Member Patterns -- the bikeshed

[Brian Goetz]

We now come to the long-awaited bikeshed discussion on what member 
patterns should look like.

Bikeshed disclaimer for EG:
   - This is likely to evoke strong opinions, so please take pains to be 
especially constructive
   - Long reply-to-reply threads should be avoided even more than usual
   - Holistic, considered replies preferred
   - Please change subject line if commenting on a sub-topic or tangential
     concern

Special reminders for Remi:
  - Use of words like "should", "must", "shouldn't", "mistake", "wrong", 
"broken"
    are strictly forbidden.
  - If in doubt, ask questions first.

Notes for external observers:
  - This is a working document for the EG; the discussion may continue for a
    while before there is an official proposal.  Please be patient.


# Pattern declaration: the bikeshed

We've largely identified the model for what kinds of patterns we need to
express, but there are still several degrees of freedom in the syntax.

As the model has simplified during the design process, the space of syntax
choices has been pruned back, which is a good thing.  However, there are 
still
quite a few smaller decisions to be made.  Not all of the considerations are
orthogonal, so while they are presented individually, this is not a 
"pick one
from each column" menu.

Some of these simplifications include:

  - Patterns with "input arguments" have been removed; another way to 
get to what
    this gave us may come back in another form.
  - I have grown increasingly skeptical of the value of the imperative 
`match`
    statement.  With better totality analysis, I think it can be eliminated.

We can discuss these separately but I would like to sync first on the broad
strokes for how patterns are expressed.

## Object model requirements

As outlined in "Towards Member Patterns", the basic model is that 
patterns are
the dual of other executable members (constructors, static methods, instance
methods.)  While they are like methods in that they have inputs, 
outputs, names,
and an imperative body, they have additional degrees of freedom that
constructors and methods lack:

  - Patterns are, in general, _conditional_ (they can succeed or fail), 
and only
    produce bindings (outputs) when they succeed.  This conditionality is
    understood by the language's flow analysis, and is used for 
computing scoping
    and definite assignment.
  - Methods can return at most one value; when a pattern completes 
successfully,
    it may bind multiple values.
  - All patterns have a _match candidate_, which is a distinguished,
    possibly-implicit parameter.  Some patterns also have a receiver, 
which is
    also a distinguished, possibly-implicit parameter.  In some such 
cases the
    receiver and match candidate are aliased, but in others these may 
refer to
    different objects.

So a pattern is a named executable member that takes a _match candidate_ 
as a
possibly-implicit parameter, maybe takes a receiver as an implicit 
parameter,
and has zero or more conditional _bindings_.  Its body can perform 
imperative
computation, and can terminate either with match failure or success.  In the
success case, it must provide a value for each binding.

Deconstruction patterns are special in many of the same ways 
constructors are:
they are constrained in their name, inheritance, and probably their
conditionality (they should probably always succeed).  Just as the 
syntax for
constructors differs slightly from that of instance methods, the syntax for
deconstructors may differ slightly from that of instance patterns.  Static
patterns, like static methods, have no receiver and do not have access 
to the
type parameters of the enclosing class.

Like constructors and methods, patterns can be overloaded, but in accordance
with their duality to constructors and methods, the overloading happens 
on the
_bindings_, not the inputs.

## Use-site syntax

There are several kinds of type-driven patterns built into the language: 
type
patterns and record patterns.  A type pattern in a `switch` looks like:

     case String s: ...

And a record pattern looks like:

     case MyRecord(P1, P2, ...): ...

where `P1..Pn` are nested patterns that are recursively matched to the
components of the record.  This use-site syntax for record patterns was 
chosen
for its similarity to the construction syntax, to highlight that a record
pattern is the dual of record construction.

**Deconstruction patterns.**  The simplest kind of member pattern, a
deconstruction pattern, will have the same use-site syntax as a record 
pattern;
record patterns can be thought of as a deconstruction pattern "acquired for
free" by records, just as records do with constructors, accessors, object
methods, etc.  So the use of a deconstruction pattern for `Point` looks 
like:

     case Point(var x, var y): ...

whether `Point` is a record or an ordinary class equipped with a suitable
deconstruction pattern.

**Static patterns.**  Continuing with the idea that the destructuring syntax
should evoke the aggregation syntax, there is an obvious candidate for the
use-site syntax for static patterns:

     case Optional.of(var e): ...
     case Optional.empty(): ...

