Where has the map method on Optional moved?

[Jed Wesley-Smith]

FlatMapper is a bizarre Frankensteinian beast, relying as it does on
mutation to do all of its things. It would be a service to all to
rename it to something – anything – else, as it does not resemble the
notion of flatMap/bind that other functional programming languages or
libraries implement.

As to consistency across the interfaces, Java doesn't have the ability
to generalise across Optional and Stream, lacking Higher Kinded Types.
The fact that Stream has named some feature that resembles a monadic
bind flatMap ideally should not then preclude the ability for Optional
to also have this feature.

As to additional potential functionality, you are quite correct, the
restriction of inheritance/OO to be the sole mechanism for adorning
functionality on a data-structure means that the temptation exists to
implement it directly, and these (filter/fold/reduce/foreach etc) are
all sensible suggestions. I do not profess to have definitive answers
to these library design decisions, given the constraints that the
designs are under. I am nevertheless obligated to point out that these
things _are_ monads, or that they form applicative functors – these
are just simple properties that arise from the data-structure.

My personal preference would be to attempt to give Optional as much of
the functionality it can support – that has been what we've tried to
do with our version*. We have certainly found that supporting monad,
functor and applicative syntax has been useful – and that things like
fold are general enough to allow you to build everything else
regardless.

cheers,
jed.

* https://bitbucket.org/atlassian/fugue/src/master/src/main/java/com/atlassian/fugue/Option.java

On 26 February 2013 21:37, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> Hi Jed,
>
> An Optional.flatMap is somewhat different to Stream.flatMap(FlatMapper ). Should we also add filter, forEach and reduce methods to have some parity to that of Stream? that is what i am getting at when i used the expression "shoe-horn".
>
> Stream.flatMap has been through a number of renames/redesigns and has very recently somewhat circled back on itself *and* we have added the following convenience method to Stream<T>:
>
>     default <R> Stream<R> flatMap(Function<T, Stream<? extends R>> mapper) {
>         return flatMap((T t, Consumer<R> sink) -> mapper.apply(t).sequential().forEach(sink));
>     }
>
> Since we are using the "flatMap" name it was felt important to have one method follow the pattern expected of it.
>
> Hmm... maybe under such circumstances it is OK to add an Optional.flatMap ? Or alternatively choose a completely different name to disassociate from Stream e.g. like Guava's Optional.transform.
>
> Paul.
>
> On Feb 26, 2013, at 6:12 AM, Jed Wesley-Smith <jed at wesleysmith.io> wrote:
>
>> Hi Paul,
>>
>> You don't get a choice, it is a (or forms a) monad, you just removed
>> the useful methods (map/flatMap aka fmap/bind). This leaves clients to
>> implement them (or the functionality) in an ad-hoc and possibly buggy
>> form themselves.
>>
>> It is a monad if there exists some pair of functions:
>>
>> A -> Option<A>
>> Option<A> -> (A -> Option<B>) -> Option<B>
>>
>> The first is Optional.of, the second is currently:
>>
>> Optional<A> a = …
>> Optional<B> b = …
>> Function<A, Optional<B> f = …
>> if (a.isPresent) {
>>  b = f.apply(a.get());
>> } else {
>>  b = Optional.empty();
>> }
>>
>> rather than:
>>
>> Optional<A> a = …
>> Function<A, Optional<B> f = …
>> final Optional<B> b = a.flatMap(f);
>>
>> cheers,
>> jed.
>>
>> On 26 February 2013 00:12, Paul Sandoz <paul.sandoz at oracle.com> wrote:
>>> Hi Dhananjay,
>>>
>>> It is not missing it was removed.
>>>
>>> java.util.Optional has a narrower scope that optional things in other languages. We are not trying to shoe-horn in an option monad.
>>>
>>> Paul.
>>>
>>> On Feb 23, 2013, at 12:27 AM, Dhananjay Nene <dhananjay.nene at gmail.com> wrote:
>>>
>>>> It seemed to be there on the Optional class in b61 but is missing now. Is
>>>> there some way to run map/flatMap operations on an Optional?
>>>>
>>>> Thanks
>>>> Dhananjay
>>>>
>>>
>>>
>
>