Records and types

LENS provides two ways of defining a custom data structure inside the script: via records and types.

Records

A record contains several named fields of different types:

record Cat
    Name : string
    Age : int
    IsCute : bool

Instances of records are created using the new keyword. Records always have two constructors defined: the default one and the one that initializes all fields in specified order.

let madTom = new Cat ()
let mrWhiskers = new Cat "Mr Whiskers" 2 true
let tardar = new Cat "Tardar Sauce" 1 false

Records also get GetHashCode and equality comparison methods for free!

Types

A type in LENS defines a list of possible values. It is similar to the enum construct from C#, but type labels in LENS may have tags associated with them:

type Suit
    Hearts
    Clubs
    Spades
    Diamonds

type Card
    Ace of Suit
    King of Suit
    Queen of Suit
    Jack of Suit
    ValueCard of Tuple<Suit, int>

If the label does not have a tag, its name is declared as a global read-only property. Otherwise a global eponymous function is created:

let sp = Spades
let ace = Ace Spades
let ten = ValueCard new (Clubs; 10)

Inheritance

Both records and types are pass-by-reference in .NET terms, deriving from System.Object. There is no way to define a custom value type in a LENS script.