**Instance patterns.**  Uses of instance patterns will likely come in 
two forms,
analogous to bound and unbound instance method references, depending on 
whether
the receiver and the match candidate are the same object.  In the 
unbound form,
used when the receiver is the same object as the match candidate, the 
pattern
name is qualified by a _type_:

```
Class<?> k = ...
switch (k) {
     // Qualified by type
     case Class.arrayClass(var componentType): ...
}
```

This means that we _resolve_ the pattern `arrayClass` starting at 
`Class` and
_select_ the pattern using the receiver, `k`.  We may also be able to 
omit the
class qualifier if the static type of the match candidate is sufficient to
resolve the desired pattern.

In the bound form, used when the receiver is distinct from the match 
candidate,
the pattern name is qualified with an explicit _receiver expression_.  As an
example, consider an interface that captures primitive widening and 
narrowing
conversions, such as those between `int` and `long`.  In the widening 
direction,
conversion is unconditional, so this can be modeled as a method from 
`int` to
`long`.  In the other direction, conversion is conditional, so this is 
better
modeled as a _pattern_ whose match candidate is `long` and which binds 
an `int`
on success.  Since these are instance methods of some class (say,
`NumericConversion<T,U>`), we need to provide the receiver instance in 
order to
resolve the pattern:

```
NumericConversion<int, long> nc = ...

switch (aLong) {
     case nc.narrowed(int i):
     ...
}
```

The explicit receiver syntax would also be used if we exposed regular 
expression
matching as a pattern on the `j.u.r.Pattern` object (the name collision on
`Pattern` is unfortunate).  Imagine we added a `matching` instance 
pattern to
`j.u.r.Pattern`; then we could use it in `instanceof` as follows:

```
static final java.util.regex.Pattern P = Pattern.compile("(a*)(b*)");
...
if (aString instanceof P.matching(String as, String bs)) { ... }
```

Each of these use-site syntaxes is modeled after the use-site syntax for a
method invocation or method reference.

## Declaration-site syntax

To avoid being biased by the simpler cases, we're going to work all the 
cases
concurrently rather than starting with the simpler cases and working 
up.  (It
might seem sensible to start with deconstructors, since they are the "easy"
case, but if we did that, we would likely be biased by their simplicity 
and then
find ourselves painted into a corner.)  As our example gallery, we will 
consider:

  - Deconstruction pattern for `Point`;
  - Static patterns for `Optional::of` and `Optional::empty`;
  - Static pattern for "power of two" (illustrating a computations where 
success
    or failure, and computation of bindings, cannot easily be separated);
  - Instance pattern for `Class::arrayClass` (used unbound);
  - Instance pattern for `Pattern::matching` on regular expressions 
(used bound).

Member patterns, like methods, have _names_.  (We can think of 
constructors as
being named for their enclosing classes, and the same for 
deconstructors.)  All
member patterns have a (possibly empty) ordered list of _bindings_, 
which are
the dual of constructor or method parameters.  Bindings, in turn, have 
names and
types.  And like constructors and methods, member patterns have a _body_ 
which
is a block statement.  Member patterns also have a _match candidate_, 
which is a
likely-implicit method parameter.

### Member patterns as inverse methods and constructors

Regardless of syntax, let us remind ourselves that that deconstructors 
are the
categorical dual to constructors (coconstructors), and pattern methods 
are the
categorical dual to methods (comethods).  They are dual in their 
structure: a
constructor or method takes N arguments and produces a result, the 
corresponding
member pattern consumes a match candidate and (conditionally) produces N
bindings.

Moreover, they are semantically dual: the return value produced by 
construction
or factory invocation is the match candidate for the corresponding member
pattern, and the bindings produced by a member pattern are the answers 
to the
_Pattern Question_ -- "could this object have come from an invocation of my
dual, and if so, with what arguments."

### What do we call them?

Given the significant overlap between methods and patterns, the first 
question
about the declaration we need to settle is how to identify a member pattern
declaration as distinct from a method or constructor declaration. _Towards
Member Patterns_ tried out a syntax that recognized these as _inverse_ 
methods
and constructors:

     public Point(int x, int y) { ... }
     public inverse Point(int x, int y) { ... }

While this is a principled choice which clearly highlights the duality, 
and one
that might be good for specification and verbal description, it is 
questionable
whether this would be a great syntax for reading and writing programs.

