Using SRTP

Jun 20, 2023

F#7 introduced a friendlier syntax for using SRTP. The constraints can now be specified in the function signature rather in the body.

  type Foo =
      { Bar: string
        Baz: string -> string }

  // before F# 7.0
  let inline runv1 foo : string * string =
      (^a: (member Bar: string) foo), (^a: (member Baz: (string -> string)) foo) "helloworld"

  // after F# 7.0
  let inline runv2<'T when 'T : (member Bar: string)
      and  'T : (member Baz: (string -> string))> (foo : 'T) : string * string =
      foo.Bar, foo.Baz "helloworld"

  [<EntryPoint>]
  let main _ =
      let foo = { Bar = "helloworld", Baz = id }
      runv1 foo |> printfn "%A"
      runv2 foo |> printfn "%A"
      0