F#在携带状态时绑定到输出
f#
我正在尝试使用计算表达式来构建操作列表。我需要绑定到从getFood操作返回的值，以便我可以注册稍后的步骤来使用它。
type Food =
| Chicken
| Rice
type Step =
| GetFood of Food
| Eat of Food
| Sleep of duration:int
type Plan = Plan of Step list
type PlanBuilder () =
member this.Bind (plan:Plan, f) =
f plan
member this.Yield _ = Plan []
member this.Run (Plan x) = Plan (List.rev x)
[<CustomOperation("eat")>]
member this.Eat (Plan p, food) =
printfn "Eat"
Plan ((Eat food)::p)
[<CustomOperation("sleep")>]
member this.Sleep (Plan p, duration) =
printfn "Sleep"
Plan ((Sleep duration)::p)
let plan = PlanBuilder()
let rng = System.Random(123)
let getFood (Plan p) =
printfn "GetFood"
let randomFood =
if rng.NextDouble() > 0.5 then
Food.Chicken
else
Food.Rice
(Plan ((GetFood randomFood)::p)), randomFood
let testPlan =
plan {
let! food = getFood // <-- This is what I am trying to get to work
sleep 10
eat food
}
我相信问题出在 ，Bind但我无法弄清楚它是什么。
(*
Example result
testPlan =
(GetFood Chicken,(
(Sleep 10,(
EatFood Chicken
))
))
*)
回答
为了使这样的事情起作用，您可能需要一种具有更多一元结构并允许您存储任何结果而不仅仅是计划的类型。我会使用这样的东西：
type Step =
| GetFood of Food
| Eat of Food
| Sleep of duration:int
type Plan<'T> = Plan of Step list * 'T
现在，Plan<'T>表示一个计算，它产生一个类型的值'T并在此过程中收集一个计划。GetFood可以制定计划，还可以返还食物：
let getFood () =
printfn "GetFood"
let randomFood =
if rng.NextDouble() > 0.5 then Food.Chicken
else Food.Rice
Plan([GetFood randomFood], randomFood)
实现计算构建器有点像魔术，但您现在可以定义Bind和自定义操作。为了能够访问参数中的变量，它需要是一个函数，如whereorselect操作：
type PlanBuilder () =
member this.For (Plan(steps1, res):Plan<'T>, f:'T -> Plan<'R>) : Plan<'R> =
let (Plan(steps2, res2)) = f res
Plan(steps1 @ steps2, res2)
member this.Bind (Plan(steps1, res):Plan<'T>, f:'T -> Plan<'R>) : Plan<'R> =
let (Plan(steps2, res2)) = f res
Plan(steps1 @ steps2, res2)
member this.Yield x = Plan([], x)
member this.Return x = Plan([], x)
member this.Run (Plan(p,r)) = Plan(List.rev p, r)
[<CustomOperation("eat", MaintainsVariableSpace=true)>]
member this.Eat (Plan(p, r), [<ProjectionParameter>] food) =
Plan((Eat (food r))::p, r)
[<CustomOperation("sleep", MaintainsVariableSpace=true)>]
member this.Sleep (Plan(p, r), [<ProjectionParameter>] duration) =
Plan ((Sleep (duration r))::p, r)
let plan = PlanBuilder()
这实际上可以让您实施您的测试计划：
let testPlan =
plan {
let! food = getFood ()
sleep 10
eat food
return ()
}
也就是说，在实践中，我不确定我是否真的想要使用它。我可能只是我们一个用于累积计划步骤的seq { .. }计算yield。