Question

I’m trying to use an OR pattern, as described here:

let foo = function
    | Some (0, x) when x > 0 | None -> "bar"
    | _ -> "baz"

However, this gives a compiler error:

error FS0010: Unexpected symbol ‘|’ in pattern matching. Expected ‘->’
or other token.

What am I doing wrong? Does it have to do with the when guard?

Answer 1

A when guard refers to a single case, regardless of how many patterns are combined. The cases need to be separated:

let foo = function
  | Some (0, x) when x > 0 -> "bar"
  | None -> "bar"
  | _ -> "baz"

For that reason, it may be better to factor out the return value, so a possibly complex expression isn’t repeated:

let foo value =
  let ret = "bar"
  match value with
  | Some (0, x) when x > 0 -> ret
  | None -> ret
  | _ -> "baz"

Using an active pattern is another way to avoid such repetition:

let (|Bar|_|) = function
  | Some(0, x) when x > 0 -> Some()
  | None -> Some()
  | _ -> None

let foo = function
  | Bar -> "bar"
  | _ -> "baz"