A more traditional option is to choose a "noun" (conditional) keyword, 
such as
`pattern`, `matcher`, `extractor`, `view`, etc:

     public pattern Point(int x, int y) { ... }

If we are using a noun keyword to identify pattern declarations, we 
could use
the same noun for all of them, or we could choose a different one for
deconstruction patterns:

     public deconstructor Point(int x, int y) { ... }

Alternately, we could reach for a symbol to indicate that we are talking 
about
an inverted member.  C++ fans might suggest

     public ~Point(int x, int y) { ... }

but this is too cryptic (it's evocative once you see it, but then it becomes
less evocative as we move away from deconstructors towards instance 
patterns.)

If we wish to offer finer-grained control over conditionality, we might
additionally need a `total` / `partial` modifier, though I would prefer 
to avoid
that.

Of the keyword candidates, there is one that stands out (for good and bad)
because it connects to something that is already in the language: 
`pattern`.  On
the one hand, using the term `pattern` for the declaration is a slight 
abuse; on
the other, users will immediately connect it with "ah, so that's how I 
make a
new pattern" or "so that's what happens when I match against this pattern."
(Lisps would resolve this tension by calling it `defpattern`.)

The others (`matcher`, `view`, `extractor`, etc) are all made-up terms that
don't connect to anything else in the language, for better or worse.  If 
we pick
one of these, we are asking users to sort out _three_ separate new things in
their heads: (use-site) patterns, (declaration-site) matchers, and the 
rules of
how patterns and matchers are connected.  Calling them both "patterns", 
despite
the mild abuse of terminology, ties them together in a way that 
recognizes their
connection.

My personal position: `pattern` is the strongest candidate here, despite 
some
flaws.

### Binding lists and match candidates

There are two obvious alternatives for describing the binding list and match
candidate of a pattern declaration, both with their roots in the 
constructor and
method syntax:

  - Pretend that a pattern declaration is like a method with multiple 
return, and
    put the binding list in the "return position", and make the match 
candidate
    an ordinary parameter;
  - Lean into the inverse relationship between constructors and methods (and
    consistency with the use-site syntax), and put the binding list in the
    "parameter list position". For static patterns and some instance 
patterns,
    which need to explicitly identify the match candidate type, there 
are several
    sub-options:
    - Lean further into the duality, putting the match candidate type in the
      "return position";
    - Put the match candidate type somewhere else, where it is less 
likely to be
      confused for a method return.

The "method-like" approach might look like this:

```
class Point {
     // Constructor and deconstructor
     public Point(int x, int y) { ... }
     public pattern (int x, int y) Point(Point target) { ... }
     ...
}

class Optional<T> {
     // Static factory and pattern
     public static<T> Optional<T> of(T t) { ... }
     public static<T> pattern (T t) of(Optional<T> target) { ... }
     ...
}
```

The "inverse" approach might look like:

```
class Point {
     // Constructor and deconstructor
     public Point(int x, int y) { ... }
     public pattern Point(int x, int y) { ... }
     ...
}

class Optional<T> {
     // Static factory and pattern (using the first sub-option)
     public static<T> Optional<T> of(T t) { ... }
     public static<T> pattern Optional<T> of(T t) { ... }
     ...
}
```

With the "method-like" approach, the match candidate gets an explicit name
selected by the author; with the inverse approach, we can go with a 
predefined
name such as `that`.  (Because deconstructors do not have receivers, we 
could by
abuse of notation arrange for the keyword `this` to refer instead to the 
match
candidate within the body of a deconstructor.  While this might seem to 
lead to
a more familiar notation for writing deconstructors, it would create a
gratuitous asymmetry between the bodies of deconstruction patterns and 
those of
other patterns.)

Between these choices, nearly all the considerations favor the "inverse"
approach:

  - The "inverse" approach makes the declaration look like the use 
site.  This
    highlights that `pattern Point(int x, int y)` is what gets invoked 
when you
    match against the pattern use `Point(int x, int y)`.  (This point is so
    strong that we should probably just stop here.)
  - The "inverse" members also look like their duals; the only 
difference is the
    `pattern` keyword (and possibly the placement of the match candidate 
type).
    This makes matched pairs much more obvious, and such matched pairs 
will be
    critical both for future language features and for library idioms.
  - The method-like approach is suggestive of multiple return or tuples, 
which is
    probably helpful for the first few minutes but actually harmful in 
the long
    term. This feature is _not_ (much as some people would like to 
believe) about
    multiple return or tuples, and playing into this misperception will 
only make
    it harder to truly understand.  So this suggestion ends up propping 
up the
    wrong mental model.

