/**
* Get the sum of the stream.
*
* This is a closing operation (it calls <code>$this->close()</code> internally).
*
* @return Optional an optional result
* @see \monad\Stream::close()
*/
public function sum() : Optional
{
return $this->bindMonad(sequence(conditionally(negation(is_empty), array_sum), optional));
}
/**
* Apply function only if the value is not NULL.
*
* @see \monad\Monad::bind($function)
* @see \monad\Chain::emptyChain()
*/
public function continue(callable $function) : Chain
{
return parent::bind(conditionally(negation(is_null), $function));
}
/**
* Map a value of the monad if it is present.
*
* See <code>isPresent()</code>.
*
* @param callable $function the mapper
* @return Optional a new Optional if the value was present, or this otherwise
*/
public function ifPresent(callable $function) : Optional
{
return $this->bind(conditionally(negation(is_null), $function));
}