F#携带状态时绑定到输出 [英] F# Binding to output while carrying the state
问题描述
我正在尝试使用计算表达式来构建动作列表.我需要绑定一个从 getFood
操作返回的值,以便我可以注册一个后续步骤来使用它.
I am trying to use a Computation Expression to build a list of actions. I need to bind to a value that gets returned from the getFood
action so that I can register a later step to consume it.
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
上,但我不知道这是什么.
I believe the problem is with the Bind
but I can't figure out what it is.
(*
Example result
testPlan =
(GetFood Chicken,(
(Sleep 10,(
EatFood Chicken
))
))
*)
推荐答案
要使此工作正常进行,您可能需要一种具有更单子结构且可以存储任何结果的类型,而不仅仅是计划.我会用这样的东西:
To make something like this work, you will probably need a type that has a more monadic structure and lets you store any result, rather than just the plan. I would use something like this:
type Step =
| GetFood of Food
| Eat of Food
| Sleep of duration:int
type Plan<'T> = Plan of Step list * 'T
现在, Plan<'T>
表示一种计算,该计算会产生类型为'T
的值,并沿途收集计划. GetFood
可以制定计划,但也可以返回食物:
Now, Plan<'T>
represents a computation that produces a value of type 'T
and collects a plan along the way. GetFood
can produce a plan, but also return the food:
let getFood () =
printfn "GetFood"
let randomFood =
if rng.NextDouble() > 0.5 then Food.Chicken
else Food.Rice
Plan([GetFood randomFood], randomFood)
实施计算构建器有点荒唐,但是您现在可以定义 Bind
和您的自定义操作.为了能够访问参数中的变量,它必须是 where
或 select
操作中的函数:
Implementing a computation builder is a bit of a black art, but you can now define Bind
and your custom operations. To be able to access variables in the argument, it needs to be a function as in the where
or select
operations:
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()
这实际上使您可以实施测试计划:
This actually lets you implement your test plan:
let testPlan =
plan {
let! food = getFood ()
sleep 10
eat food
return ()
}
也就是说,实际上,我不确定我是否真的想使用它.我可能只是一个使用 seq {..}
计算.
That said, in practice, I'm not sure I would actually want to use this. I would probably just us a seq { .. }
computation that uses yield
to accumulat the steps of the plan.
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