F# - Pattern matching discriminated union and accessing object's properties

Question

from F# tour i have this example

type Person = {
    First : string
    Last  : string
 }

/// A Discriminated Union of 3 different kinds of employees
type Employee =
    | Engineer of engineer: Person
    | Manager of manager: Person * reports: List<Employee>
    | Executive of executive: Person * reports: List<Employee> * assistant: Employee

let rec findDaveWithOpenPosition(emps: List<Employee>) = 
    emps
    |> List.filter(function 
                    | Manager({First =  "Dave"}, []) -> true
                    | Executive({First = "Dave"}, [], _) -> true
                    | _ -> false
                    )

However I would like to get access to object after matching object, something like this:

let rec findDaveWithOpenPos2(emps: List<Employee>) =
    List.filter (fun (e:Employee) ->
                    match e with
                        | Manager({First = "Dave"}, []) -> e.Last.Contains("X") //Does not compile
                        | Executive({First = "Dave"}, [], _) -> true
                        | _ -> false
                    ) emps

So i would like to have statically typed "e" as Person or Employee or Manager variable on right hand side with access to it's properties. Is it possible? Is there a better construction?

Lee Lee · Accepted Answer · 2019-01-11T15:37:27

You can name the Person instance within the Manager case using as:

let rec findDaveWithOpenPos2(emps: Employee list) =
    List.filter (fun (e:Employee) ->
                    match e with
                        | Manager({First = "Dave"} as p, []) -> p.Last.Contains("X")
                        | Executive({First = "Dave"}, [], _) -> true
                        | _ -> false
                    ) emps

F# - Pattern matching discriminated union and accessing object's properties

1 Answers