Isorecursive_fibonacci_record.fs

open System;

(* The type
     'a IsoRec record
   is the same as
     type 'a isorec_record = Roll of { n : int; a : int; b : int; f : 'a isorec_record -> 'a }
   but apparenty that way of specifying types is deprecated
*)
type 'a record = { n : int; a : int; b : int; f : 'a }
type 'a IsoRec = Roll of ('a IsoRec record -> 'a)
let unroll (Roll a) = a

let make_rec n =
    { n=n
    ; a=0
    ; b=1
    ; f=Roll
        (function tup ->
            if tup.n = 0
            then tup.a
            else unroll tup.f { n=tup.n-1
                              ; a=tup.a+tup.b
                              ; b=tup.a
                              ; f=tup.f})

    }

[<EntryPoint>]
let main argv = 
    printf "Which fibonacci number would you like? "
    (* Make the record
       The function "(|>)" (parentheses are used to denote that it is infix)
       is the application operator with the arguments taken in the other order,
       that is
         "foo" |> Console.Write
       is the same as
         Console.Write "foo"
    *)
    let record = Console.ReadLine () |>
                 int |>
                 make_rec
    printfn "%d" (unroll record.f record) (* Evaluate the record *)
    Console.ReadLine () |> ignore         (* Just so that we don't close *)
    0                                     (* Return an integer exit code *)

(* P.S.: One of the cool things about ML (F#) is that comments can be nested
   so you can do this: (* *), whereas /* /* */ */ would not work in C, as the first
   "*/" would end the comment, not the second one.
*)