How to do argument validation of F# records

Question

F# makes it easy to define types such as

type coords = { X : float; Y : float }

but how do I define constraints/check arguments for the con

Answer 1

You can make the implementation private. You still get structural equality but you lose direct field access and pattern matching. You can restore that ability using active patterns.

//file1.fs

type Coords = 
  private { 
    X: float
    Y: float 
  }

[]
module Coords =
  ///The ONLY way to create Coords
  let create x y =
    check x
    check y
    {X=x; Y=y}

  let (|Coords|) {X=x; Y=y} = (x, y)

//file2.fs

open Coords
let coords = create 1.0 1.0
let (Coords(x, y)) = coords
printfn "%f, %f" x y