Boxed collections

Description

Is it possible to have code like this:

    | Shape.FSharpList s ->
        s.Accept
          { new IFSharpListVisitor<'T -> V> with
              member __.Visit<'a> () =
                wrap (fun (ts: 'T list) ->
                  ts
                  |> List.map (fun t -> x2V (t.GetType()) t)
                  |> List.toArray
                  |> fun args -> V.ForList(args))
          }

when the list is e.g. string list, but is only known to the caller as obj, which is then reflected over?

…without getting the dreaded exception:

System.InvalidCastException: Unable to cast object of type 'Microsoft.FSharp.Collections.FSharpList`1[System.String]' to type 'Microsoft.FSharp.Collections.FSharpList`1[System.Object]'.
   at Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicFunctions.UnboxGeneric[T](Object source)
   at [e-mail](a inp) /logary/src/targets/Logary.Targets.Stackdriver/Target_Stackdriver.fs:line 169

That is

[eiriktsarpalis]

It is possible, but you need to use custom lambda types:

type IBoxedLambda =
    abstract InputType : System.Type
    abstract InvokeBoxed : obj -> obj

type MyCustomLambda<'T,'V>(f : 'T -> 'V) =
    interface IBoxedLambda with
       member __.InputType = typeof<'T>
       member __.InvokeBoxed o = f (o :?> 'T) :> obj

You then just replace your wrap function with MyCustomLambda(f) :> IBoxedLambda

[eiriktsarpalis]

You can define the following

let toJsonUntyped (t:Type) : IBoxedLambda =
    let shape = TypeShape.Create t
    shape.Accept { new ITypeShapeVisitor<IBoxedLambda> with
         member __.Visit<'t> () = toJson<'t>() :> IBoxedLambda }

then in the relevant toJson code:

fun value ->
    match value with
    | null -> Json.Null
    | o -> toJsonUntyped (o.GetType()) . InvokeBoxed o

It is advisable to memoize the generated lambdas, otherwise you’ll end up paying expensive reflection querying logic on every serialization cycle.