Saturn框架拆分模块到独立文件出现Value restriction报错如何解决

使用Saturn和Giraffe进行F# Web开发时，将UiRoutes模块从主程序文件移动到独立文件后出现编译报错，需要明确模块放在同一文件和独立文件中的编译差异。

Value restriction. The value 'uiRouter' has been inferred to have generic type
val uiRouter : (HttpFunc -> '_a -> HttpFuncResult) when '_a :> AspNetCore.Http.HttpContext
Either make the arguments to 'uiRouter' explicit or, if you do not intend for it to be generic, add a type annotation.

open Saturn
// MongoDB.FSharp.Serializers.Register()

module ApiRoutes =

    open Giraffe

    let apiRoutes =
        router {
            //Routers in here
        }


module UiRoutes =

    open Giraffe
    open Giraffe.ViewEngine
    open DBApi

    let carsView =
        html [] [
            //Some HTML
        ]

        let uiRouter = htmlView carsView //HttpFunc -> AspNetCore.Http.HttpContext -> HttpFuncResult  



let appRouter =
    router {
        forward "/api" ApiRoutes.apiRoutes
        forward "" UiRoutes.uiRouter
    }

let myApp = application { use_router appRouter }
run myApp

F#的值限制规则要求顶级无参数值不能是未确定的泛型类型：

• 模块放在同一文件时，属于同一个编译单元，编译器可以通过同一文件内appRouter的forward调用，推断出uiRouter的泛型参数'_a实际为Microsoft.AspNetCore.Http.HttpContext，不会触发报错
• 模块移动到独立文件后，每个文件是独立的编译单元，编译UiRoutes模块时无法获取其他文件的调用信息，编译器无法确定泛型参数的具体类型，就会抛出值限制错误

给uiRouter显式添加类型注解，锁定参数类型即可解决问题：
修改前代码：
let uiRouter = htmlView carsView

修改后代码：

let (uiRouter: HttpFunc -> Microsoft.AspNetCore.Http.HttpContext -> HttpFuncResult) = htmlView carsView

内容的提问来源于stack exchange，提问作者Chilli