无法定义计算表达式自定义 "condition" 运算符

Question

我很难在下面的代码中给出 condition 的定义。希望有一个例子和见解：

// a computation expression builder class
type Builder() =
    .
    .
    .

    [<CustomOperation( "condition",
      MaintainsVariableSpaceUsingBind = true )>]
    member this.Condition(p, [<ProjectionParameter>] b) = 
        condition p b

let attemp = AttemptBuilder()

let test =
    attempt { let x, y = exp1, exp2
              condition booleanExpr(x, y)   
              return (x, y) }

我假设 b 是隐含的 ( fun x, y -> booleanExpr(x, y) )。术语 booleanExpr(x, y) 只是一些涉及 x 和 y 的布尔表达式。

Answer 1

找到了：

let condition p guard = ( fun () ->
    match p() with
    | Some x when guard x -> Some x
    | _ -> None )

无法定义计算表达式自定义 "condition" 运算符

Cannot define a computation expression custom "condition" operator

f#

computation-expression

custom-operator