Question

I have an Ocaml function that is giving me errors.

What I am trying to do:

Recursively create a List of random numbers (0-2) of size “limit”.

Here’s what I have:

let rec carDoorNumbers = fun limit -> 
match limit with
| [1] -> Random.int 3
| [] -> Random.int 3 :: carDoorNumbers (limit-1);;

I am getting this error:

Error: This expression has type 'a list
   but an expression was expected of type int

Answer 1

Think about what your function has to do: given a limit, you have to create a list of numbers. So your type is something like carDoorNumbers : int -> int list.

Looking at that, it seems you have two errors. First, you’re matching limit (which should be an int) against a list pattern. [1] -> ... matches a list containing only the element 1 and [] matches the empty list; you really want to match against the number 1 and any other number n.

The second error is that you return two different types in your match statement. Remember that you are supposed to be returning a list. In the first case, you are returning Random.int 3, which is an int rather than an int list. What you really want to return here is something like [Random.int 3].

The error you got is a little confusing. Since the first thing you returned was an int, it expects your second thing to also be an int. However, your second case was actually correct: you do return an int list! However, the compiler does not know what you meant, so its error is backwards; rather than changing the int list to an int, you need to change the int to an int list.