From: Tom Hirschowitz <tom.hirschowitz@ens-lyon.fr>
To: Aaron Bohannon <bohannon@cis.upenn.edu>
Cc: Caml List <caml-list@inria.fr>
Subject: [Caml-list] exceptions in recursive modules
Date: Tue, 7 Dec 2004 07:58:14 +0100 [thread overview]
Message-ID: <16821.21638.982534.207888@gargle.gargle.HOWL> (raw)
In-Reply-To: <41B514EB.7030707@cis.upenn.edu>
> Here is the example. This is essentially the example in the manual, but
> I have added an exception to the module:
>
> module rec A : sig
> type t = Leaf of int | Node of ASet.t
> exception Fail
> val compare : t -> t -> int
> end = struct
> type t = Leaf of int | Node of ASet.t
> exception Fail
> let rec compare = (* suitable definition *)
> end
> and ASet : Set.S with type elt = A.t = Set.Make(A)
>
> Then we can try to use it:
>
> # let x = A.Leaf(3);;
> val x : A.t = A.Leaf 3
> # let s = ASet.add x ASet.empty;;
> val s : ASet.t = <abstr>
> # let s' = ASet.add x s;;
> Exception: Undefined_recursive_module ("recmodtest.ml", 6, 6).
>
> If we remove the "exception Fail" from the signature, everything works
> just fine. Is this behavior correct? (I am using OCaml 3.08.1)
I think this is due to the necessary coercion between your A and the
OrderedType argument expected by Set.Make. Your code is compiled to
something like
module rec A : sig
type t = Leaf of int | Node of ASet.t
exception Fail
val compare : t -> t -> int
end = struct
type t = Leaf of int | Node of ASet.t
exception Fail
let rec compare = (* suitable definition *)
end
and ASet : Set.S with type elt = A.t = Set.Make(
struct
type t = A.t
let compare = A.compare
end)
According to the (brief) explanation in the manual,
"Evaluation of a recursive module definition proceeds by building
initial values for the safe modules involved, binding all (functional)
values to fun x -> raise Undefined_recursive_module. The defining
module expressions are then evaluated, and the initial values for the
safe modules are replaced by the values thus computed."
Here, the argument passed to Set.Make is never replaced with a proper
value. If you explicit the coercion with some A' as below, the code
works. Is it all well this way, or did I miss something?
module rec A : sig
type t = Leaf of int | Node of ASet.t
exception Fail
val compare: t -> t -> int
end
= struct
type t = Leaf of int | Node of ASet.t
exception Fail
let compare t1 t2 =
match (t1, t2) with
(Leaf s1, Leaf s2) -> Pervasives.compare s1 s2
| (Leaf _, Node _) -> 1
| (Node _, Leaf _) -> -1
| (Node n1, Node n2) -> ASet.compare n1 n2
end
and A' : Set.OrderedType with type t = A.t = struct
type t = A.t
let compare = A.compare
end
and ASet : Set.S with type elt = A'.t
= Set.Make(A')
let x = A.Leaf(3)
let s = ASet.add x ASet.empty
let s' = ASet.add x s
prev parent reply other threads:[~2004-12-07 7:55 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-12-07 2:26 Aaron Bohannon
2004-12-07 6:58 ` Tom Hirschowitz [this message]
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