Member Patterns -- the bikeshed

[Guy Steele]

Rémi,

I get the impression that, in introducing the notion of a “carrier”, you seem to be focused on how deconstructors and patterns will necessarily be implemented in terms of the current definitions of Java and the JVM, or at least trying to explain it to the user (Java programmer) in terms of such an implementation.

But taking the point of view of such a user, I just don’t see the need to introduce a new notion of “carrier" to explain the set of match results (an ordered sequence) from a pattern, just as I don't see any need to introduce a new notion of “carrier" to explain the set of arguments (an ordered sequence) to a method.

There are other languages that treat a sequence of arguments as a first-class object, and treat methods or functions as simply always taking one argument, which may be one of those sequence objects; conceptually a function body first deconstructs the argument-sequence object. And the same approach works for the value returned, and that is how multiple results are addressed in such a language.

But Java has historically not been that kind of language. Like C and C++, from the start it has supported the idea of a function/method call that takes a sequence of arguments. That sequence of arguments is not a first-class object, and is not considered to have a type or any associated methods. The way that sequence is represented at run time is really of no concern to the programmer, and that fact has historically made it easier to allocate them on the stack rather than the heap. Yes, sometimes we wish that sequence were really a record (so that we could pass it around as a single object) or a map of some kind (so that we could pass argument values tagged by keywords rather than presenting them in a specific order), but that’s just not the way Java is.

And I suggest that in a Java-based model where patterns are regarded as duals of methods, the same observations apply to sequences of match results. There is no need for such a sequence to be an object, or even to be given a special name such as “carrier”. The representation of such a sequence at run time is not the programmer’s concern, and that in turn may make it easier to allocate them on the stack rather than the heap in some situations. All I care about as a user of patterns is that I supply a match candidate, a pattern that does not fail produces an ordered sequence of match results, and those results are then bound, in order, to variables I specify at the point of pattern use.

For me, a pattern that returns a sequence of match results is something that can be, but need not be, desugared into classical Java elements.

—Guy

On Apr 3, 2024, at 10:23 AM, forax at univ-mlv.fr wrote:



________________________________
From: "Brian Goetz" <brian.goetz at oracle.com>
To: "Remi Forax" <forax at univ-mlv.fr>
Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
Sent: Wednesday, April 3, 2024 2:48:40 PM
Subject: Re: Member Patterns -- the bikeshed
I would summarize your comments below as: Let's throw the entire model in the garbage, and replace it with something like Scala's "return an Optional<Tuple>" instead.

We've been discussing the model for several years; you've been asking (and waiting patiently) for "when are we going to talk about declaration syntax", and now that we're there, you want to throw it all out and start over?

My makeup job was too big so you do not recognize your model behind :)

There are two parts, the declaration part and the use-site part.
Correct me if i'm wrong but apart the support of a method pattern with no prefix, we are in agreement here.

For the declaration part, I think that
  carrier(int x, int y) asCartesian()
is more readable than
  inverse () asCartesian(int x, int y)

The inverse notation is a leaky abstraction in a leat two cases
- when a modifier or an annotation is used. For an annotation, there is a notion of a target and the parameter target is at the wrong place,
- when declaring a lambda, because in that case the parameters are not inversed.

Now, what i call a carrier type is what you call a list of bindings.
In terms of syntax, I think it is important to put a name in front of that list of bindings, i've proposed "carrier" so we provide a name for that feature, it's easier when discussing about it it or google it.
That does not change the fact that a method that returns a carrier is a special method because it requires at least a special erasure (because overloading), and a special reflection API,

But I hope, we will not cross the line and have to use new opcodes in the bytecode.
For me, a method that returns a carrier is something that can be desugared classical Java elements like an enum or a record is desugared to a class.


We've discussed how strategies that rely on "ask the user to declare a record for every API point" feel clever for about five minutes, but start to feel old quickly.

yes, this is what you have to do actually if you simulate the feature with Java nowadays. Not, what you should have to do in the future.
And the idea is to do better, among other things, we want to suport overloading.



The "carrier" concept in your examples seems to be just another way of reinventing multiple return -- with the added dis-bonus of being like but not quite the same as records.  We've been pretty clear that "multiple return" is not the design center here.

The idea behind a carrier is to let users define their binding list is a way that does not feel too strange, that why I propose to add a name/keyword in front of the binding list.

And I do not know how you define what a binding list is but multiple return + components description is a good definition for me.

Rémi



The use of ! for indicating totality is interesting, that's worth thinking about.



On 4/3/2024 6:21 AM, Remi Forax wrote:
I think that by not starting from the deconstructor, the notion of inverse methods make less sense.
I think that the notion of carrier / carrier type is less disruptive that the notion of member patterns.




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