OCaml中的递归帮助

Question

I'm trying to make a recursive function with Ocaml, but I keep getting the same error code.

let rec get x =
if x > 7 then
get x-7;;

And I get the very useful error message of:

Error: This expression has type int but an expression was expected of type unit

I'm a complete beginner at OCaml, and studying it for a module at university. And this is one of my assignments, and I'm a bit stuck!

I originally wanted to do it by a while loop, (as I'm a predominantly imperative programmer), but I couldn't get that to work, so I thought I'd try recursive!

Thanks

Answer 1

There's two problems with this code. First, the spacing of x-7 indicates that you would like to pass x - 7 to get , but it will actually be parsed as (get x) - 7 . That's easily fixed with parentheses:

let rec get x =
  if x > 7 then get (x - 7)

The second problem is that you don't have a second arm for the if , so the function doesn't have much of a chance of returning anything. (One arm if is taken to be of type unit , only useful for effects.)

You probably want to return something if x is less than 7, maybe:

let rec get x =
  if x > 7 then get (x - 7) else x

Writing this with a while loop is possible, but you should understand that variables in OCaml are not mutable locations, only names. You'll have to introduce and manipulate mutable places explicitly:

let get x =
  let y = ref x in
  while !y > 7 do
    y := !y - 7;
  done;
  !y

Hope that helps.