Hi,
I played some more with GADTs, this time with an universal list. Well,
not truely universal as you can't store arbitrary types. Only Int and
Float are allowed here:
type _ kind = (* used for searching in the list *)
| Int_kind : int kind
| Float_kind : float kind
type value = (* box the values so we have runtime type information *)
| Int of int
| Float of float
type list = (* the universal list *)
| Nil : list
| Cons : value * list -> list
(* find the first value in the list of a given kind *)
let rec get : type a . list -> a kind -> a = fun l k ->
match (k, l) with
| (_, Nil) -> raise Not_found
| (Int_kind, Cons (Int x, xs)) -> x
| (Float_kind, Cons (Float x, xs)) -> x
| (_, Cons (_, xs)) -> get xs k
(* print out list *)
let rec print = function
| Nil -> print_newline ()
| Cons ((Int x), xs) -> Printf.printf "%d " x; print xs
| Cons ((Float x), xs) -> Printf.printf "%f " x; print xs
(* testing *)
let empty = Nil
let l1 = Cons ((Int 1), empty)
let l2 = Cons ((Float 2.), l1)
let () = print l2
let i = get l2 Int_kind
let f = get l2 Float_kind;;
(*
2.000000 1
type _ kind = Int_kind : int kind | Float_kind : float kind
type value = Int of int | Float of float
type list = Nil : list | Cons : value * list -> list
val get : list -> 'a kind -> 'a = <fun>
val print : list -> unit = <fun>
val empty : list = Nil
val l1 : list = Cons (Int 1, Nil)
val l2 : list = Cons (Float 2., Cons (Int 1, Nil))
val i : int = 1
val f : float = 2.
*)
At first glance you might think: Why does that need GADTs? Why not
simply use
type value = Int of int | Float of float
for this?
Take a close look at the get funktion:
val get : list -> 'a kind -> 'a = <fun>
It does not return a value but directly the unboxed type. Because the
value is unboxed you can directly use it and ocaml will detect if you
screw up the type like in:
let () = Printf.printf "%d\n" (get l2 Float_kind);;
Error: This expression has type float but an expression was expected
of type int
MfG
Goswin
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