Stream generators

[Remi Forax]

On 11/30/2012 06:51 PM, Brian Goetz wrote:
>> I think it would be beneficial for comparison to show a bit of their
>> implementations.
>
> Here's iterate(seed, UnaryOperator):
>
>     public static<T> Stream<T> iterate(final T seed, final 
> UnaryOperator<T> f) {
>         Objects.requireNonNull(f);
>         final InfiniteIterator<T> iterator = new InfiniteIterator<T>() {
>             T t = null;
>
>             @Override
>             public T next() {
>                 return t = (t == null) ? seed : f.operate(t);
>             }
>         };
>         return stream(new StreamSource.ForIterator<>(iterator), 
> StreamOpFlag.IS_ORDERED);
>     }
>
> Not too difficult.  But, the idea is to make things that are easy in 
> the header of a for-loop to be easy as the source of a stream.

this doesn't work if seed is null.

If InfiniteIterator is defined like this:
public interface InfiniteIterator<T> extends Iterator<T> {
     public default boolean hasNext() {
       return true;
     }
   }

then the code can be written like this:
   @SuppressWarnings("unchecked")
   public <T> Iterator<T> iterate(T seed, UnaryOperator<T> op) {
     Objects.requireNonNull(f);
     Object[] array = new Object[] { seed };
     InfiniteIterator<T> iterator = () -> {
       Object value = array[0];
       array[0] = op.operate(value);
       return (T)value;
     };
     return stream(new StreamSource.ForIterator<>(iterator), 
StreamOpFlag.IS_ORDERED);
   }


>
>> repeat(n) in Scheme is about 10 characters.
>
> Yeah, well this is Java...

and the way iterators are defined is different. In Java there is no 
allocation if f does no allocation,
this is not the case in Scheme.

Rémi