Question

I want to write a function which accepts a list of sequence in which each sequence will be used in for comprehension expression.

eg.

for (x <- (1 to 10); y <- (1 to 10)) yield List(x,y)

In the above expression, I must know before hand that I want a combination of x and y. WHat if I want a combination of x, y, z …etc… (an unknown number of combination)? If I want 10 combinations, I want to paste 10 snippets “x <- (1 to 10)” into the expression. I think I can do this with macro in Clojure (a way to paste code). How can I do this in Scala?

The function I want to write has the signature like this:

combine(list: List[List[Int]])

The body of the function will use each item in the list to paste into the for comprehension.

Hope you understand my intention.

Answer 1

If you want to compute the cartesian product of a list of lists, you can do it by chaining map and flatMap calls together recursively. That is what for expressions do anyway.

  def prod(l:List[List[Int]]):List[List[Int]] = l match {
    case Nil => List(Nil)
    case l::ls => l.flatMap(i => prod(ls).map(t =>i::t))
  }