gen.ml

(*
 https://blog.lighty.ch/2020/07/20/fpe-05-generalized-generator/
*)

type 'a gen = Gen of 'a * (unit -> 'a gen)

let rec counter i : int gen = Gen (i, fun () -> counter (i + 1))

let take n gen =
  let rec take_iter n (Gen (v, f)) acc =
    if n = 0 then acc else take_iter (n - 1) (f ()) (acc @ [ v ])
  in
  take_iter n gen []

let _ = take 10 (counter 1)

let iterate fn =
  let rec gen from = Gen (from, fun () -> gen (fn from)) in
  gen

let inc x = x + 1
let _ = take 10 (iterate inc 0)

let generate gn fn =
  let rec gen from = Gen (gn from, fun () -> gen (fn from)) in
  gen

let fact x =
  let rec fact_iter x acc =
    if x = 0 then acc else fact_iter (x - 1) (x * acc)
  in
  fact_iter x 1

let _ = take 10 (generate fact inc 0)