The main downside of the "inverse" approach is the one-time speed bump 
of the
unfamiliarity of the inverted syntax.  (The "method-like" syntax also 
has its
own speed bumps, it is just unfamiliar in different ways.)  But unlike the
advantages of the inverse approach, which continue to add value forever, 
this
speed bump is a one-time hurdle to get over.

To smooth out the speed bumps of the inverse approach, we can consider 
moving
the position of the match candidate for static and (suitable) instance 
pattern
declarations, such as:

```
class Optional<T> {
     // the usual static factory
     public static<T> Optional<T> of(T t) { ... }

     // Various ways of writing the corresponding pattern
     public static<T> pattern of(T t) for Optional<T> { ... }
     // or ...
     public static<T> pattern(Optional<T>) of(T t) { ... }
     // or ...
     public static<T> pattern(Optional<T> that) of(T t) { ... }
     // or ...
     public static<T> pattern<Optional<T>> of(T t) { ... }
     ...
}
```

(The deconstructor example looks the same with either variant.) Of these,
treating the match candidate like a "parameter" of "pattern" is probably the
most evocative:

```
public static<T> pattern(Optional<T> that) of(T t) { ... }
```

as it can be read as "pattern taking the parameter `Optional<T> that` called
`of`, binding `T`, and is a short departure from the inverse syntax.

The main value of the various rearrangements is that users don't need to 
think
about things operating in reverse to parse the syntax.  This trades some 
of the
secondary point (patterns looking almost exactly like their inverses) for a
certain amount of cognitive load, while maintaining the most important
consideration: that the declaration site look like the use site.

For instance pattern declarations, if the match candidate type is the 
same as
the receiver type, the match candidate type can be elided as it is with
deconstructors.

My personal position: the "multiple return" version is terrible; all the
sub-variants of the inverse version are probably workable.

### Naming the match candidate

We've been assuming so far that the match candidate always has a fixed name,
such as `that`; this is an entirely workable approach.  Some of the 
variants are
also amenable to allowing authors to explicitly select a name for the match
candidate.  For example, if we put the match candidate as a "parameter" 
to the `pattern` keyword, there is an obvious place to put the name:

```
static<T> pattern(Optional<T> target) of(T t) { ... }
```

My personal opinion: I don't think this degree of freedom buys us much, 
and in
the long run readability probably benefits by picking a fixed name like 
`that`
and sticking with it.  Even with a fixed name, if there is a sensible 
position
for the name, allowing users to type `that` for explicitness is fine (as 
we do
with instance methods, though many people don't know this.)  We may even 
want to
require it.

## Body types

Just as there are two obvious approaches for the declaration, there are two
obvious approaches we could take for the body (though there is some coupling
between them.)  We'll call the two body approaches _imperative_ and
_functional_.

The imperative approach treats bindings as initially-DU variables that 
must be
DA on successful completion, getting their value through ordinary 
assignment;
the functional approach sets all the bindings at once, positionally.  Either
way, member patterns (except maybe deconstructors) also need a way to
differentiate a successful match from a failed match.

Here is the `Point` deconstructor with both imperative and functional 
style. The
functional style uses a placeholder `match` statement to indicate a 
successful
match and provision of bindings:

```
class Point {
     int x, y;

     Point(int x, int y) {
         this.x = x;
         this.y = y;
     }

     // Imperative style, deconstructor always succeeds
     pattern Point(int x, int y) {
         x = that.x;
         y = that.y;
     }

     // Functional style
     pattern Point(int x, int y) {
         match(that.x, that.y);
     }
}
```

There are some obvious differences here.  In the imperative style, the 
dtor body
looks much more like the reverse of the ctor body. The functional style 
is more
concise (and amenable to further concision via the "concise method bodies"
mechanism in the future), as well as a number of less obvious 
differences.  For
deconstructors, the imperative approach is likely to feel more natural 
because
of the obvious symmetry with constructors.

In reality, it is _premature at this point to have an opinion_, because we
haven't yet seen the full scope of the problem; deconstructors are a special
case in many ways, which almost surely is distorting our initial 
opinion.  As we
move towards conditional patterns (and pattern lambdas), our opinions 
may flip.

Regardless of which we pick, there are some additional syntactic choices 
to be
made -- what syntax to use to indicate success (we used `match` in the above
example) or failure.  (We should be especially careful around trying to 
reuse
words like `return`, `break`, or `yield` because, in the case where 
there are
zero bindings (which is allowable), it becomes unclear whether they mean 
"fail"
or "succeed with zero bindings".)

