No 'fold left' without a combiner ?

[Remi Forax]

On 09/09/2013 03:48 PM, Paul Sandoz wrote:
> On Sep 8, 2013, at 6:18 PM, Remi Forax <forax at univ-mlv.fr> 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 = advices.stream().reduce(impl, (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 = advices.stream().reduce(impl,
>>       (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.

Rémi