RFR: 8255398: Add a dropReturn MethodHandle combinator [v3]

[Jorn Vernee]

On Mon, 26 Oct 2020 18:33:11 GMT, Rémi Forax <github.com+828220+forax at openjdk.org> wrote:

>>> …`dropReturn` seemed like a good choice since we already have `dropArguments`. WRT changing to `MethodHandle::changeReturnType`...
>> 
>> That's a very reasonable point.  People might look for `dropRT` after they find `dropAs`.  And `MHs` is designed as a large-ish set of utility methods.
>> 
>> I agree that adding `MH::changeRT` is a slippery slope; it tends to lift more of the MT API onto MH, and it starts to put new stuff into the smaller MH API; that was my bad.
>> 
>> But (in the spirit of brainstorming, and agreeing with your present proposal), I'll suggest a separate idea to think about.  Use a HOFP API to give easy access to the entire `MT` API all in one go, by reifying the `MH.type` property as a temporary (lambda argument):
>> 
>> MethodHandle mapType(MethodHandle mh, UnaryOperator<MethodType> fn) {
>>    return mh.asType(fn.apply(mh.type()));
>> }
>> 
>> This also suggests a possible argument transform utility, a sort of mini-version of Charlie Nutter's MH builder API:
>> 
>> MethodHandle mapArguments(MethodHandle mh, UnaryOperator<List<ArgumentValue<?>>> fn) {
>>   var args = mh.type().argumentList().stream().map(t -> makeArg(t)).asList();
>>   args = fn.apply(args);
>>   … do stuff to re-weave mh with the resulting argument transforms …
>> }
>> public sealed interface ArgumentValue<T> {
>>   Class<T> type();
>>   <U> ArgumentValue<U> asType(Class<U> newType);
>>   … other stuff for pre-applying MHs … 
>> }
>
> instead of mapArguments(), i believe it's better to have a method map on a MethodType that takes two mapping functions, one for the return type and one for the prameter types.
> 
> so one can write:
>   MHs.mapType(mh, mt -> mt.map(Function.identity(), t-> t.isPrimitive? t: Object.class))
> to not change the return type and erase the parameter type to Object if they are not primitives
> 
> With that, changing the return type to void can be done that way too
>   MHs.mapType(mh, mt -> mt.map(__ -> void.class, Function.identity())

It's an interesting idea. It turns something like:

target = target.asType(target.type().changeReturnType(void.class));

Into 

target = target.asType(mt -> mt.changeReturnType(void.class));

I.e. it removes the need to reference the target handle/type twice, which prevents them from potentially going out of sync (which seems more likely when you have multiple MethodTypes and MethodHandles flying around). Also, it feels unnecessary that, I have to re-specify `target`'s type as a base, when I'm asking `target` to change it's type. This also solves that.

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PR: https://git.openjdk.java.net/jdk/pull/866