ClientSource.fs

(Server.login emailEntry.Text passwordEntry.Text) |> Async.map (fun res ->
    match res with
    | Server.Success           -> f() // login
    | Server.IncorrectPassword -> outputLabel.Text <- "Incorrect password"
    | Server.EmailNotFound     -> outputLabel.Text <- "Email not found"
    | Server.NotActivated      -> outputLabel.Text <- "Account not activated"
    | _                        -> outputLabel.Text <- "Error, please try again"
) |> Async.StartImmediate

ClientSourceAlt.fs

// If ServerWrapper just returned the raw server output
(Server.login emailEntry.Text passwordEntry.Text) |> Async.map (fun res ->
    match res with
    | "Success"           -> f() // login
    | "IncorrectPassword" -> outputLabel.Text <- "Incorrect password"
    | "EmailNotFound"     -> outputLabel.Text <- "Email not found"
    | "NotActivated"      -> outputLabel.Text <- "Account not activated"
    | _                   -> outputLabel.Text <- "Error, please try again"
) |> Async.StartImmediate

ServerWrapper.fs

type LoginResult =
    | Success
    | IncorrectPassword
    | EmailNotFound
    | NotActivated
    | Error

let loginFake _ _ = async { return Success }

let login email password =
    sprintf "login?email=%s&password=%s" email password
    |> fetchUrlAsync
    |> Async.map (fun res ->
        match res with
        | "Success"           -> Success
        | "IncorrectPassword" -> IncorrectPassword
        | "EmailNotFound"     -> EmailNotFound
        | "NotActivated"      -> NotActivated
        | _                   -> Error
    )