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Final Tagless in OCaml
(*
* An OCaml implementation of final tagless, inspired from this article by Oleksandr Manzyuk:
* https://oleksandrmanzyuk.wordpress.com/2014/06/18/from-object-algebras-to-finally-tagless-interpreters-2/
*)
module FinalTagless = struct
type eval = { eval : int }
type view = { view : string }
module type ExpT = sig
type t
val lit : int -> t
val add : t -> t -> t
end
module ExpEval = struct
type t = eval
let lit n = { eval = n }
let add x y = { eval = x.eval + y.eval }
end
module ExpView = struct
type t = view
let lit n = { view = string_of_int n }
let add x y = let s = "(" ^ x.view ^ " + " ^ y.view ^ ")"
in { view = s }
end
module type MulT = sig
include ExpT
val mul : t -> t -> t
end
module MulEval = struct
include ExpEval
let mul x y = { eval = x.eval * y.eval }
end
module MulView = struct
include ExpView
let mul x y = let s = "(" ^ x.view ^ " * " ^ y.view ^ ")"
in { view = s }
end
let e1 (type s) (module M : ExpT with type t = s) =
M.add (M.add (M.lit 10) (M.lit 12)) (M.lit 40)
let v1 = e1 (module ExpEval)
let s1 = e1 (module ExpView)
let e2 (type s) (module M : MulT with type t = s) =
M.mul (M.add (M.lit 10) (M.lit 12)) (M.lit 40)
let v2 = e2 (module MulEval)
let s2 = e2 (module MulView)
let () =
Printf.printf "%s = %d\n" s1.view v1.eval; (* ((10 + 12) + 40) = 62 *)
Printf.printf "%s = %d\n" s2.view v2.eval (* ((10 + 12) * 40) = 880 *)
end
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