索引

笔记

Computation Expression:

作用：改变代码执行逻辑；减少重复代码；扩展F#语言

限制：

  • 1.不能在Computation Expression中定义新类型
  • 2.不能在Computation Expression中使用mutable变量，用ref变量代替

下面定义一个允许使用let!和return关键字的computation expression

type DefinedBuilder() =
    member this.Bind((x:int),(rest:int->int)) = //允许使用let!
        printfn "x:%d" x
        match x with
        | x when x>0 ->rest x
        | _ -> x
    member this.Return(x:'a) = x //允许使用return

let defined = DefinedBuilder()

let expr a b c =
    defined{
        let! x = a *2 //只有当x>0时才会执行以下剩余语句，否则返回x
        let! y = b - 5//只有当y>0时才会执行以下剩余语句，否则返回y
        let! z = c % 3//只有当z>0时才会执行以下剩余语句，否则返回z
        return x + y + z
    }

printfn "expr -2 3 5 = %d" (expr -2 3 5) //返回x
printfn "expr 1 3 5 = %d" (expr 1 3 5) //返回y
printfn "expr 1 6 3 = %d" (expr 1 6 3) //返回z
printfn "expr 1 6 5 = %d" (expr 1 6 5) //返回x+y+z

Computation Expresssion所有方法： 方法 描述

member For:
seq<'a> * ('a -> Result<unit>) ->
Result<unit>
允许使用for循环，参数是循环的所有数据项和循环的主体代码

member Zero:
unit -> Result<unit>
允许执行unit表达式

member Combine:
Result<unit> * Result<'a> ->
Result<'a>
把computation expression各部分连接起来

member While:
(unit -> bool) * Result<unit> ->
Result<unit>
允许使用while循环

member Return:
'a -> Result<'a>
允许使用return关键字

member ReturnFrom:
'a -> Result<'a>
允许使用return!关键字

member Yield:
'a -> Result<'a>
允许使用yield关键字

member YieldFrom:
seq<'a> -> Result<'a>
允许使用yield!关键字

member Delay:
(unit -> Result<'a>) ->
Result<'a>
这个操作通常和Combine一起使用，以确保操作以正确的顺序执行

member Run:
Result<'a> -> Result<'a>
在Computation Expression的开始执行时被调用

member Using:
'a * ('a -> Result<'b>) ->
Result<'b> when 'a :>
System.IDisposable
允许使用use!关键字，负责IDisposable.Dispose()

member Bind:
Result<'a> * ('a -> Result<'b>) ->
Result<'b>
允许使用let!关键字(do!是let!的特殊形式，Bind返回

Result<unit>)

member TryFinally:
Result<'a> * (unit -> unit) ->
Result<'a>
允许使用try finally表达式，参数:try代码块的结果和finally代码块代表的函数

member TryWith:
Result<'a> * (exn -> Result<'a>) ->
Result<'a>
允许使用try with表达式，参数:try代码块的结果和with代码块代表的函数

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