General reducees F# implementation, based on this http://blog.plataformatec.com.br/2015/05/introducing-reducees/

Reducees.fsx

module Reducees = 

  type Instruction = Cont | Done | Halt

  let rec reduce source instruction func =
    match source, instruction with
    | head::tail, (Cont,   acc) -> reduce tail (func head acc) func
    | [],         (Cont,   acc) -> Done, acc
    | _                         -> instruction

open Reducees

let map coll func =
  let result = reduce coll (Cont, []) (fun x acc -> Cont, ((func x)::acc))
  match result with Done, res -> List.rev res | _ ->  []

printf "%A" <| map [1;2;3;4] ((*)2)

let take coll n =
  let nextInstruction n = if n = 1 then Halt else Cont
  let result = reduce coll (Cont, ([], n)) (fun x (acc, count) -> (nextInstruction count), (x::acc, n-1))
  match result with _, (res, _) -> List.rev res

printf "%A" <| take [1;2;3;4;5] 2