diff --git a/containers.mllib b/containers.mllib
index 614d2e65..29136e4b 100644
--- a/containers.mllib
+++ b/containers.mllib
@@ -32,3 +32,4 @@ ActionMan
 BV
 QCheck
 BencodeOnDisk
+Show
diff --git a/show.ml b/show.ml
new file mode 100644
index 00000000..892acf55
--- /dev/null
+++ b/show.ml
@@ -0,0 +1,130 @@
+
+(*
+copyright (c) 2013, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {1 GADT Description of Printers}
+
+This module provides combinators to build printers for user-defined types.
+It doesn't try to do {b pretty}-printing (see for instance Pprint for this),
+but a simple way to print complicated values without writing a lot of code.
+*)
+
+type 'a sequence = ('a -> unit) -> unit
+
+type 'a t = Buffer.t -> 'a -> unit
+  (** A printer for the type ['a] *)
+
+(** {2 Combinators} *)
+
+let unit buf () = ()
+let int buf i = Buffer.add_string buf (string_of_int i)
+let string buf s = Buffer.add_string buf s
+let bool buf b = Printf.bprintf buf "%B" b
+let float3 buf f = Printf.bprintf buf "%.3f" f
+let float buf f = Buffer.add_string buf (string_of_float f)
+
+let list ?(start="[") ?(stop="]") ?(sep=", ") pp buf l =
+  let rec pp_list l = match l with
+  | x::((y::xs) as l) ->
+    pp buf x;
+    Buffer.add_string buf sep;
+    pp_list l
+  | x::[] -> pp buf x
+  | [] -> ()
+  in
+  Buffer.add_string buf start;
+  pp_list l;
+  Buffer.add_string buf stop
+  
+let array ?(start="[") ?(stop="]") ?(sep=", ") pp buf a =
+  Buffer.add_string buf start;
+  for i = 0 to Array.length a - 1 do
+    (if i > 0 then Buffer.add_string buf sep);
+    pp buf a.(i)
+  done;
+  Buffer.add_string buf stop
+  
+let arrayi ?(start="[") ?(stop="]") ?(sep=", ") pp buf a =
+  Buffer.add_string buf start;
+  for i = 0 to Array.length a - 1 do
+    (if i > 0 then Buffer.add_string buf sep);
+    pp buf (i, a.(i))
+  done;
+  Buffer.add_string buf stop
+
+let seq ?(start="[") ?(stop="]") ?(sep=", ") pp buf seq =
+  Buffer.add_string buf start;
+  let first = ref true in
+  seq (fun x ->
+    (if !first then first := false else Buffer.add_string buf sep);
+    pp buf x);
+  Buffer.add_string buf stop
+
+let opt pp buf x = match x with
+  | None -> Buffer.add_string buf "none"
+  | Some x -> Printf.bprintf buf "some %a" pp x
+
+let pair ppa ppb buf (a, b) =
+  Printf.bprintf buf "(%a, %a)" ppa a ppb b
+
+let triple ppa ppb ppc buf (a, b, c) =
+  Printf.bprintf buf "(%a, %a, %a)" ppa a ppb b ppc c
+
+let quad ppa ppb ppc ppd buf (a, b, c, d) =
+  Printf.bprintf buf "(%a, %a, %a, %a)" ppa a ppb b ppc c ppd d
+
+let map f pp buf x =
+  pp buf (f x);
+  ()
+
+(** {2 IO} *)
+
+let output oc pp x =
+  let buf = Buffer.create 64 in
+  pp buf x;
+  Buffer.output_buffer oc buf
+
+let to_string pp x =
+  let buf = Buffer.create 64 in
+  pp buf x;
+  Buffer.contents buf
+
+let sprintf format =
+  let buffer = Buffer.create 64 in
+  Printf.kbprintf
+    (fun fmt -> Buffer.contents buffer)
+    buffer
+    format
+
+let fprintf oc format =
+  let buffer = Buffer.create 64 in
+  Printf.kbprintf
+    (fun fmt -> Buffer.output_buffer oc buffer)
+  buffer
+  format
+
+let printf format = fprintf stdout format
+let eprintf format = fprintf stderr format
+
diff --git a/show.mli b/show.mli
new file mode 100644
index 00000000..960fad0a
--- /dev/null
+++ b/show.mli
@@ -0,0 +1,74 @@
+
+(*
+copyright (c) 2013, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {1 GADT Description of Printers}
+
+This module provides combinators to build printers for user-defined types.
+It doesn't try to do {b pretty}-printing (see for instance Pprint for this),
+but a simple way to print complicated values without writing a lot of code.
+*)
+
+type 'a sequence = ('a -> unit) -> unit
+
+type 'a t = Buffer.t -> 'a -> unit
+  (** A printer for the type ['a] *)
+
+(** {2 Combinators} *)
+
+val unit : unit t  (* prints nothing! *)
+val int : int t
+val string : string t
+val bool : bool t
+val float3 : float t (* 3 digits after . *)
+val float : float t
+
+val list : ?start:string -> ?stop:string -> ?sep:string -> 'a t -> 'a list t
+val array : ?start:string -> ?stop:string -> ?sep:string -> 'a t -> 'a array t
+val arrayi : ?start:string -> ?stop:string -> ?sep:string -> (int * 'a) t -> 'a array t
+val seq : ?start:string -> ?stop:string -> ?sep:string -> 'a t -> 'a sequence t
+
+val opt : 'a t -> 'a option t
+
+val pair : 'a t -> 'b t -> ('a * 'b) t
+val triple : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t
+val quad : 'a t -> 'b t -> 'c t -> 'd t -> ('a * 'b * 'c * 'd) t
+
+val map : ('a -> 'b) -> 'b t -> 'a t
+
+
+(** {2 IO} *)
+
+val output : out_channel -> 'a t -> 'a -> unit
+val to_string : 'a t -> 'a -> string
+
+val sprintf : ('a, Buffer.t, unit, string) format4 -> 'a
+  (** print into a string *)
+
+val fprintf : out_channel -> ('a, Buffer.t, unit, unit) format4 -> 'a
+  (** Print on a channel *)
+
+val printf : ('a, Buffer.t, unit, unit) format4 -> 'a
+val eprintf : ('a, Buffer.t, unit, unit) format4 -> 'a