The type 'XmlProvider<…>.Parameter' does not support the operator 'get_Value'?

问题

How to fix the code? Is the inline/Statically Resolved Type the same powerful as structural typing?

The type 'XmlProvider<...>.Parameter' does not support the operator 'get_Value'?

let input1 = """<r1><parameters><parameter name="token">1</parameter><parameter name="other">xxx</parameter></parameters><othersOf1>..sample....</othersOf1></r1>"""
let xml1 = XmlProvider<"""<r1><parameters><parameter name="token">1</parameter><parameter name="other">xxx</parameter></parameters><othersOf1>...</othersOf1></r1>""">.Parse(input1)

let inline get name parameters =
    parameters |> Seq.tryFind (fun x -> (^P : (member Name : 'a) x) = name)
    |> Option.map (fun v -> (^P : (member Value : 'b) v))

get "token" xml1.Parameters

回答1:

Value is only defined for nodes that have a single type (or types the type provider can unify, e.g. 2 and 3.0). In your example the second value is the string xxx, so a parameter gets two properties: Number and String, each returning an option of the respective type. You can either

• change your input to have one single consistent value type (xxx &rightarrow; 2)

  let xml1 = XmlProvider<"""<r1><parameters><parameter name="token">1</parameter><parameter name="other">2</parameter></parameters><othersOf1>...</othersOf1></r1>""">.Parse(input1)

• turn them into a single output type (e.g. string)

  let inline get name parameters =
      parameters |> Seq.tryFind (fun x -> (^P : (member Name : 'a) x) = name)
      |> Option.bind (fun v ->
          match (^P : (member Number : int option) v) with
          | Some number -> Some (string number)
          | None -> (^P : (member String : string option) v))
  

• create an appropriate DU

  type Value = Number of int | Name of string
  
  let inline get name parameters =
      parameters |> Seq.tryFind (fun x -> (^P : (member Name : 'a) x) = name)
      |> Option.map (fun v ->
          match (^P : (member Number : int option) v) with
          | Some number -> Number number
          | None ->
              match (^P : (member String : string option) v) with
              | Some s -> Name s
              | _ -> failwith "Either number or string should be Some(value)")
  

• if you don't know the values upfront, you can also tell the type provider to not infer them at all:

  XmlProvider<"""...""", InferTypesFromValues=false>
  

  this will cause parameters to have a Value : string property.

来源：https://stackoverflow.com/questions/38337854/the-type-xmlprovider-parameter-does-not-support-the-operator-get-value