I’m working on building a website using the F# MVC 5 template, and I noticed that the template creator uses formally declared types for the route defaults. In VB.Net and C#, this is typically done with anonymous classes, but these don’t exist in F# my current work around is to use an Expando Object, like this:

[<RequireQualifiedAccess>]
module RouteConfig =

    let private createDefaults (propValuePairs : seq<string * obj>) : obj =
        let defaults = ExpandoObject() :> IDictionary<string,obj>
        propValuePairs |> Seq.iter (fun (key,value) -> defaults.Add(key,value) )
        box defaults  

    /// create a pair, boxing the second item
    let inline private (=>) (key : string) value = key, box value

    let RegisterWebApi =
        Action<_> (fun (config: HttpConfiguration) ->
            // Configure routing
            config.MapHttpAttributeRoutes()
            config.Routes.MapHttpRoute(
                "DefaultApi", // Route name
                "api/{controller}/{id}", // URL with parameters
                createDefaults [ "controller" => "{controller}"; 
                                 "id" => RouteParameter.Optional]
            ) |> ignore

            // Configure serialization
            config.Formatters.XmlFormatter.UseXmlSerializer <- true
            config.Formatters.JsonFormatter.SerializerSettings.ContractResolver
                <- Newtonsoft.Json.Serialization.CamelCasePropertyNamesContractResolver()

            // Additional Web API settings        
        )

    let RegisterRoutes(routes:RouteCollection) =

        routes.IgnoreRoute("{resource}.axd/{*pathInfo}")
        routes.MapRoute(
            "Default", // Route name
            "{controller}/{action}/{id}", // URL with parameters
            createDefaults [ "controller" => "Home" ; 
                             "action" => "Index"; 
                             "id" => UrlParameter.Optional; ]
        ) |> ignore

I’m mainly wondering if type providers can do something like this at compile time instead of runtime.