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Command pattern

Category:
Design patterns
Description: Command pattern to demostrate a redo-undo framework. This implementation group the commands under Do/Undo category. For more information, please go to http://en.wikipedia.org/wiki/Command_pattern
Code:
type CommandType = 
| Deposit
| Withdraw
type TCommand =
| Command of CommandType * int

let CommandPatternSample2() =
let result = ref 7
let deposit x = result := !result + x
let withdraw x = result := !result - x
let Do = fun (cmd:TCommand) ->
match cmd with
| Command(CommandType.Deposit, n) -> deposit n
| Command(CommandType.Withdraw,n) -> withdraw n
let Undo = fun (cmd:TCommand) ->
match cmd with
| Command(CommandType.Deposit, n) -> withdraw n
| Command(CommandType.Withdraw,n) -> deposit n
printfn "current balance %d" !result
let depositCmd = Command(Deposit, 3)
Do depositCmd
printfn "after deposit: %d" !result
Undo depositCmd
printfn "after undo: %d" !result

Execution Result:
current balance 7
after deposit: 10
after undo: 7

Last edited Sep 14, 2011 at 4:20 AM by ttliu2000, version 1

Comments

Thorium Mar 27, 2012 at 3:51 AM 
Command-pattern is just a way to represent function as a type.
In F# functions are types by default.
So there is no reason to use this pattern. You can use a function.