Pattern assignment

Brian Goetz brian.goetz at
Fri Mar 25 15:38:52 UTC 2022

We still have a lot of work to do on the current round of pattern 
matching (record patterns), but let's take a quick peek down the road.  
Pattern assignment is a sensible next building block, not only because 
it is directly useful, but also because it will be required for 
_declaring_ deconstruction patterns in classes (that's how one pattern 
delegates to another.)  What follows is a rambling sketch of all the 
things we _could_ do with pattern assignment, though we need not do all 
of them initially, or even ever.

# Pattern assignment

So far, we've got two contexts in the language that can accommodate 
patterns --
`instanceof` and `switch`.  Both of these are conditional contexts, 
designed for
dealing with partial patterns -- test whether a pattern matches, and if so,
conditionally extract some state and act on it.

There are cases, though, when we know a pattern will always match, in 
which case
we'd like to spare ourselves the ceremony of asking.  If we have a 3d 
asking if it is a `Point` is redundant and distracting:

Point p = ...
if (p instanceof Point(var x, var y, var z)) {
     // use x, y, z

In this situation, we're asking a question to which we know the answer, and
we're distorting the structure of our code to do it.  Further, we're 
ourselves of the type checking the compiler would willingly do to 
validate that
the pattern is total.  Much better to have a way to _assert_ that the 

## Let-bind statements

In such a case, where we want to assert that the pattern matches, and 
bind it, we'd rather say so directly.  We've experimented with a few ways to
express this, and the best approach seems to be some sort of `let` 

let Point(var x, var y, var z) p = ...;
// can use x, y, z, p

Other ways to surface this might be to call it `bind`:

bind Point(var x, var y, var z) p = ...;

or even use no keyword, and treat it as a generalization of assignment:

Point(var x, var y, var z) p = ...;

(Usual disclaimer: we discuss substance before syntax.)

A `let` statement takes a pattern and an expression, and we statically 
that the pattern is exhaustive on the type of the expression; if it is 
not, this is a
type error at compile time.  Any bindings that appear in the pattern are
definitely assigned and in scope in the remainder of the block that 
encloses the
`let` statement.

Let statements are also useful in _declaring_ patterns; just as a subclass
constructor will delegate part of its job to a superclass constructor, a
subclass deconstruction pattern will likely want to delegate part of its 
job to
a superclass deconstruction pattern.  Let statements are a natural way 
to invoke
total patterns from other total patterns.

#### Remainder

Let statements require that the pattern be exhaustive on the type of the 
For total patterns like type patterns, this means that every value is 
including `null`:

let Object o = x;

Whatever the value of `x`, `o` will be assigned to `x` (even if `x` is null)
because `Object o` is total on `Object`.  Similarly, some patterns are 
not total on some types:

Object o = ...
let String s = o;  // compile error

Here, `String s` is not total on `Object`, so the `let` statement is not 
But as previously discussed, there is a middle ground -- patterns that are
_total with remainder_ -- which are "total enough" to be allowed to be 
exhaustive, but which in fact do not match on certain "weird" values. An
example is the record pattern `Box(var x)`; it matches all box 
instances, even
those containing null, but does not match a `null` value itself (because to
deconstruct a `Box`, we effectively have to invoke an instance member on the
box, and we cannot invoke instance members on null receivers.) 
Similarly, the
pattern `Box(Bag(String s))` is total on `Box<Bag<String>>`, with remainder
`null` and `Box(null)`.

Because `let` statements guarantee that its bindings are definitely assigned
after the `let` statement completes normally, the natural thing to do when
presented with a remainder value is to complete abruptly by reason of 
(This is what `switch` does as well.)  So the following statement:

Box<Bag<String>> bbs = ...
let Box(Bag(String s)) = bbs;

would throw when encountering `null` or `Box(null)` (but not 
because that matches the pattern, with `s=null`, just like a switch 
only this case would.