### Success and failure

Except for possibly deconstructors, which we may require to be total, a 
pattern
declaration needs a way to indicate success and failure.  In the 
examples above,
we posited a `match` statement to indicate success in the functional 
approach,
and in both examples leaned on the "implicit success" of deconstructors 
(under
the assumption they always succeed).  Now let's look at the more general 
case to
figure out what else is needed.

For a static pattern like `Optional::of`, success is conditional. Using
`match-fail` as a placeholder for "the match failed", this might look like
(functional version):

```
public static<T> pattern(Optional<T> that) of(T t) {
     if (that.isPresent())
         match (that.get());
     else
         match-fail;
}
```

The imperative version is less pretty, though.  Using `match-success` as a
placeholder:

```
public static<T> pattern(Optional<T> that) of(T t) {
     if (that.isPresent()) {
         t = that.get();
         match-success;
     }
     else
         match-fail;
}
```

Both arms of the `if` feel excessively ceremonial here.  And if we chose 
to not
make all deconstruction patterns unconditional, deconstructors would 
likely need
some explicit success as well:

```
pattern Point(int x, int y) {
     x = that.x;
     y = that.y;
     match-success;
}
```

It might be tempting to try and eliminate the need for explicit success by
inferring it from whether or not the bindings are DA or not, but this is
error-prone, is less type-checkable, and falls apart completely for patterns
with no bindings.

### Implicit failure in the functional approach

One of the ceremonial-seeming aspects of `Optional::of` above is having 
to say
`else match-fail`, which doesn't feel like it adds a lot of value.  
Perhaps we
can be more concise without losing clarity.

Most conditional patterns will have a predicate to determine matching, 
and then
some conditional code to compute the bindings and claim success. Having 
to say
"and if the predicate didn't hold, then I fail" seems like ceremony for the
author and noise for the reader.  Instead, if a conditional pattern 
falls off
the end without matching, we could treat that as simply not matching:

```
public static<T> pattern(Optional<T> that) of(T t) {
     if (that.isPresent())
         match (that.get());
}
```

This says what we mean: if the optional is present, then this pattern 
succeeds
and bind the contents of the `Optional`.  As long as our "succeed" construct
strongly enough connotes that we are terminating abruptly and 
successfully, this
code is perfectly clear.  And most conditional patterns will look a lot like
`Optional::of`; do some sort of test and if it succeeds, extract the 
state and
bind it.

At first glance, this "implicit fail" idiom may seem error-prone or 
sloppy.  But
after writing a few dozen patterns, one quickly tires of saying "else
match-fail" -- and the reader doesn't necessarily appreciate reading it 
either.

Implicit failure also simplifies the selection of how we explicitly indicate
failure; using `return` in a pattern for "no match" becomes pretty much 
a forced
move.  We observe that (in a void method), "return" and "falling off the 
end"
are equivalent; if "falling off the end" means "no match", then so should an
explicit `return`.  So in those few cases where we need to explicitly 
signal "no
match", we can just use `return`.  It won't come up that often, but 
here's an
example where it does:

```
static pattern(int that) powerOfTwo(int exp) {
     int exp = 0;

     if (that < 1)
         return; // explicit fail

     while (that > 1) {
         if (that % 2 == 0) {
             that /= 2;
             ++exp;
         }
         else
             return; // explicit fail
     }
     match (exp);
}
```

As a bonus, if `return` as match failure is a forced move, we need only 
select a
term for "successful match" (which obviously can't be `return`). We 
could use
`match` as we have in the examples, or a variant like `matched` or 
`matches`.
But rather than just creating a new control operator, we have an 
opportunity to
lean into the duality a little harder, by including the pattern syntax 
in the
match:

```
matches of(that.get());
```

or the (optionally?) qualified (inferring type arguments, as we do at 
the use
site):

```
matches Optional.of(that.get());
```

These "use the name" approaches trades a small amount of verbosity to gain a
higher degree of fidelity to the pattern use site (and to evoke the comethod
completion.)

If we don't choose "implicit fail", we would have to invent _two_ new 
control
flow statements to indicate "success" and "failure".

My personal position: for the functional approach, implicit failure both 
makes
the code simpler and clearer, and after you get used to it, you don't 
want to go
back.  Whether we say `match` or `matches` or `matches <pattern-name>` 
are all
workable, though I like some variant that names the pattern.

