[External] : Re: Pattern assignment

Brian Goetz brian.goetz at oracle.com
Fri Apr 1 13:48:05 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.

Remainder is hard; the idea that our definition of "exhaustive" is 
intentionally defective is subtle, and will surely elicit "lol java" 
reactions among those disinclined to think very hard.  I wonder if a 
better term than "exhaustive" would help, one that doesn't promise so much.

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

(I assume you mean you don't see the benefit of *let* with partial 
patterns, since if all patterns were total this would just be multiple 

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

Because of scoping, and because you can't have a pattern just write to a 
local, even a blank final.  (This could of course be made to work, but I 
would really rather avoid going there if we at all can. (Yes Remi, I 
know you're in favor of going there.))

> when I could instead write (I assume blank finals are valid pattern 
> variables?)
> final Foo foo;
> if (!(x instanceof Optional.of(foo))) foo = defaultFoo;

Not currently, and I'd like to avoid it.  One reason is that this looks 
too much like a factory invocation; another is that, if we ever have 
constant patterns, then it won't be clear whether `foo` above is a 
variable into which to write the answer, or a constant that is being 
matched to the result of the binding.  Both of these are fighting (with 
method invocation) for the concise syntax, and I'm not sure I want any 
of them to win, but they can't all win, and I am not ready to pick that 
winner yet.  But, we will probably have to confront this  in some form 
when we get to dtor declaration.

But yes, the main value of the `else` is so that bindings can be via a 
fallback path and be in scope for the rest of the method.  The rest of 
`else` and `when` is mostly along for the ride.  And its likely that we 
wouldn't do all these forms initially, but I wanted to sketch out the 
whole design space before doing anything.

> 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".

OK, so you see this as being mostly "for unconditional patterns".

> 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.

Let expressions would alleviate some but not all of the cases for which 
general statement-expressions would.  They are not quite as good for "f 
= new Foo(); f.setX(3); yield f;", but (IMO) better for pulling common 
subexpressions into variables whose scope is confined to the expression.

> 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.

Yes, but I didn't come up with a syntax I liked enough for both lambdas 
and let.  Perhaps I'll try some more.

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