Active Patterns. with Pattern matching

ap-pattern.fs

let (|IsOneOf|_|) x y = if x = y then Some() else None

type SomeRecord =
    { ExternalEnum: int
      SomeStringData: string }

let toProcess =
    [ { ExternalEnum = 1
        SomeStringData = "I contain xml" }
      { ExternalEnum = 2
        SomeStringData = "I contain json" }
      { ExternalEnum = 3
        SomeStringData = "I contain csv" }
      { ExternalEnum = 4
        SomeStringData = "I also contain json" } ]

let processJsonFiles (record: SomeRecord) =
    match record with
    | { ExternalEnum = IsOneOf 2 | IsOneOf 4
        SomeStringData = _ } -> Some "parsed json"
    | _ -> None

toProcess
|> List.map processJsonFiles
|> List.iter (printfn "%A")

Credit to @TheAngryByrd. Saving as a gist so I do not forget about this