Pattern assignment

Alan Malloy amalloy at
Fri Apr 1 00:31:15 UTC 2022

I'm certainly on board with a pattern-matching context that doesn't require
a vacuous conditional. Remainder, as it often does to me, seems like the
most likely point of confusion, but if we believe Java developers can get
their heads around the idea of remainder in other contexts, I don't think
this one is a novel problem.

I don't immediately see the benefit of partial patterns: why should I write

let Optional.of(foo) = x;
else foo = defaultFoo;

when I could instead write (I assume blank finals are valid pattern

final Foo foo;
if (!(x instanceof Optional.of(foo))) foo = defaultFoo;

Obviously it's shorter, but I'm not sure that's worth giving up the
promised simplicity from earlier that `let` is for when "we know a pattern
will always match".

Let-expressions seem like a reasonable extension, though who knows how
popular it will be. Of course, we could always generalize and add
statement-expressions instead...alas, such a change will have to wait quite
a while longer, I'm sure.

Did you consider allowing pattern parameters only in lambdas, not in
methods in general? Since a lambda is generally "internal implementation"
and a method is often API-defining, it might be reasonable to allow
implementation details to leak into lambda definitions if it makes them
more convenient to write, while keeping the more formal separation of
implementation and API for method parameters.

On Fri, Mar 25, 2022 at 8:39 AM Brian Goetz <brian.goetz at> wrote:

> 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
> `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.
> #### 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`.
> ## 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.
> #### 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.
> #### 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; }
> ```
> #### 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);
> ```
> #### 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.
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