Proposed API for JEP 259: Stack-Walking API

[forax at univ-mlv.fr]

----- Mail original -----
> De: "Mandy Chung" <mandy.chung at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "Paul Sandoz" <paul.sandoz at oracle.com>, core-libs-dev at openjdk.java.net
> Envoyé: Jeudi 5 Novembre 2015 22:48:45
> Objet: Re: Proposed API for JEP 259: Stack-Walking API
> 
> 
> > On Nov 4, 2015, at 5:00 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> > 
> >> 
> >> Good point. Damn, i don’t like wildcards :-)
> >> 
> >> The following works fine:
> >> 
> >>  static <T> Function<Stream<T>, Long> counter() {
> >>      return Stream::count;
> >>  }
> >> 
> >> But there could also cases where one is stuck using a wildcard:
> >> 
> >>  Function<Stream<?>, Long> f = Stream::count;
> > 
> > 
> > Wildcards are not that complex, but because they are use-side annotations,
> > it's really easy to forget them, and IDEs do not help here :(
> > 
> > I really hope that the JEP about declaration site variance [1] will be
> > implemented at the beginning of jdk 10, it will remove the need of
> > wildcards for functional interfaces.
> > 
> 
> Remi, Paul,
> 
> The current StackWalker::walk method:
>     <T> T walk(Function<Stream<StackFrame>, T> function)
> 
> This would mean that the StackWalker API is intended to work with a function
> taking Stream<StackFrame> but not Stream<?>.
> 
> Changing to
>     <T> T walk(Function<? super Stream<StackFrame>, ? extends T> function)
> 
> will allow use of some existing functions taking Stream<?>.  I don’t object
> changing it to use wildcard.
> 
> However, the StackWalker API is specific for reading StackFrame and I would
> expect it’s reasonable if it doesn’t work with a function taking Stream<?>
> as the useful functions such as Stream::count,
> Stream::collect(Collectors.toList(…)) can be used.
> 
> Just want to double confirm the advice which way to go.

yes, it is.

Stream<?> is perhaps not the best example, using a Function<? super Stream<StackFrame>, ? extends T> also allow calls that use a Stream<Object> (no wildcard here !),
like this:
  Function<Stream<Object>, Void> fun = stream -> { stream.forEach(System.out::println); return null; };
  stackWalker.walk(fun);

> 
> Mandy

regards,
Rémi