Inspect the last value evaluated in FSI

it.fsx

// Run this as: fsi --use:it.fsx --quiet
open System
open System.Reflection
open Microsoft.FSharp.Compiler.Interactive

let (getSavedItType : unit -> Type, getSavedIt : unit -> obj) =
    let t = typeof<IEventLoop>
    let t = t.Assembly.GetType(t.Namespace + ".RuntimeHelpers", true)
    let bf = BindingFlags.Static ||| BindingFlags.NonPublic
    let itTypeMethod = t.GetMethod(name = "GetSavedItType", bindingAttr = bf)
    let itMethod = t.GetMethod(name = "GetSavedIt", bindingAttr = bf)
    (fun () -> unbox (itTypeMethod.Invoke(null, [| |]))), (fun () -> itMethod.Invoke(null, [| |]))

let printIt () =
    match getSavedItType () with
    | t when t = typeof<unit> -> printfn "No value"
    | t -> printfn "Got value: %A : %O" (getSavedIt ()) t

(*
1 + 1
;;
printIt ();;

let one = 1 + 1
;;
printIt ();;

let rec fib n =
    if n < 2 then
        1
    else
        fib (n - 2) + fib (n - 1)
;;
printIt ();;

fib 6
;;
printIt ();;
*)