Trying to port Liam's simplified controller example to F#: http://withouttheloop.com/articles/2013-10-20-are-we-doing-mvc-wrong/ Untested.

BorrowABook.fs

module BorrowABook = 
    type BookLoanProblem = 
        NotInCatalogue 
        | OutOnLoan
        | InsufficientPermission
        | Failed of string

    type Book = Book
    type BookModel = BookModel

    let borrow (b : BookModel) : Choice<Book, BookLoanProblem> = Choice1Of2 Book

LibraryController.fs

open System.Web
open System.Web.Mvc
open FSharpx
open BorrowABook
[<HandleError>]
type LibraryController() =
    inherit Controller()

    member this.viewForProb (prob : BookLoanProblem) =
        match prob with
        | NotInCatalogue         -> setFlash "Not in catalogue"; this.redirect "Catalogue"
        | OutOnLoan              -> setFlash "Out on loan"; this.redirect "Catalogue"
        | InsufficientPermission -> setFlash "Don't have permission"; this.redirect "Catalogue"
        | Failed s               -> setFlash s; this.viewByName("BorrowFailed")

    member this.Borrow (model : BookModel) : ActionResult =
        borrow model
        |> Choice.choice (this.view "Borrowed") this.viewForProb

    let setFlash msg = ()
    // Helpers for partial app etc. Put in f#-specific controller base class?
    member this.view (s : string) (m:obj) : ActionResult = upcast this.View(s, m)
    member this.viewByName (s : string) : ActionResult = upcast this.View(s)
    member this.redirect s : ActionResult = upcast this.RedirectToAction(s)