Decorate pattern

implement the decorate pattern in F#. The decorate pattern is to add new featuures to an object at runtime.

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type Divide() = 
    let mutable divide = fun (a,b) -> a / b
    member this.Function
        with get() = divide
        and set(v) = divide <- v
    member this.Invoke(a,b) = divide (a,b)

let decorate() = 
    let d = Divide()
    let checkZero (a,b) = if b = 0 then failwith "a/b and b is 0" else (a,b)
    
    try 
        d.Invoke(1, 0) |> ignore
    with e -> printfn "without check, the error is = %s" e.Message

    d.Function <- checkZero >> d.Function 
    try
        d.Invoke(1,0) |> ignore
    with e -> printfn "after add check, error is = %s" e.Message

Multiple items
type Divide =
  new : unit -> Divide
  member Invoke : a:int * b:int -> int
  member Function : (int * int -> int)
  member Function : (int * int -> int) with set

Full name: Script.Divide

--------------------
new : unit -> Divide

val mutable divide : (int * int -> int)

val a : int

val b : int

val this : Divide

member Divide.Function : (int * int -> int) with set

Full name: Script.Divide.Function

val set : elements:seq<'T> -> Set<'T> (requires comparison)

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

val v : (int * int -> int)

member Divide.Invoke : a:int * b:int -> int

Full name: Script.Divide.Invoke

val decorate : unit -> unit

Full name: Script.decorate

val d : Divide

val checkZero : ('a * int -> 'a * int)

val a : 'a

val failwith : message:string -> 'T

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

member Divide.Invoke : a:int * b:int -> int

val ignore : value:'T -> unit

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

val e : exn

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

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

property System.Exception.Message: string

property Divide.Function: int * int -> int

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More information

Link:	http://fssnip.net/7i
Posted:	13 years ago
Author:	Tao Liu
Tags:	design patterns