I'm trying to make a function in OCaml which does the summation function in math. I tried this:

sum n m f =
    if n = 0 then 0
    else if n > m then f
    else f + sum (n + 1) m f;;

However, I get an error - "Characters 41-44: else f * sum(n + 1) m f;; Error: Unbound value sum and sum is underlined (has carrot signs pointing to it)

I looked at this: Simple OCaml exercise It's the same question, but I see a lot of other things that I do not have. For example, for my n = m case, I do not have f n and then in the else case, I do not have f m.

Why do you need f n if you want the function to return an integer? D: What's the problem!? Thanks in advance.