Lightweight interfaces instead of function types

[Alex Buckley]

Stephen Colebourne wrote:
> On 19 February 2010 17:47, Neal Gafter <neal at gafter.com> wrote:
>> Tmp.java:5: [error] generic array creation
>>        Callable<String>[] array = new Callable<String>[10];
>> This is a hard error, not a warning.
> 
> True. But if I want a Callable<String>[] I write:
> 
>       Callable<String>[] array = new Callable[10];
> 
> This works, and is safe /in practice/ (rather than type-system safe).

No, it isn't safe in practice. It causes heap pollution which can cause 
programs to crash. It is a hack allowed because migration compatibility 
was so very, very important. (It is not particularly helpful of IDE 
vendors to hide the unchecked warning by default.) Reified generics 
don't help either; they just make the topic 5x more complicated.

> Again, its a trade-off. The benefit for ParallelArray is that
> Predicate or Transformer is IMO more meaningful to someone just
> casually coming to the API than a random function type.

I actually agree with this. Long nominal type names are the Java way. 
Unfortunately, we don't have reified generics over primitive type 
arguments (Predicate<int>, Ops.Reducer<int,int>). This is because we 
don't know how to implement them efficiently, even if strides have been 
made over the past decade w.r.t. implementing reified generics over 
reference type arguments.

Reifying function types as MethodHandles still has no specialization for 
primitive types, so won't give Doug what he needs. Long story short: 
first-class functions in the Java language may be doomed without fully 
reified structural types in the Java VM. (Other languages can avoid 
primitive types or box them to their hearts' content, but again, that 
doesn't help Java programmer or Doug's library.)

Hence my "peering over the edge of a very tall cliff" comment last week.

Alex