Question

I have a function that generates Fibonacci numbers:

let rec fib n =
    match n with
    | (0 | 1) -> 1
    | x when x > 0 -> (fib (x-2) + fib (x-1))
    | _ -> raise (Invalid_argument "Negative value supplied to fib");;

but what I really want is for it to return a list of said numbers. I tried this:

let rec fib n list =
    match n with
    | (0 | 1) -> 1 :: []
    | x when x > 0 -> (fib (x-2) list + fib (x-1) list) :: list
    | _ -> raise (Invalid_argument "Negative value supplied to fib");;

But ocamlc says

File “main.ml”, line 2, characters 4-174:
Error: This expression has type int list
but an expression was expected of type int

(Line 2 characters 4-174 corresponds to the match block). I want this to return type “int list”, why is it inferring type int?

Answer 1

The expression fib (x - 2) list + fib (x - 1) list requires fib to return int, because (+) takes int as parameters.