dictionary - Use functional combinators on Scala Tuples?

Question

'map' preserves the number of elements, so using it on a Tuple seems sensible.

My attempts so far:

scala> (3,4).map(_*2)    
error: value map is not a member of (Int, Int)
       (3,4).map(_*2)
             ^
scala> (3,4).productIterator.map(_*2)
error: value * is not a member of Any
       (3,4).productIterator.map(_*2)
                                  ^
scala> (3,4).productIterator.map(_.asInstanceOf[Int]*2)
res4: Iterator[Int] = non-empty iterator

scala> (3,4).productIterator.map(_.asInstanceOf[Int]*2).toList
res5: List[Int] = List(6, 8)

It looks quite painful... And I haven't even begun to try to convert it back to a tuple.
Am I doing it wrong? Could the library be improved?

Answer 1

In general, the element types of a tuple aren't the same, so map doesn't make sense. You can define a function to handle the special case, though:

scala> def map[A, B](as: (A, A))(f: A => B) = 
     as match { case (a1, a2) => (f(a1), f(a2)) } 
map: [A,B](as: (A, A))(f: (A) => B)(B, B)

scala> val p = (1, 2)    
p: (Int, Int) = (1,2)

scala> map(p){ _ * 2 }
res1: (Int, Int) = (2,4)

You could use the Pimp My Library pattern to call this as p.map(_ * 2).

UPDATE

Even when the types of the elements are not the same, Tuple2[A, B] is a Bifunctor, which can be mapped with the bimap operation.

scala> import scalaz._
import scalaz._

scala> import Scalaz._
import Scalaz._

scala> val f = (_: Int) * 2
f: (Int) => Int = <function1>

scala> val g = (_: String) * 2
g: (String) => String = <function1>

scala> f <-: (1, "1") :-> g
res12: (Int, String) = (2,11)

UPDATE 2

http://gist.github.com/454818