### Implicit success in the imperative approach

In the imperative approach, we can be implicit as well, but it feels more
natural (at least, initially) to choose implicit success rather than 
failure.
This works great for unconditional patterns:

```
pattern Point(int x, int y) {
     x = that.x;
     y = that.y;
     // implicit success
}
```

but not quite as well for conditional patterns:

```
static<T> pattern(Optional<T> that) of(T t) {
     if (that.isPresent()) {
         t = that.get();
     }
     else
         match-fail;
     // implicit success
}
```

We can eliminate one of the arms of the if, with the more concise (but
convoluted) inversion:

```
static<T> pattern(Optional<T> that) of(T t) {
     if (!that.isPresent())
         match-fail;
     t = that.get();
     // implicit success
}
```

Just as with the functional approach, if we choose imperative and "implicit
success", using `return` to indicate success is pretty much a forced move.

### Imperative is a trap

If we assume that functional implies implicit failure, and imperative 
implies
implicit success, then our choices become:

```
class Optional<T> {
     public static<T> Optional<T> of(T t) { ... }

     // imperative, implicit success
     public static<T> pattern(Optional<T> that) of(T t) {
         if (that.isPresent()) {
             t = that.get();
         }
         else
             match-fail;
     }

     // functional, implicit failure
     public static<T> pattern(Optional<T> that) of(T t) {
         if (that.isPresent())
             matches of(that.get());
     }
}
```

Once we get past deconstructors, the imperative approach looks worse by
comparison because we need to assign all the bindings (which is _O(n)_
assignments) _and also_ indicate success or failure somehow, whereas in the
functional style all can be done together with a single `matches` statement.

Looking at the alternatives, except maybe for unconditional patterns, the
functional example above seems a lot more natural.  The imperative approach
works with deconstructors (assuming they are not conditional), but does not
scale so well to conditionality -- which is the essence of patterns.

 From a theoretical perspective, the method-comethod duality also gives us a
forceful nudge towards the functional approach.  In a method, the method
arguments are specified as a positional list of expressions at the use 
site:

     m(a, b, c)

and these values are invisibly copied into the parameter slots of the method
prior to frame activation.  The dual to that for a comethod to similarly 
convey
the bindings in a positional list of expressions (as they must either all be
produced or none), where they are copied into the slots provided at the use
site, as is indicated by `matches` in the above examples.

My personal position: the imperative style feels like a trap.  It seems
"obvious" at first if we start with deconstructors, but becomes increasingly
difficult when we get past this case, and gets in the way of other
opportunities.  The last gasp before acceptance is the discomfort that 
dtor and
ctor bodies are written in different styles, but in the rear-view 
mirror, this
feels like a non-issue.

### Derive imperative from functional?

If we start with "functional with implicit failure", we can possibly rescue
imperative by deriving a version of imperative from functional, by 
"overloading"
the match-success operator.

If we have a pattern whose binding names are `b1..bn` of types `B1..Bn`, 
then
the `matches` operator must take a list of expressions `e1..en` whose 
arity and
types are compatible with `B1..Bn`.  But we could allow `matches` to 
also have a
nilary form, which would have the effect of being shorthand for

     matches <pattern-name>(b1, b2, ..., bn)

where each of `b1..bn` must be DA at the point of matching.  This means 
that we
could express patterns in either form:

```
class Optional<T> {
     public static<T> Optional<T> of(T t) { ... }

     // imperative, derived from functional with implicit failure
     public static<T> pattern(Optional<T> that) of(T t) {
         if (that.isPresent()) {
             t = that.get();
             matches of;
         }
     }

     public static<T> pattern(Optional<T> that) of(T t) {
         if (that.isPresent())
             matches of(that.get());
     }
}
```

This flexibility allows users to select a more verbose expression in 
exchange
for a clearer association of expressions and bindings, though as we'll 
see, it
does come with some additional constraints.

### Wrapping an existing API

Nearly every library has methods (sometimes sets of methods) that are 
patterns
in disguise, such as the pair of methods `isArray` and `getComponentType` in
`Class`, or the `Matcher` helper type in `java.util.regex`. Library 
maintainers
will likely want to wrap (or replace) these with real patterns, so these can
participate more effectively in conditional contexts, and in some cases,
highlight their duality with factory methods.

