Decorator Pattern with Object Expressions

A simple example of a decorator pattern using object expressions and the pipeline operator for a fluent interface

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type Coffee =
  abstract member getCost: unit -> double
  abstract member getIngredients: unit -> string list
let SimpleCoffee = 
  { new Coffee with 
    member this.getCost() = 1.0
    member this.getIngredients() = ["Coffee"]
  }
let WithMilk(coffee:Coffee) =
  { new Coffee with 
    member this.getCost() = coffee.getCost() + 0.5
    member this.getIngredients() = "milk" :: coffee.getIngredients() }
let WithSyrup (syrup:string) (coffee:Coffee) =
  { new Coffee with 
    member this.getCost() = coffee.getCost() + 0.25
    member this.getIngredients() = syrup :: coffee.getIngredients() }

// Let's try it out
let myCoffee = WithMilk(SimpleCoffee)
// We can pipe them too

abstract member Coffee.getCost : unit -> double

Full name: Script.Coffee.getCost

type unit = Unit

Full name: Microsoft.FSharp.Core.unit

Multiple items
val double : value:'T -> float (requires member op_Explicit)

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.double

--------------------
type double = System.Double

Full name: Microsoft.FSharp.Core.double

abstract member Coffee.getIngredients : unit -> string list

Full name: Script.Coffee.getIngredients

Multiple items
val string : value:'T -> string

Full name: Microsoft.FSharp.Core.Operators.string

--------------------
type string = System.String

Full name: Microsoft.FSharp.Core.string

type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>

val SimpleCoffee : Coffee

Full name: Script.SimpleCoffee

type Coffee =
  interface
    abstract member getCost : unit -> double
    abstract member getIngredients : unit -> string list
  end

Full name: Script.Coffee

val this : Coffee

abstract member Coffee.getCost : unit -> double

abstract member Coffee.getIngredients : unit -> string list

val WithMilk : coffee:Coffee -> Coffee

Full name: Script.WithMilk

val coffee : Coffee

val WithSyrup : syrup:string -> coffee:Coffee -> Coffee

Full name: Script.WithSyrup

val syrup : string

val myCoffee : Coffee

Full name: Script.myCoffee

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More information

Link:	http://fssnip.net/oh
Posted:	10 years ago
Author:	Steve Goguen
Tags:	patterns