get the same label from different types of records

output.txt

Mechanical Pencil's diametric rating: 1.500000
Black Pen's diametric rating: 2.000000

pen.fs

type Colors = Black | Red | Blue

type Pen = {
  Diameter: float
  Rating: int
  InkColor: Colors
}

type Pencil = {
  Diameter: float
  Hardness: int
  Blackness: int
  Rating: int
}

let inline getDiametricRating (
                t:^q when 
                  ^q : (member Diameter:float) and 
                  ^q : (member Rating:int)
  ) = 
  let diameter = (^q : (member Diameter:float) t)
  let rating = float(^q : (member Rating:int) t)
  diameter * rating


let mechanical_pencil = {
  Diameter = 0.5
  Hardness = 1
  Blackness = 2
  Rating = 3
}

let black_pen = {
  Diameter = 1.0
  Rating = 2
  InkColor = Red
}

printfn "Mechanical Pencil's diametric rating: %f" (getDiametricRating mechanical_pencil)

printfn "Black Pen's diametric rating: %f" (getDiametricRating black_pen)