regular_vs_active_pattern.fs

type A =
  | A of int
  | B of string
  
> List.choose (|A|_|) [ A 3; B "foo"];;

  List.choose (|A|_|) [ A 3; B "foo"];;
  -------------^^^^^

/vagrant/stdin(6,14): error FS0039: The value or constructor '|A|_|' is not defined  

let (|IsA|_|) x = match x with | A i -> Some i | _ -> None;;

> List.choose (|IsA|_|) [ A 3; B "foo"];;                     
val it : int list = [3]