F#: downcast a discriminated union

Question

I have a discriminated union type:

type F =
| A of int
| B of float

Suppose I have a list of F that has been filtered to yield only objects of type A:

let listOfAs=list.filter (fun f -> match f with | A(f') -> true | _ -> false)

How can I work with the resulting list of F without requiring pattern matches everywhere in my code? The compiler doesn't like a direct cast, eg

list.map (fun f -> int f) listOfAs

Answer 1

You cannot really cast discriminated union value - the type of F is a different thing than the type int (it is not like C union where they have the same binary representation).

So, the easiest solution is to write a function that takes list<F> and returns list<int> containing only the int values that were wrapped in the A case.

To do this, you can use List.choose (instead of List.filter). This lets you specify a projection where you can return None (meaning skip the value) or Some v (meaning return value v as part of the resulting list):

let listOfAs = List.choose (fun f -> 
  match f with 
  | A(f') -> Some f'
  | _ -> None)