No 'fold left' without a combiner ?

Paul Sandoz paul.sandoz at
Mon Sep 9 06:48:00 PDT 2013

On Sep 8, 2013, at 6:18 PM, Remi Forax <forax at> wrote:

> Did I say that I will send you an email to the EG lists each time I will be annoyed by visibility of lambda parameter rule ?
> ArrayList<Advice> advices = ...
> AnnotatedElement annotatedElement = ...
> MethodHandle mh =, (mh, advice) -> advice.chain(annotatedElement, mh), null);
> In fact, there is another issue, (the real reason of this email), this code throws a NPE at runtime because I use null as combiner.
> It seems there is no way to express a left fold on values (different from T) when the values can not be combined.
> I've fixed the issue by writing,
>  MethodHandle target =,
>      (mh, advice) -> advice.chain(annotatedElement, mh),
>      (_1, _2) -> { throw new AssertionError(); } );
> but it's clearly ugly.

Yes, and it will barf for parallel streams.

You might be better off using an explicit forEach or writing a static foldLeft utility method that internally calls sequential().


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