OCaml: Matching with any negative

Question

Is there a way to get pattern matching to match my value with any negative number? It does not matter what the negative number is I just need to match with any negative.

I have accomplished what I want with this simple code:
let y = if(n < 0) then 0 else n in
match y with
0 -> []
| _ -> [x] @ clone x (n - 1)

But I want to eliminate that if statement and just get it to check it as another case in the match statement

Answer 1

There is the keyword when. By head (I can't test right now)

let y = match n with | when n < 0 -> 0 | 0 -> [] | _ -> [x] @ clone x (n - 1)

However, even your example shouldn't work. As on one side you return an int, and on the other a list.

Answer 2

Yes, use a guard:

match n with
    _ when n < 0 -> []
  | _ -> [x] @ clone x (n - 1)

Answer 3

You can make your code a little cleaner like this:

match n < 0 with
| true -> []
| false -> [x] @ clone x (n - 1)

Even better would be:

if n < 0 then [] else [x] @ clone x (n - 1)

Generally, if statements are clearer than matches for simple logical tests.

While we're at it, we might as well use :: in place of @:

if n < 0 then [] else x :: clone x (n - 1)