f# - 我可以为 Builder Like DSL 创建嵌套的计算表达式吗？

Question

这是我想做的:

type DirectionBuilder() =
    member self.Yield(()) = []

    [<CustomOperation("left")>]
    member self.Left (acc, degree) = None

    [<CustomOperation("right")>]
    member self.Right (acc, degree) = None 

    [<CustomOperation("velocity")>]
    member self.Velocity (acc, ()) = new VelocityBuilder()


and VelocityBuilder() = 
    member self.Yield(()) = []

    [<CustomOperation("accelerate")>]
    member self.Accesslarate (acc, v) = None

    [<CustomOperation("decelerate")>]
    member self.Decelerate (acc, v) = None


let direction () = new DirectionBuilder()

direction() {
    right 90
    left 30
    velocity() {
        accelerate 1
    }
}

这行崩溃了

      velocity() {
  ----^^^^^^^^
  stdin(952,5): error FS3099: 'velocity' is used with an incorrect number of 
  arguments. This is a custom operation in this query or computation 
  expression. Expected 1 argument(s), but given 2.
  > 

有没有办法向 F# 解释这个自定义操作确实应该接受一个计算表达式？

Answer 1

我想有一种方法几乎可以得到你想要的语法。您使 velocity 操作接受 VelocityBuilder 生成的类型 - 在您的情况下这似乎是一个 选项。然后创建一个单独的计算并将其传入。

所以你得到这样的东西:

type DirectionBuilder() =
    member self.Yield(()) = []

    [<CustomOperation("left")>]
    member self.Left (acc, degree) = None

    [<CustomOperation("right")>]
    member self.Right (acc, degree) = None 

    [<CustomOperation("velocity")>]
    member self.Velocity (acc, odd: 'a option) = None


type VelocityBuilder() = 
    member self.Yield(()) = []

    [<CustomOperation("accelerate")>]
    member self.Accelerate (acc, v) = None

    [<CustomOperation("decelerate")>]
    member self.Decelerate (acc, v) = None

let dir = new DirectionBuilder()
let vel = new VelocityBuilder()

dir {
    right 90
    left 30
    velocity (vel {
        accelerate 1
    })
}

也就是说，如果您着手编写计算工作流，您可能应该首先设计一个类型来表示您的计算状态。现在你有语法糖，但没有肉 ;)

一旦您有了类型，如果它被证明是有用的，就可以遵循它的工作流程。