Suffering from Nothing

Question

How does the A turn to be Nothing in the process?

def seq2map[A](src: Seq[A]): Map[A, A] = {
    def pair = for {
        f <- src.headOption
        s <- src.headOption
    } yield (f, s)
    Stream continually pair takeWhile(_ isDefined) toMap
}

error: Expression of type Map[Nothing, Nothing] doesn't conform to expected type Map[A, A]

Thank you!

score 4 · Accepted Answer · answered Oct 12 '14 at 00:05

I get

<console>:12: error: Cannot prove that Option[(A, A)] <:< (T, U).
           Stream continually pair takeWhile(_ isDefined) toMap
                                                          ^

because

scala> val src = (1 to 10).toSeq
src: scala.collection.immutable.Range = Range(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

scala>     def pair = for {
     |         f <- src.headOption
     |         s <- src.headOption
     |     } yield (f, s)
pair: Option[(Int, Int)]

is not a pair, but an Option.

scala> (Stream continually pair takeWhile (_.isDefined)).flatten
res0: scala.collection.immutable.Stream[(Int, Int)] = Stream((1,1), ?)

is a stream of pairs.

Just waiting for the game to start.

Suffering from Nothing

1 Answers1