Question

I am trying to zip multiple sequences to form a long tuple:

val ints = List(1,2,3)
val chars = List('a', 'b', 'c')
val strings = List("Alpha", "Beta", "Gamma")
val bools = List(true, false, false)

ints zip chars zip strings zip bools

What I get:

List[(((Int, Char), String), Boolean)] =
  List((((1,a),Alpha),true), (((2,b),Beta),false), (((3,c),Gamma),false))

However I would like to get a sequence of flat tuples:

List[(Int, Char, String, Boolean)] = 
  List((1,a,Alpha,true), (2,b,Beta,false), (3,c,Gamma,false))

I now I can do:

List(ints, chars, strings, bools).transpose

But it returns weakly typed List[List[Any]]. Also I can do (ints, chars, strings).zipped, but zipped works only on 2-tuples and 3-tuples.

Is there a way to zip (arbitrary) number of equal-length sequences easily?

Answer 1

Here’s one way to solve your example, but this is not for an arbitrary number of sequences.

val ints = List(1,2,3)
val chars = List('a', 'b', 'c')
val strings = List("Alpha", "Beta", "Gamma")
val bools = List(true, false, false)

val input = ints zip chars zip strings zip bools

// Flattens a tuple ((A,B),C) into (A,B,C)
def f2[A,B,C](t: ((A,B),C)) = (t._1._1, t._1._2, t._2)

// Flattens a tuple ((A,B),C,D) into (A,B,C,D)
def f3[A,B,C,D](t: ((A,B),C,D)) = (t._1._1, t._1._2, t._2, t._3)

input map f2 map f3

I don’t think it is possible to do it generically for tuples of arbitrary length, at least not with this kind of solution. Tuples are strongly-typed, and the type system doesn’t allow you to specify a variable number of type parameters, as far as I know, which makes it impossible to make a generalized version of f2 and f3 that takes a tuple of arbitrary length ((A,B),C,D,...) (that would return a tuple (A,B,C,D,...)).

If there were a way to specify a variable number of type parameters, we wouldn’t need traits Tuple1, Tuple2, … Tuple22 in Scala’s standard library.