type id = string

type ty_annot = id * Types.ty

type lam =
  Fun of ty_annot * lam
  | App of lam * lam
  | Var of id
  | Exf of lam * Types.ty

(** alpha renaming in a deterministic way *)
let alpha_convert (e : lam) : lam =
  let cpt_var = ref 0 in (* id cpt for variables *)
  let generator (cpt : int ref) : id = (* id generator *)
    let v = "__"^(string_of_int !cpt)
    in incr cpt; v

  in let renamed = ref []
  in let new_ren_var (cpt : int ref) (var : id) : id = (* checks if a variable already has a new id, or generates one *)
    let var' = generator cpt in (
      renamed := (var, var')::!renamed;
      var'
    )

  in let get_ren_var (var : id) : id = (* returns var if not found *)
    match List.assoc_opt var !renamed with
      | Some var' -> var'
      | None -> var
  
  in let rec alpha_aux (e : lam) : lam = match e with (* actual renaming *)
    Fun ((v, t), e) -> Fun ((new_ren_var cpt_var v, t), alpha_aux e)
    | App (e1, e2) -> App (alpha_aux e1, alpha_aux e2)
    | Var v -> Var (get_ren_var v)
    | Exf (e, t) -> Exf (alpha_aux e, t)
  in alpha_aux e


(** alpha equivalence *)
let (=~) (e1 : lam) (e2 : lam) : bool =
  alpha_convert e1 = alpha_convert e2