src/duplication_detection.ml

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open Lang

module Env = Set.Make (struct
  type t = Lang.expr

  (* TODO: compare them modulo alpha-equivalence *)
  let compare = Stdlib.compare
end)

module Make (M : sig
  module Error : sig
    exception Duplication of Lang.expr

    exception Known_value of Lang.expr

    exception Useless_if of Lang.expr

    exception Useless_match of Lang.expr
  end
end) =
struct
  let tbl = Hashtbl.create 512

  let add e =
    (* TODO: compare them modulo alpha-equivalence *)
    match Hashtbl.find_opt tbl e with
    | Some () ->
        raise @@ M.Error.Duplication e
    | None ->
        Hashtbl.add tbl e ()

  let rec expr env e =
    if Env.mem e env then raise @@ M.Error.Known_value e ;
    match e with
    | Const _ ->
        ()
    | Bind (_, e1, e2) ->
        expr env e1 ; expr env e2 ; add e
    | Abstract (_, _, e1) ->
        expr env e1 ; add e
    | Apply (e1, e2) ->
        expr env e1 ; expr env e2 ; add e
    | Match (orig, match_expr, cases) as e ->
        expr env match_expr ;
        let diff = ref false in
        List.iter
          (function
            | x, Const (Literal y) ->
                if x <> y then diff := true
            | _ ->
                diff := true)
          cases ;
        (* TODO: M.Error.Useless_match *)
        if not !diff then
          raise
          @@
          if orig = Generated then M.Error.Useless_if e
          else M.Error.Useless_match e ;
        let env' = Env.add match_expr env in
        List.iter (expr env') (List.map snd cases) ;
        add e
    | Type (_id, _cons, e) ->
        expr env e

  let expr e = expr Env.empty e
end