Matching a string against a `j.u.r.Pattern` regular expression has all 
the same
elements as a pattern, just with an ad-hoc API (and one that I have to 
look up
every time).  But we can fairly easily wrap a true pattern around the 
existing
API.  To match against a `Pattern` today, we pass the match candidate to
`Pattern::matcher`, which returns a `Matcher` with accessors 
`Matcher::matches`
(did it match) and `Matcher::group` (conditionally extract a particular 
capture
group.)  If we want to wrap this with a pattern called `regexMatch`:

```
pattern(String that) regexMatch(String... groups) {
     Matcher m = this.matcher(that);
     if (m.matches())
         matches Pattern.regexMatch(IntStream.range(1, m.groupCount())
                                             .map(Matcher::group)
.toArray(String[]::new));
     // whole lotta matchin' goin' on
}
```

This says that a `j.u.r.Pattern` has an instance pattern called `regex`, 
whose
match candidate is `String`, and which binds a varargs of `String` 
corresponding
to the capture groups.  The implementation simply delegates to the existing
`j.u.r.Matcher` API.  This means that `j.u.r.Pattern` becomes a sort of 
"pattern
object", and we can use it as a receiver at the use site:

```
static Pattern As = Pattern.compile("(a*)");
static Pattern Bs = Pattern.compile("(b*)");
...
switch (string) {
     case As.regexMatch(var as): ...
     case Bs.regexMatch(var bs): ...
     ...
}
```

### Odds and ends

There are a number of loose ends here.  We could choose other names for the
match-success and match-fail operations, including trying to reuse 
`break` or
`yield`.  But, this reuse is tricky; it must be very clear whether a 
given form
of abrupt completion means "success" or "failure", because in the case of
patterns with no bindings, we will have no other syntactic cues to help
disambiguate.  (I think having a single `matches`, with implicit failure and
`return` meaning failure, is the sweet spot here.)

Another question is whether the binding list introduces corresponding 
variables
into the scope of the body.  For imperative, the answer is "surely yes"; for
functional, the answer is "maybe" (unless we want to do the trick where we
derive imperative from functional, in which case the answer is "yes" again.)

If the binding list does not correspond to variables in the body, this 
may be
initially discomforting; because they do not declare program elements, 
they may
feel that they are left "dangling".  But even if they are not declaring
_program_ elements, they are still declaring _API_ elements (similar to the
return type of a method.)  We will want to provide Javadoc on the 
bindings, just
like with parameters; we will want to match up binding names in 
deconstructors
with parameter names in constructors; we may even someday want to support
by-name binding at the use site (e.g., `case Foo(a: var a)`).  The names are
needed for all of these, just not for the body. Names still matter.  My take
here is that this is a transient "different is scary" reaction, one that we
would get over quickly.

A final question is whether we should consider unqualified names as 
implicitly
qualified by `that` (and also `this`, for instance patterns, with some 
conflict
resolution).  Users will probably grow tired of typing `that.` all the 
time, and most of the time, the unqualified use is perfectly readable.

## Exhaustiveness

There is one last syntax question in front of us: how to indicate that a 
set of
patterns are (claimed to be) exhaustive on a given match candidate 
type.  We see
this with `Optional::of` and `Optional::empty`; it would be sad if the 
compiler
did not realize that these two patterns together were exhaustive on 
`Optional`.
This is not a feature that will be used often, but not having it at all 
will be
a repeated irritant.

The best I've come up with is to call these `case` patterns, where a set of
`case` patterns for a given match candidate type in a given class are 
asserted
to be an exhaustive set:

```
class Optional<T> {
     static<T> Optional<T> of(T t) { ... }
     static<T> Optional<T> empty() { ... }

     static<T> case pattern of(T t) for Optional<T> { ... }
     static<T> case pattern empty() for Optional<T> { ... }
}
```

Because they may not be truly exhaustive, `switch` constructs will have 
to back
up the static assumption of exhaustiveness with a dynamic check, as we 
do for
other sets of exhaustive patterns that may have remainder.

I've experimented with variants of `sealed` but it felt more forced, so 
this is
the best I've come up with.

## Example: patterns delegating to other patterns

Pattern implementations must compose.  Just as a subclass constructor 
delegates
to a superclass constructor, the same should be true for deconstructors.
Here's a typical superclass-subclass pair:

```
class A {
     private final int a;

     public A(int a) { this.a = a; }
     public pattern A(int a) { matches A(that.a); }
}

class B extends A {
     private final int b;

     public B(int a, int b) {
         super(a);
         this.b = b;
     }

     // Imperative style
     public pattern B(int a, int b) {
         if (that instanceof super(var aa)) {
             a = aa;
             b = that.b;
             matches B;
         }
     }

     // Functional style
     public pattern B(int a, int b) {
         if (that instanceof super(var a))
             matches B(a, b);
     }
}
```

(Ignore the flow analysis and totality for the time being; we'll come 
back to
this in a separate document.)

