[External] : Re: Declared patterns -- translation and reflection

Brian Goetz brian.goetz at oracle.com
Wed Mar 30 00:47:45 UTC 2022

>     The mangling has to be stable across compilations with respect to
>     any source- and binary-compatible changes to the pattern
>     declaration.  One mangling that works quite well is to use the
>     "symbolic-freedom encoding" of the erasure of the pattern
>     descriptor.  Because the erasure of the descriptor is exactly as
>     stable as any other method signature derived from source
>     declarations, it will have the desired binary compatibility
>     properties, overriding will work as expected, etc. 
> I think we need a least to use a special name like <deconstructor> the 
> same way we have <init>.

Yes.  Instance/static patterns will have names, so for them, we'll use 
the name as declared in the source.  Dtors have no names, just like 
ctors, so we have to invent something to stand in for that. <dtor> or 
similar is fine.

> I agree that we also need to encode the method type descriptor (the 
> carrier type) into the name, so the name of the method in the 
> classfile should be <deconstructor+mangle> or <name+mangle> (or 
> perhaps <pattern+name+mangle> ofr the pattern methods).

The key constraint is that the mangled name be stable with respect to 
compatible changes in the declaration.  The rest is just "classfile 

>     #### Return value
>     In an earlier design, we used a pattern object (which was a bundle
>     of method handles) as the return value of the pattern.  This
>     enabled clients to invoke these via condy and bind method handles
>     into the constant pool for deconstruction and static patterns.
>     Either way, we make use of some sort of carrier object to carry
>     the bindings from the pattern to the client; either we return the
>     carrier from the pattern method, or there is a method on the
>     pattern object that we invoke to get a carrier.  We have a few
>     preferences about the carrier; we'd like to be able to late-bind
>     to the actual implementation (i.e., we don't want to freeze the
>     name of a carrier class in the method descriptor), and at least
>     for records, we'd like to let the record instance itself be the
>     carrier (since it is immutable and we can just invoke the
>     accessors to get the bindings.)
> So the return type is either Object (too hide the type of the carrier) 
> or a lambda that returns an Object (PatternObject or PatternCarrier 
> acting like a glorified lambda).

If the pattern method actually runs the match, then I think Object is 
right.  If the method returns a constant bundle of method handles, then 
it can return something like PatternHandle or a matcher lambda.  But I 
am no longer seeing the benefit in this extra layer of indirection, 
given how the other translation work has played out.

>         Pattern {
>             u2 attr_name;
>             u4 attr_length;
>             u2 patternFlags; // bitmask
>             u2 patternName;  // index of UTF8 constant
>             u2 patternDescr; // index of MethodType (or alternately
>     UTF8) constant
>             u2 attributes_count;
>             attribute_info attributes[attributes_count];
>         }
>     This says that "this method is a pattern", reifies the name of the
>     pattern (patternName), reifies the pattern descriptor
>     (patternDescr) which encodes the types of the bindings as a method
>     descriptor or MethodType, and has attributes which can carry
>     annotations, parameter metadata, and signature metadata for the
>     bindings.   The existing attributes (e.g. Signature,
>     ParameterNames, RVAA) can be reused as is, with the interpretation
>     that this is the signature (or names, or annos) of the *bindings*,
>     not the input parameters.  Flags can carry things like
>     "deconstructor pattern" or "partial pattern" as needed. 
> From the classfile POV, a constructor is a method with a funny name in 
> between brackets, i think deconstructor and pattern methods should 
> work the same way.

Be careful of extrapolating from one data point.  Dtor are only one form 
of declared patterns; we also have to accomodate static and instance 

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