Really, this is a lot of fuss over nothing.
There is actually no fundamental difference between Scala's Option, Guava's
Optional, Fugue's Option, Java's Optional and Haskell's Maybe – they are
modelling the same thing, the possibility of a value not being present.
The fact that there may be minor differences in api or semantics around whether
null is a legal value are minor in the scheme of things (and yes, null is a
pretty stupid legal value of a Some IMHO).
Stephen's example is ludicrous, why have a list of optional values? You'd
flatten down into just a list – and an optional list only makes sense if the
enclosed list is guaranteed to be non-empty, otherwise you just return an empty
list!
If we are going to use potential straw-men as arguments we can stall all
progress. Please concentrate on the important matters, let's disavow null as a
valid value and save us all a billion dollars.
cheers,
jed.
On 06/03/2013, at 8:47 PM, Remi Forax <fo...@univ-mlv.fr> wrote:
> Ok, let be nuclear on this,
> There is no good reason to introduce Optional<T> in java.util.
>
> It doen't work like Google's Guava Optional despite having the same
> name, it doesn't work like Scala's Option despite having a similar name,
> moreover the lambda pipeline face a similar issue with the design of
> collectors (see stream.collect()) but solve that similar problem with a
> different design, so the design of Optional is not even consistent with
> the rest of the stream API.
>
> So why do we want something like Optional, we want it to be able to
> represent the fact that as Mike states a returning result can have no
> value by example Colections.emptyList().stream().findFirst() should
> 'return' no value.
>
> As Stephen Colebourne said, Optional is a bad name because Scala uses
> Option [1] which can used in the same context, as result of a filter/map
> etc. but Option in Scala is a way to mask null. Given the name
> proximity, people will start to use Optional like Option in Scala and we
> will see methods returning things like Optional<List<Optional<String>>>.
>
> Google's Guava, which is a popular library, defines a class named
> Optional, but allow to store null unlike the current proposed
> implementation, this will generate a lot of confusions and frustrations.
>
> In fact, we don't need Optional at all, because we don't need to return
> a value that can represent a value or no value,
> the idea is that methods like findFirst should take a lambda as
> parameter letting the user to decide what value should be returned by
> findFirst if there is a value and if there is no value.
> So instead of
> stream.findFirst().orElse(null)
> you will write
> stream.findFirst(orNull)
> with orNull() defined as like that
> public static <T> Optionalizer orNull() {
> return (isPresent, element) -> isPresent? element: null;
> }
>
> The whole design is explained here [2] and is similar to the way
> Collectors are defined [3],
> it's basically the lambda way of thinking, instead of creating an object
> representing the different states resulting of a call to findFirst,
> findFirst takes a lambda as parameter which is fed with the states of a
> call.
>
> cheers,
> Rémi
>
> [1] http://www.scala-lang.org/api/current/index.html#scala.Option
> [2]
> http://mail.openjdk.java.net/pipermail/lambda-libs-spec-observers/2013-February/001470.html
> [3]
> http://hg.openjdk.java.net/lambda/lambda/jdk/file/tip/src/share/classes/java/util/stream/Collectors.java
>
>
> On 03/04/2013 09:29 PM, Mike Duigou wrote:
>> Hello All;
>>
>> This patch introduces Optional container objects to be used by the lambda
>> streams libraries for returning results.
>>
>> The reference Optional type, as defined, intentionally does not allow null
>> values. null may be used with the Optional.orElse() method.
>>
>> All of the Optional types define hashCode() and equals implementations. Use
>> of Optional types in collections should be generally discouraged but having
>> useful equals() and hashCode() is ever so convenient.
>>
>> http://cr.openjdk.java.net/~mduigou/JDK-8001642/0/webrev/
>>
>> Mike
>>
>
>