The first thing that jumps out at us is that, in the imperative version, 
we had
to create a "garbage" variable `aa` to receive the binding, because `a` was
already in scope, and then we have to copy the garbage variable into the 
real
binding variable. Users will surely balk at this, and rightly so. In the
functional version (depending on the choices from "Odds and Ends") we 
are free
to use the more natural name and avoid the roundabout locution.

We might be tempted to fix the "garbage variable" problem by inventing 
another
sub-feature: the ability to use an existing variable as the target of a 
binding,
such as:

```
pattern Point(int a, int b) {
     if (this instanceof A(__bind a))
         b = this.b;
}
```

But, I think the language is stronger without this feature, for two reasons.
First, having to reason about whether a pattern match introduces a new 
binding
or assigns to an existing variables is additional cognitive load for 
users to
reason about, and second, having assignment to locals happening through
something other than assignment introduces additional complexity in finding
where a variable is modified.  While we can argue about the general 
utility of
this feature, bringing it in just to solve the garbage-variable problem is
particularly unattractive.

## Pattern lambdas

One final consideration is is that patterns may also have a lambda 
form.  Given
a single-abstract-pattern (SAP) interface:

```
interface Converter<T,U> {
     pattern(T t) convert(U u);
}
```

one can implement such a pattern with a lambda. Such a lambda has one 
parameter
(the match candidate), and its body looks like the body of a declared 
pattern:

```
Converter<Integer, Short> c =
     i -> {
         if (i >= Short.MIN_VALUE && i <= Short.MAX_VALUE)
             matches Converter.convert((short) i);
     };
```

Because the bindings of the pattern lambda are defined in the interface, 
not in
the lambda, this is one more reason not to like the imperative version: 
it is
brittle, and alpha-renaming bindings in the interface would be a
source-incompatible change.

## Example gallery

Here's all the pattern examples so far, and a few more, using the suggested
style (functional, implicit fail, implicit `that`-qualification):

```
// Point dtor
pattern Point(int x, int y) {
     matches Point(x, y);
}

// Optional -- static patterns for Optional::of, Optional::empty
static<T> case pattern(Optional<T> that) of(T t) {
     if (isPresent())
         matches of(t);
}

static<T> case pattern(Optional<T> that) empty() {
     if (!isPresent())
         matches empty();
}

// Class -- instance pattern for arrayClass (match candidate type inferred)
pattern arrayClass(Class<?> componentType) {
     if (that.isArray())
         matches arrayClass(that.getComponentType());
}

// regular expression -- instance pattern in j.u.r.Pattern
pattern(String that) regexMatch(String... groups) {
     Matcher m = matcher(that);
     if (m.matches())
         matches Pattern.regexMatch(IntStream.range(1, m.groupCount())
                                             .map(Matcher::group)
.toArray(String[]::new));
}

// power of two (somewhere)
static pattern(int that) powerOfTwo(int exp) {
     int exp = 0;

     if (that < 1)
         return;

     while (that > 1) {
         if (that % 2 == 0) {
             that /= 2;
             exp++;
         }
         else
             return;
     }
     matches powerOfTwo(exp);
}
```

## Closing thoughts

I came out of this exploration with very different conclusions than I 
expected
when going in.  At first, the "inverse" syntax seemed stilted, but over 
time it
started to seem more obvious.  Similarly, I went in expecting to prefer the
imperative approach for the body, but over time, started to warm to the
functional approach, and eventually concluded it was basically a forced 
move if
we want to support more than just deconstructors.  And I started out 
skeptical
of "implicit fail", but after writing a few dozen patterns with it, 
going back
to fully explicit felt painful.  All of this is to say, you should hold your
initial opinions at arm's length, and give the alternatives a chance to 
sink in.

For most _conditional_ patterns (and conditionality is at the heart of 
pattern
matching), the functional approach cleanly highlights both the match 
predicate
and the flow of values, and is considerably less fussy than the imperative
approach in the same situation; `Optional::of`, `Class::arrayClass`, and 
`regex`
look great here, much better than the would with imperative.  None of these
illustrate delegation, but in the presence of delegation, the gap gets even
wider.
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