Question

My code (below) falls over with a stack overflow exception. Im assuming F# isnt like haskell and dosent play well with recursive lists. Whats the correct way of dealing with recursive lists like this in F# ? Should i pass it an int so it has a determined size?

let rec collatz num =
    match num with 
        |x when x % 2 = 0 ->num :: collatz (x/2)                                            
        |x ->               num :: collatz ((x * 3) + 1)

let smallList = collatz(4) |> Seq.take(4)

Answer 1

For an infinite list like this, you want to return a sequence. Sequences are lazy; lists are not.

let rec collatz num = 
  seq {
    yield num
    match num with 
    | x when x % 2 = 0 -> yield! collatz (x/2)                                            
    | x -> yield! collatz ((x * 3) + 1)
  }

let smallList = 
  collatz 4
  |> Seq.take 4
  |> Seq.toList //[4; 2; 1; 4]