Pattern assignment

[Remi Forax]

> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Sent: Friday, March 25, 2022 4:38:52 PM
> Subject: Pattern assignment

> 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.
And obviously (but let state the obvious hidden behind "directly useful") we have introduced records as named tuples and _declaring_deconstruction_ is a way deconstruct that tuple. 

Construction: 
var x = ... 
var y = ... 
var point = new Point(x, y); 

De-construction 
var point = ... 
__let__ Point(var x, var y) = point; 

> # 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 `Point`,
> 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 depriving
> 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 pattern
> matches.

> ## Let-bind statements

> In such a case, where we want to assert that the pattern matches, and forcibly
> 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` statement:

> ```
> 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 verify
> 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.
yes ! 

> #### Remainder

> Let statements require that the pattern be exhaustive on the type of the
> expression.
> For total patterns like type patterns, this means that every value is matched,
> 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 clearly
> 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 valid.
> But as previously discussed, there is a middle ground -- patterns that are
> _total with remainder_ -- which are "total enough" to be allowed to be
> considered
> 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 exception.
> (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 `Box(Bag(null))`,
> because that matches the pattern, with `s=null`, just like a switch containing
> only this case would.

> #### Conversions

> JLS Chapter 5 ("Conversions and Contexts") outlines the conversions (widening,
> 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 context
> 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 local
> variable declarations (especially if we choose to go without a keyword); this
> 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`.
There are another different between assignment and _let_, a _let_ creates new fresh local variables (binding) while assignment is able to reuse an existing local variable. 

In Java, the if statement is used a lot (too much IMO but i don't think we should fight to change that) so it may make sense to be able to reuse an existing local variables. 

Here is an example with a special keyword _ASSIGN_ indicating that the existing local variable is assigned. 

var box = ... 
var x = 0; 
if (...) { 
_let_ Box(_ASSIGN_ x) = box; 
} 

or with an if/else 

var minmax = ... 
int value; 
if (...) { 
_let_ MinMax(_ASSIGN_ value, _) = minmax; 
} else { 
_let_ MinMax(_, _ASSIGN_ value) = minmax; 
} 

> ## 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 even
> recover by assigning the bindings.
I don't like that because in that case "let pattern else ..." is equivalent of "if instanceof pattern else ... " with the former being expression oriented and the later statement oriented. 
As i said earlier, i don't think we should fight the fact that Java is statement oriented by adding expression oriented variations of existing constructs. 

> #### What about recovery?

> If we're supporting partial patterns, we might want to allow the `else` clause
> 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 continue:

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

> This allows us to continue, while preserving the invariant that when the `let`
> statement completes normally, all bindings are DA.
It fails if the "then" part or the "else" part need more than one instruction. 
Again, it's statement vs expression. 

> #### What about guards

> If we're supporting partial patterns, we also need to consider the case where
> 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 directly.
> We've already introduced a means to do this for switch cases -- a `when` clause
> -- and this works equally well in `let`:

> ```
> let Point(var x, var y) = aPoint
> when x >= 0 && y >= 0
> else { x = y = 0; }
> ```
It can be re-written using an if instanceof, so i do not think we need a special syntax 

int x, y; 
if (!(aPoint instanceof Point(_ASSIGN_ x, _ASSIGN_ y) && x >= 0 && y >= 0)) { 
x = 0; 
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 single
> expression. This is not an accident! It is quite useful when the same expression
> 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`
> clause:

> ```
> 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);
> ```
"Let ... in" is useful but i don't think it's related to the current proposal, for me it's orthogonal. We can introduce "let ... in" independently to the pattern assignment idea, 
and if the pattern assignment is already in the language, then "let ... in" will support it. 

> #### 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 pattern
> 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() - lowerLeft.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.
Technically, a lambda is implementation not API because a lambda is using the functional interface as API, but as you said it feels like a bridge too far at least for now. When Valhalla will land people may use value records more and we may have to introduce this kind of syntax. 

Rémi