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|
type variable = string
and term =
| Var of variable
| Lam of variable * term
| App of term * term
| Const of constant
and constant =
| CInt of int
| CString of string
;;
class fold = Camlp4Filters.GenerateFold.generated;;
(* class fold = Camlp4FoldGenerator.generated;; *)
module VarSet = Set.Make(String);;
(* Compute free variables with the fold class *)
let free_variables_v1 =
let o =
object (self)
inherit fold as super
val fv = VarSet.empty
method fv = fv
method empty_fv = {< fv = VarSet.empty >}
method term t =
match t with
| Var(v) -> {< fv = VarSet.add v fv >}
| Lam(v, t) ->
let fv1 = VarSet.remove v (self#empty_fv#term t)#fv in
{< fv = VarSet.union fv fv1 >}
| _ -> super#term t
end
in fun t -> VarSet.elements (o#term t)#fv
;;
(* Let's try to abstract that a little *)
let fold_term f t init =
let o =
object (self)
inherit fold as super
val acc = init
method get = acc
method reset = {< acc = init >}
method term t =
{< acc = f t acc (fun t -> (self#reset#term t)#get)
(fun t -> (super#term t)#get) >}
end
in
(o#term t)#get
;;
(* A nicer version of free_variables *)
let free_variables_v2 t =
VarSet.elements begin
fold_term begin fun t fv self next ->
match t with
| Var(v) -> VarSet.add v fv
| Lam(v, t) -> VarSet.union fv (VarSet.remove v (self t))
| _ -> next t
end t VarSet.empty
end
;;
let term1 =
App(
App(Var"x1",
Lam("x",
App(Var"x", App(Var"y", (Lam("y", Lam("z", (App(Var"y", App(Var"x4",Var"z")))))))))),
Var"x3")
;;
let fv1 = free_variables_v1 term1;;
let fv2 = free_variables_v2 term1;;
(* Low cost syntax *)
let ( ^-> ) v t = Lam(v, t)
let ( @ ) t1 t2 = App(t1, t2)
let ( ! ) s = Var s
let term2 =
!"x1" @
("x" ^-> !"x" @ !"y" @ ("y" ^-> ("z" ^-> !"y" @ !"x4" @ !"z"))) @
!"x3"
;;
let fv1' = free_variables_v1 term2;;
let fv2' = free_variables_v2 term2;;
|
