods.)
When we invoke a method, sometimes we are able to refer to the method
with an
unqualified name (e.g., m(3)
), and sometimes the method must be
qualified
with a type name, package name, or a receiver object. The same is true for
declared patterns.
Constructors for classes that are in the same package, or have been imported, can be referred to with an unqualified name; constructors can also be qualified with a package name. The same is true for deconstruction patterns:
case Foo(int x, int y): // unqualified
case com.foo.Bar(int x, int y): // qualified by package
Static methods that are declared in the current class or an enclosing class, or are statically imported, can be referred to with an unqualified name; static methods can also be qualified with a type name. The same is true for static patterns:
case powerOfTwo(int exp): // unqualified
case Optional.of(var e): // qualified by class
Instance methods invoked on the current object can be referred to with an unqualified name; instance methods can also be qualified by a receiver object. The same is true for instance patterns:
case match(String s): // unqualified
case As.match(String s): // qualified by receiver
In a qualified pattern x.y
, x
might be a package name, a class name,
or a
(effectively final) receiver variable; we use the same rules for
choosing how to
interpret a qualifier for patterns as we do for method invocations.
Declaring method-pattern pairs whose structure and name are the same yields many benefits. It means that we take things apart using the same abstractions used to put them together, which makes code more readable and less error-prone.
Referring to a inverse pair of operations by a single name is simpler than
having separate names for each direction; not only don't we need to come
up with
a name for the other direction, we also don't need to teach clients that
"these
two names are inverses", because the inverses have the same name
already. What
we know about the method Integer::toString
immediately carries over to its
inverse.
Further, thinking about a method-pattern pair provides a normalizing
force to
actually ensuring the two are inverses; if we just had two related methods
xToY
and yToX
, they might diverge subtly because the connection
between the
two members is not very strong.
Finally, this gives the language permission to treat the pair of members as a thing in some cases, such as the use of ctor-dtor pairs in "withers" or serialization.
The explicit duality takes a little time to get used to. We have many
years of
experience of naming a method for its directionality, so people's first
reaction
is often "the pattern should be called Integer.fromString
, not
Integer.toString
". So people will initially bristle at giving both
directions
the same name, especially when one implies a directionality such as
toString
.
(In these cases, we can fall back on a convention that says that we
should name
it for the total direction.)
Interfaces with a single abstract method (SAM) are called functional interfaces and we support a conversion (historically called SAM conversion) from lambdas to functional interfaces. Interfaces with a single abstract pattern can benefit from a similar conversion (call this "SAP" conversion.)
In the early days of Streams, people complained about processing a stream using instanceof and cast:
Stream<Object> objects = ...
Stream<String> strings = objects.filter(x -> x instanceof String)
.map(x -> (String) x);
This locution is disappointing both for its verbosity (saying the same
thing in
two different ways) and its efficiency (doing the same work basically
twice.) Later, it became possible to slightly simplify this using
mapMulti
:
objects.mapMulti((x, sink) -> { if (x instanceof String s)
sink.accept(s); })
But, ultimately this stream pipeline is a pattern match; we want to
match the
elements to the pattern String s
, and get a stream of the matched string
bindings. We are now in a position to expose this more directly. Suppose we
had the following SAP interface:
interface Match<T, U> {
inverse U match(T t);
}
then Stream
could expose a match
method:
<U> Stream<T> match(Match<T, U> pattern);
We can SAP-convert a lambda whose yielded bindings are compatible with the sole abstract pattern in the SAP interface::
Match<Object, String> m = o -> { if (o instanceof String s) __yield(s); };
... stream.match(s) ...
And we can do the same with pattern references to existing patterns
that are
compatible with the sole pattern in a SAP interface. As a special
case, we can
also support a conversion from type patterns to a compatible SAP type
with an
instanceof
pattern reference (analogous to a new
method reference):
objects.match(String::instanceof)
where String::instanceof
means the same as the previous lambda
example. This
means that APIs like Stream
can abstract over conditional behavior as
well as
unconditional.