No 'fold left' without a combiner ?

Remi Forax forax at
Mon Sep 9 08:16:12 PDT 2013

On 09/09/2013 03:48 PM, Paul Sandoz wrote:
> 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.

yes, here I control th stream creation so no big deal
> You might be better off using an explicit forEach or writing a static foldLeft utility method that internally calls sequential().

or better, have a foldLeft (or whatever it's name) on Stream that throws 
an IllegalStateException if the stream is parallel.

> Paul.


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