#### Conversions

JLS Chapter 5 ("Conversions and Contexts") outlines the conversions 
narrowing, boxing, unboxing, etc) that are permitted in various contexts
(assignment, loose method invocation, strict method invocation, cast, etc.)
We need to define the set of conversions we're willing to perform in the 
of a `let` statement as well; which of the following do we want to support?

let int x = aShort;     // primitive widening
let byte b = 0;         // primitive narrowing
let Integer x = 0;      // boxing
let int x = anInteger;  // unboxing

The above examples -- all of which use type patterns -- look a lot like 
variable declarations (especially if we choose to go without a keyword); 
strongly suggests we should align the valid set of conversions in `let`
statements with those permitted in assignment context.  The one place 
where we
have to exercise care is conversions that involve unboxing; a null in such
circumstances feeds into the remainder of the pattern, rather than having
matching throw (we're still likely to throw, but it affects the timing 
of how
far we progress in a pattern switch before we do so.)  So for example, the
the pattern `int x` is exhaustive on `Integer`, but with remainder `null`.

## Possible extensions

There are a number of ways we can extend `let` statements to make it more
useful; these could be added at the same time, or at a later time.

#### What about partial patterns?

There are times when it may be more convenient to use a `let` even when 
we know
the pattern is partial.  In most cases, we'll still want to complete 
abruptly if the
pattern doesn't match, but we may want to control what happens. For example:

let Optional.of(var contents) = optName
else throw new IllegalArgumentException("name is empty");

Having an `else` clause allows us to use a partial pattern, which receives
control if the pattern does not match.  The `else` clause could choose 
to throw,
but could also choose to `break` or `return` to an enclosing context, or 
recover by assigning the bindings.

#### What about recovery?

If we're supporting partial patterns, we might want to allow the `else` 
to provide defaults for the bindings, rather than throw.  We can make 
the bindings of the
pattern in the `let` statement be in scope, but definitely unassigned, 
in the
`else` clause, which means the `else` clause could initialize them and 

let Optional.of(var contents) = optName
else contents = "Unnamed";

This allows us to continue, while preserving the invariant that when the 
statement completes normally, all bindings are DA.

#### What about guards

If we're supporting partial patterns, we also need to consider the case 
the pattern matches but we still want to reject the content. This could of
course be handled by testing and throwing after the `let` completes, but 
if we
want to recover via the `else` clause, we might want to handle this 
We've already introduced a means to do this for switch cases -- a `when` 
-- and this works equally well in `let`:

let Point(var x, var y) = aPoint
when x >= 0 && y >= 0
else { x = y = 0; }

#### What about expressions?

The name `let` conjures up the image of `let` expressions in functional
languages, where we introduce a local binding for use in the scope of a 
expression.  This is not an accident!  It is quite useful when the same 
is going to be used multiple times, or when we want to limit the scope 
of a local
to a specific computation.

It is a short hop to `let` being usable as an expression, by providing 
an `in`

String lastThree =
     let int len = s.length()
     in s.substring(len-3, len);

The scope of the binding `len` is the expression to the right of the `in`,
nothing else.  (As with `switch` expressions, the expression to the right
of the `in` could be a block with a `yield` statement.)

It is a further short hop to permitting _multiple_ matches in a single `let`
statement or expression:

int area = let Point(var x0, var y0) = lowerLeft,
                Point(var x1, var y1) = upperRight
            in (x1-x0) * (y1-y0);

#### What about parameter bindings?

Destructuring with total patterns is also useful for method and lambda
parameters.  For a lambda that accepts a `Point`, we could include the 
in the lambda parameter list, and the bindings would automatically be in 
scope in the body.  Instead of:

areaFn = (Point lowerLeft, Point upperRight)
          -> (upperRight.x() - lowerLeft.x()) * (upperRight.y() - 

we could do the destructuring in the lambda header:

areaFn = (let Point(var x0, var y0) lowerLeft,
           let Point(var x1, var y1) upperRight)
          -> (x1-x0) * (y1-y0);

This allows us to treat the derived values to be "parameters" of the 
lambda.  We
would enforce totality at compile time, and dynamically reject remainder 
as we
do with `switch` and `let` statements.

I think this one may be a bridge too far, though.  The method header should
probably be reserved for API declaration, and destructuring only serves the
implementation.  I think I'd prefer to move the `let` into the body of the
method or lambda.

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