General 'map' function for Scala tuples?

问题

I would like to map the elements of a Scala tuple (or triple, ...) using a single function returning type R. The result should be a tuple (or triple, ...) with elements of type R.

OK, if the elements of the tuple are from the same type, the mapping is not a problem:

scala> implicit def t2mapper[A](t: (A,A)) = new { def map[R](f: A => R) = (f(t._1),f(t._2)) }
t2mapper: [A](t: (A, A))java.lang.Object{def map[R](f: (A) => R): (R, R)}

scala> (1,2) map (_ + 1)
res0: (Int, Int) = (2,3)

But is it also possible to make this solution generic, i.e. to map tuples that contain elements of different types in the same manner?

Example:

class Super(i: Int)
object Sub1 extends Super(1)
object Sub2 extends Super(2)

(Sub1, Sub2) map (_.i)

should return

(1,2): (Int, Int)

But I could not find a solution so that the mapping function determines the super type of Sub1 and Sub2. I tried to use type boundaries, but my idea failed:

scala> implicit def t2mapper[A,B](t: (A,B)) = new { def map[X >: A, X >: B, R](f: X => R) = (f(t._1),f(t._2)) }
<console>:8: error: X is already defined as type X
       implicit def t2mapper[A,B](t: (A,B)) = new { def map[X >: A, X >: B, R](f: X => R) = (f(t._1),f(t._2)) }
                                                                    ^
<console>:8: error: type mismatch;
 found   : A
 required: X
 Note: implicit method t2mapper is not applicable here because it comes after the application point and it lacks an explicit result type
       implicit def t2mapper[A,B](t: (A,B)) = new { def map[X >: A, X >: B, R](f: X => R) = (f(t._1),f(t._2)) }

Here X >: B seems to override X >: A. Does Scala not support type boundaries regarding multiple types? If yes, why not?

回答1:

I think this is what you're looking for:

implicit def t2mapper[X, A <: X, B <: X](t: (A,B)) = new {
  def map[R](f: X => R) = (f(t._1), f(t._2))
}

scala> (Sub1, Sub2) map (_.i)                             
res6: (Int, Int) = (1,2)

A more "functional" way to do this would be with 2 separate functions:

implicit def t2mapper[A, B](t: (A, B)) = new { 
  def map[R](f: A => R, g: B => R) = (f(t._1), g(t._2)) 
}       

scala> (1, "hello") map (_ + 1, _.length)                                         
res1: (Int, Int) = (2,5)

回答2:

I’m not a scala type genius but maybe this works:

implicit def t2mapper[X, A<:X, B<:X](t: (A,B)) = new { def map[A, B, R](f: X => R) = (f(t._1),f(t._2)) }

回答3:

The deeper question here is "why are you using a Tuple for this?"

Tuples are hetrogenous by design, and can contain an assortment of very different types. If you want a collection of related things, then you should be using ...drum roll... a collection!

A Set or Sequence will have no impact on performance, and would be a much better fit for this kind of work. After all, that's what they're designed for.

回答4:

For the case when the two functions to be applied are not the same

scala> Some((1, "hello")).map((((_: Int) + 1 -> (_: String).length)).tupled).get
res112: (Int, Int) = (2,5)

The main reason I have supplied this answer is it works for lists of tuples (just change Some to List and remove the get).

回答5:

This can easily be achieved using shapeless, although you'll have to define the mapping function first before doing the map:

object fun extends Poly1 {
  implicit def value[S <: Super] = at[S](_.i) 
}

(Sub1, Sub2) map fun // typed as (Int, Int), and indeed equal to (1, 2)

(I had to add a val in front of i in the definition of Super, this way: class Super(val i: Int), so that it can be accessed outside)

来源：https://stackoverflow.com/questions/4022436/general-map-function-for-scala-tuples