Type Class / Witness

[David Alayachew]

Can we get the slides for the talk? Or will they be linked in the video
description?

On Sat, Aug 9, 2025, 2:13 PM David Alayachew <davidalayachew at gmail.com>
wrote:

> Glad to hear it. Looking forward to seeing that video.
>
> And thank you all for discussing these topics out in the open -- whether
> as conference videos, these mailing lists, as reddit comments, etc.
>
> On Fri, Aug 8, 2025, 5:28 PM Remi Forax <forax at univ-mlv.fr> wrote:
>
>> In the future, we want to retrofit primitive to be value types.
>>
>> Type classes are one of the solutions for that.
>>
>> Rémi
>>
>>
>> On August 8, 2025 9:12:28 PM UTC, David Alayachew <
>> davidalayachew at gmail.com> wrote:
>>
>>> Woah, Type Classes!
>>>
>>> Ok, design talk aside, are you saying type classes are something you all
>>> have decided to do? Is that a feature, even if only for value classes, that
>>> we can safely assume is on the road map?
>>>
>>> All previous discussion on this made it sound like it's being strongly
>>> considered, but not on the road map.
>>>
>>>
>>> On Fri, Aug 8, 2025, 4:02 PM Brian Goetz <brian.goetz at oracle.com> wrote:
>>>
>>>> This is a valid observation, but you are getting way ahead of
>>>> yourself.  My talk this week was about general mechanisms; we are not
>>>> designing this feature yet, and discussions about it are premature (and
>>>> likely counterproductive, as people chime in with questions about something
>>>> that is not yet designed.)
>>>>
>>>> > On Aug 8, 2025, at 12:06 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>>>> >
>>>> > Hello,
>>>> > for those who where not at the JVMLS, it may make sense to re-read
>>>> this email later,
>>>> > when the talk of Brian on Type class / witness is visible on youtube.
>>>> > I write this now because i'm sure i will totally forget about that in
>>>> few days.
>>>> >
>>>> > During the talk, Brian said that witness are restricted to be used
>>>> only by value classes so we avoid the operator overloading hell that you
>>>> can see in other languages.
>>>> >
>>>> > I think this restriction can be bypassed.
>>>> >
>>>> > The problem is that you can always wrap any class into a value class
>>>> organized as a monad (containing a superposition of states) thus bypassing
>>>> that restriction.
>>>> >
>>>> >
>>>> > As an example, let say we want to abuse of the >> operator to emulate
>>>> C++ iostream.
>>>> > Let suppose we have this witness interface defined in the JDK (it is
>>>> by example used by Integer)
>>>> >
>>>> > witness interface BitwiseNumeric<V> {
>>>> >  V rightShift(V v1, V v2);
>>>> >
>>>> >  // other methods
>>>> > }
>>>> >
>>>> > And the witness interface for exact conversion also defined in the JDK
>>>> >
>>>> > witness interface Conversion<U, V> {
>>>> >  U convert(V value);
>>>> > }
>>>> >
>>>> >
>>>> > So the trick is to create a value class that encode either a Writer
>>>> or any value you want to add to the writer as a monad,
>>>> > The exact conversion is used to wrap any value into the IOStream, the
>>>> righShift is used to consume the wrapped value using a side-effect.
>>>> >
>>>> > value class IOStream {
>>>> > private final Writer writer;
>>>> > private final Object o;
>>>> >
>>>> > private IOStream(Writer writer, Object o) { this.writer = writer;
>>>> this.o = o; super(); }
>>>> >
>>>> > public IOWriter(Writer! writer) { this(writer, null); }
>>>> >
>>>> > public static final witness BitwiseNumeric<IOStream!> BITWISE_NUMERIC
>>>> = new BitwiseNumeric<>() {
>>>> >   public IOStream! rightShift(IOStream! v1, IOStream! v2) {
>>>> >     if (v1.writer == null || v2.writer != null) {
>>>> >       throw new ISE();
>>>> >     }
>>>> >     v1.writer.append("" + v2.o);
>>>> >     return v1;
>>>> >   }
>>>> >
>>>> >   // other methods throw UnsupportedOperationException
>>>> > };
>>>> >
>>>> > public static final witness Conversion<IOStream!, Integer!>
>>>> FROM_INTEGER = new Conversion<>() {
>>>> >   IOStream! convert(Integer! value) {
>>>> >     return new IOStream(null, value);
>>>> >   }
>>>> > };
>>>> > }
>>>> >
>>>> > So one can write
>>>> >  IOStream stream = new IOStream(writer);
>>>> >  var _ = stream >> 2 >> 3;
>>>> >
>>>> > I think this is a general mechanism so any operator can be abused
>>>> that way.
>>>> >
>>>> > regards,
>>>> > Rémi
>>>>
>>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/amber-spec-observers/attachments/20250811/8f0e9a16/attachment.htm>