anonymous method call

[Remi Forax]

On 06/02/2013 12:11 AM, Marc Petit-Huguenin wrote:
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> On 06/01/2013 12:32 PM, Remi Forax wrote:
>> On 06/01/2013 08:10 PM, Marc Petit-Huguenin wrote: Thanks Brian and Rémi
>> for the response to my previous question.
>>
>> Now that we established my lack of knowledge about the lambda type
>> inference, I have another question, but please note that it is not a
>> suggestion for improvement, merely a question about the limits of what
>> would be possible.
>>
>> A functional interface has only one abstract method, so is there any reason
>> it would not be possible to use an anonymous call to this method?
>>
>> For example:
>>
>> IntUnaryOperator operator = f -> f + 1;
>>
>> and instead of
>>
>> int i = operator.applyAsInt(1);
>>
>> being able to do
>>
>> int i = operator(1);
>>
>>
>> Thanks.
>>
>>> It's possible to implement that but it exhibits a nasty property given
>>> the way Java scope works.
>>> In Java, the scope that contains local variables and the one that
>>> contains method calls are separated, when the compiler see
>>> int i = operator(1);
>>> it knowns that this is a method call, and it will not check if there is a
>>> local variable named 'operator'.
>>> So to support the new syntax and still be able to compile old code, the
>>> compiler has to first check if a method is available and then check if a
>>> local variable exist if no method was found.
>>> This rule is awful because it means that if you add a method in a class
>>> (or worst in a super class) you may broke an already existing code
>>> because you introduce a method that will be used instead of calling the
>>> lambda.
>>> so this syntax was rejected by the Expert Group :)
> It makes sense, thanks.
>
> FYI, I thought of this when trying to adapt the code in
> http://www.righto.com/2009/03/y-combinator-in-arc-and-java.html.  The result
> is far simpler than the original code, but I was wondering if there was a way
> to make it even better:
>
> interface HigherOrder<T> extends Function<HigherOrder<T>, T> {}
>
> Function<UnaryOperator<IntUnaryOperator>, IntUnaryOperator> y = r ->
> ((HigherOrder<IntUnaryOperator>)(f -> f.apply(f))).apply(f -> r.apply(x ->
> f.apply(f).applyAsInt(x)));
> IntUnaryOperator factorial = y.apply(f -> n -> n == 0 ? 1 : n * f.applyAsInt(n
> - - 1));
> assert factorial.applyAsInt(10) == 3628800;
> IntUnaryOperator fibonacci = y.apply(f -> n -> n == 0 ? 0 : n == 1 ? 1 :
> f.applyAsInt(n - 1) + f.applyAsInt(n - 2));
> assert fibonacci.applyAsInt(30) == 832040;

Apart from adding a local variable for f -> f.apply(f) to avoid the cast
and make the code a little more readable, I don't think so but maybe I'm 
wrong.

     Function<UnaryOperator<IntUnaryOperator>, IntUnaryOperator> y = r -> {
       HigherOrder<IntUnaryOperator> hop = f -> f.apply(f);
       return hop.apply(f -> r.apply(x -> f.apply(f).applyAsInt(x)));
     };

     IntUnaryOperator factorial =
         y.apply(f -> n -> n == 0 ? 1 : n * f.applyAsInt(n - 1));
     System.out.println(factorial.applyAsInt(10)); // 3628800;

     IntUnaryOperator fibonacci =
         y.apply(f -> n -> n == 0 ? 0 : n == 1 ? 1 : f.applyAsInt(n - 1) 
+ f.applyAsInt(n - 2));
     System.out.println(fibonacci.applyAsInt(30)); // 832040;

cheers,
Rémi

>
> - -- 
> Marc Petit-Huguenin
> Email: marc at petit-huguenin.org
> Blog: http://blog.marc.petit-huguenin.org
> Profile: http://www.linkedin.com/in/petithug
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