Is it possible to enforce that a Record respects some invariants?

Question

Suppose I wanted to create a Record type that represents acceptable min/max bounds:

type Bounds = { Min: float; Max: float }

Is there a way to enforce that Min < Max? It is easy to write a validateBounds function, I was just wondering if there was a better way to do this.

Edit: I realized that for this specific example I could probably get away with exposing two properties and re-order the arguments, so let's say we were trying to do

type Person = { Name: string }

and Name needs to have at least one character.

Answer 1

Here's another solution based on protection levels:

module MyModule =
    type Bounds = private { _min: float; _max: float } with
        // define accessors, a bit overhead
        member public this.Min = this._min
        member public this.Max = this._max
        static member public Make(min, max) =
            if min > max then raise (ArgumentException("bad values"))
            {_min=min; _max=max}

    // The following line compiles fine,
    // e.g. within your module you can do "unsafe" initialization
    let myBadBounds = {_min=10.0; _max=5.0}

open MyModule
let b1 = Bounds.Make(10.0, 20.0) // compiles fine
let b1Min = b1.Min
let b2 = Bounds.Make(10.0, 5.0) // throws an exception
// The following line does not compile: the union cases of the type 'Bounds'
// are not accessible from this code location
let b3 = {_min=10.0; _max=20.0}
// The following line takes the "bad" value from the module
let b4 = MyModule.myBadBounds

Answer 2

I think your best bet is a static member:

type Bounds = { Min: float; Max: float }
    with
        static member Create(min: float, max:float) =
            if min >= max then
                invalidArg "min" "min must be less than max"

            {Min=min; Max=max}

and use it like

> Bounds.Create(3.1, 2.1);;
System.ArgumentException: min must be less than max
Parameter name: min
   at FSI_0003.Bounds.Create(Double min, Double max) in C:\Users\Stephen\Documents\Visual Studio 2010\Projects\FsOverflow\FsOverflow\Script2.fsx:line 5
   at <StartupCode$FSI_0005>.$FSI_0005.main@()
Stopped due to error
> Bounds.Create(1.1, 2.1);;
val it : Bounds = {Min = 1.1;
                   Max = 2.1;}

However, as you point out, the big down-side of this approach is that there is nothing preventing the construction of an "invalid" record directly. If this is a major concern, consider using a class type for guaranteeing your invariants:

type Bounds(min:float, max:float) = 
    do
        if min >= max then
            invalidArg "min" "min must be less than max"

    with
        member __.Min = min
        member __.Max = max

together with an active pattern for convenience similar to what you get with records (specifically with regard to pattern matching):

let (|Bounds|) (x:Bounds) =
    (x.Min, x.Max)

all together:

> let bounds = Bounds(2.3, 1.3);;
System.ArgumentException: min must be less than max
Parameter name: min
   at FSI_0002.Bounds..ctor(Double min, Double max) in C:\Users\Stephen\Documents\Visual Studio 2010\Projects\FsOverflow\FsOverflow\Script2.fsx:line 4
   at <StartupCode$FSI_0003>.$FSI_0003.main@()
Stopped due to error
> let bounds = Bounds(1.3, 2.3);;

val bounds : Bounds

> let isMatch = match bounds with Bounds(1.3, 2.3) -> "yes!" | _ -> "no";;

val isMatch : string = "yes!"

> let isMatch = match bounds with Bounds(0.3, 2.3) -> "yes!" | _ -> "no";;

val isMatch : string = "no"