输出列表中的前 n 个元素

Question

How do you write a f# recursive function that accepts a positive integer n and a list xs as input, and returns a list containing only the first n elements in xs.

let rec something n xs =.. something 3 [1..10] = [1;2;3]

Answer 1

The short answer is: Don't, just use Seq.take .

A simple version would be something like:

let rec take n list = 
  match n with
  | 0 -> []
  | _ -> List.head list :: take (n - 1) (List.tail list)

A tail-recursive could look like:

let rec take n list =
  let rec innertake m innerlist acc = 
    match m with
    | 0 -> List.rev acc
    | _ -> innertake (m - 1) (List.tail innerlist) ((List.head innerlist) :: acc)
  innertake n list []

Note that neither of these does anything to handle the case that the input list is shorter than the requested number of items.