Login with reactive piglets + Bootstrap

namespace LoginWithReactivePiglets

open WebSharper
open WebSharper.UI.Next
open WebSharper.UI.Next.Html

[<JavaScript>]
module Client =
    open WebSharper.JavaScript
    open WebSharper.UI.Next.Client
    open WebSharper.UI.Next.Piglets
    open WebSharper.Forms.Bootstrap
    
    let cls = Attr.Class

let loginForm =
        Piglet.Return (fun user pass check -> user, pass, check)
        <*> (Piglet.Yield ""
            |> Validation.IsNotEmpty "Must enter a username")
        <*> (Piglet.Yield ""
            |> Validation.IsNotEmpty "Must enter a password")
        <*> Piglet.Yield false
        |> Piglet.WithSubmit
        |> Piglet.Run (fun (user, pass, check) ->
            JS.Alert("Welcome, " + user + "!")
        )
        |> Piglet.Render (fun user pass check submit ->
            form [
                Controls.Input "Username" [] (user, [cls "sr-only"], [cls "input-lg"; attr.readonly ""])
                Controls.Simple.TextAreaWithError "Username - echoed" user submit.View
                Controls.TextAreaWithError "Username - echoed" [] (user, [], []) submit.View
                Controls.Simple.InputWithError "Username - echoed" user submit.View
                Controls.Simple.InputPasswordWithError "Password" pass submit.View
                Controls.Checkbox "Keep me logged in" [] (check, [cls "input-lg"], [])
                Controls.Radio "This is a radio button" [] (check, [], [])
                Controls.Button "Log in" [cls "btn btn-primary"] submit.Trigger
                Controls.ShowErrors [attr.style "margin-top:1em;"] submit
            ]
        )
        |> fun s -> s.RunById "main"