Smart constructor

Purpose

🔗 Making illegal states unrepresentable, F♯ for fun and profit

• Design to prevent invalid states

  • Encapsulate state (all primitives) in an object

• Smart constructor guarantees a valid initial state

  • Validates input data

  • If Ko, returns "nothing" (Option) or an error (Result)

  • If Ok, returns the created object wrapped in an Option / a Result

Encapsulate the state in a type

→ Single-case (discriminated) union: Type X = private X of a: 'a... 🔗 Designing with types: Single case union types, F♯ for fun and profit

→ Record: Type X = private { a: 'a... } 🔗 You Really Wanna Put a Union There? You Sure?, by Paul Blasucci

☝ private keyword:

• Hide object content

• Fields and constructor no longer visible from outside

• Smart constructor defined in companion module or static method

Example #1

Smart constructor : → tryCreate function in companion module → Returns an Option

module Xxx.Types                            // 👈 Top-level module❗

type Latitude = private { Latitude: float } with
    member this.Value = this.Latitude       // 👈 Required because `.Latitude` is private too

[<RequireQualifiedAccess>]                  // 👈 Optional but recommended
module Latitude =
    let tryCreate (latitude: float) =
        if latitude >= -90. && latitude <= 90. then
            Some { Latitude = latitude }    // 👈 Constructor accessible here
        else
            None

Usages:

module Xxx.Usages

open Xxx.Types

let lat_ok = Latitude.tryCreate 45.  // Some { Latitude = 45.0 }
let lat_ko = Latitude.tryCreate 115. // None

Access control

private keyword has not exactly the same meaning in F♯ as in C♯!

• In F♯, private indicates that the entity can be accessed only from the enclosing type or module.

• In our example, private is applied on the Latitude definition that is on the Xxx.Types module.

• { Latitude = ... } and latitude.Latitude are accessible in Xxx.Types module as if there were no private keyword.

• In the 2nd code block, we are in another module. The Latitude definition is not accessible.

• We can use only Latitude.tryCreate and latitude.Value.

Example #2

Smart constructor with:

• Static method Of

• Returns a Result with a string in the Error track.

type Tweet =
    private { Tweet: string }

    static member Of tweet =
        if System.String.IsNullOrEmpty tweet then
            Error "Tweet shouldn't be empty"
        elif tweet.Length > 280 then
            Error "Tweet shouldn't contain more than 280 characters"
        else Ok { Tweet = tweet }

let tweet1 = Tweet.Of "Hello world" // Ok { Tweet = "Hello world" }