F#：究竟应该如何使用 ToString？

Question

I am learning F# but I just don't understand how I am supposed to use ToString. Below are a few attempts. The syntax errors are saying it is expecting type string but that it is actually type uint -> string. So it doens't actually appear to be invoking a function? Could this be explained? This seems like such a simple thing to do but I can't figure it out.

open System
open System.IO
open FSharp.Data

[<EntryPoint>]
let main (args: string[]) =
    let htmlPage = HtmlDocument.Load("https://scrapethissite.com/")

    printfn  "%s" htmlPage.ToString // This causes a syntax error
    
    htmlPage.ToString
    |> (fun x -> printfn "%s" x) // This also causes a syntax error

    0

Answer 1

.ToString is a method, not a value. In F# every method and every function has a parameter. In fact, that's how functions differ from values (and methods from properties): by having a parameter.

Unlike in C#, F# methods and functions cannot be parameterless. If there is nothing meaningful that you'd want to pass to the method, that method would still have one parameter of type unit . See how this is visible in the error message? unit -> string is the type.

To call such method, you have to pass it the parameter. The sole value of type unit is denoted () . So to call the method you should do:

    htmlPage.ToString ()
    |> printfn "%s"

Your first example is a bit more complicated. The following would not work:

printfn "%s" htmlPage.ToString ()

Why? Because according to F# syntax this looks like calling printfn and passing it three parameters: first "%s" , then htmlPage.ToString , and finally () . To get the correct order of calls you have to use parentheses:

printfn "%s" (htmlPage.ToString ())

---

And finally, general piece of advice: when possible try to avoid methods and classes in F# code. Most things can be done with functions. In this particular case, the ToString methods can be replaced with the equivalent function string :

printfn "%s" (string htmlPage)