OCaml - Implement nested functions

Question

I want to implement the nested functions which are recursive. How to do that? I am getting an error in the following code "an operator expected"

let powerset_cf f1 = if (let rec f lst = match lst with 
                                  [] -> true
                                  | h::t ->  if (f1 h) then (f t) 
                                             else false ) then true
                     else false;;

Here, lst = any list (e.g [1;2;3]) f1 any characteristic function which returns true if the element is in the set defined by characteristic function. (e.g. let f1 x = (x mod 3 == 0 && x<=15);; ) The powerset_cf function returns true if the lst is the member of the power set of the characteristic function.

Please help fixing this. Thanks in advance.

The internal let rec should be a let rec ... in ... and you are lacking the in ... part. BTW, be sure to indent properly (for readability reasons) your code. — Basile Starynkevitch

V. Michel V. Michel · Accepted Answer · 2016-08-29T17:58:53

let powerset_cf f1 lst =
  let rec f lst = 
    match lst with
      | []   -> true
      | h::t ->  if f1 h then f t else false
  in
  if f lst then true else false
;;

Of course we can simplify the code, but that's not the question asked.

OCaml - Implement nested functions

1 Answers