F# Smart Constructors for Union Type.

Neulich über Hakell Smart Constructors gelesen.
Das funktioniert teilweise auch mit F# Union Type.

Die Signaturdatei mit verdeckten Resistor Konstruktoren.

//Resistor.fsi 
namespace SmartConstructors
module Resistor =
    type Bands = int

    // Union Type with hiding constructors
    type Resistor = private Metal of Bands | Ceramic of Bands
    //smart constructor.
    val metalResistor : Bands -> Resistor
    val (|Metal|Ceramic|) : Resistor -> Choice<Bands,Bands> 

Implementierung.

//Resistor.fs
namespace SmartConstructors
open System
module Resistor  =
    type Bands = int
    
    type  Resistor = Metal of Bands | Ceramic of Bands with
        override x.ToString()= 
            match x with
            | (Metal v) -> "Metal " + v.ToString() 
            | (Ceramic v) -> "Ceramic " + v.ToString()
    //smart constructor.
    let metalResistor (b:Bands) =
        assert ( b >= 4 && b <= 8)
        Metal b
    let (|Metal|Ceramic|) n =
        match n with
        | (Metal v) -> Metal v
        | (Ceramic v) -> Ceramic v

Aufrufen.

// other F# Console Project
// with reference to SmartConstructors.dll
// Program.fs
open SmartConstructors.Resistor

//don't compile
//let failResistor = Metal 5

//only way to build a metal resistor
let resistor6 = metalResistor 6

let getBands r = 
    match r with
    | Metal v -> v
    | Ceramic v -> v
printfn "%A" (resistor6.ToString())
printfn "%A" (getBands resistor6)

//Assertion failed
let resistor10 = metalResistor 10
printfn "%A" (resistor10.ToString())