Question

Is it possible to express the following code in such a way that the map and null skipping is expressed in one call?

list.map(_.accept(this, arg).asInstanceOf[T]).filter(_ != null)

Answer 1

list flatMap { i => Option(i.accept(this, arg).asInstanceOf[T]) }

or alternatively, if you like, (though this will be converted more or less to your original expression)

for {
  item <- list
  itemConverted = item.accept(this, arg).asInstanceOf[T]
  itemNonNull = itemConverted if itemConverted != 0
} yield itemNonNull

Using collect would be possible but it would likely call accept twice on most arguments because of the isDefinedAt test of the partial function:

list collect {
  case i if i.accept(this, arg).asInstanceOf[T] != null => i.accept(this, arg).asInstanceOf[T]
}

One would need to use some memoising (or smart extractors) to avoid this.