diff --git a/_oasis b/_oasis
index 6251d22a..da7316d4 100644
--- a/_oasis
+++ b/_oasis
@@ -39,7 +39,7 @@ Flag "bench"
 Library "containers"
   Path:             core
   Modules:          CCVector, CCDeque, CCGen, CCSequence, CCFQueue, CCMultiMap,
-                    CCMultiSet, CCBV, CCPrint, CCPersistentHashtbl,
+                    CCMultiSet, CCBV, CCPrint, CCPersistentHashtbl, CCError,
                     CCLeftistheap, CCList, CCOpt, CCPair, CCFun, CCHash,
                     CCKList, CCInt, CCBool, CCArray, CCBatch, CCOrd, CCLinq
   FindlibName:      containers
diff --git a/core/CCError.ml b/core/CCError.ml
new file mode 100644
index 00000000..6d92b531
--- /dev/null
+++ b/core/CCError.ml
@@ -0,0 +1,124 @@
+
+(*
+copyright (c) 2013-2014, 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 Error Monad} *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a equal = 'a -> 'a -> bool
+type 'a ord = 'a -> 'a -> int
+type 'a printer = Buffer.t -> 'a -> unit
+type 'a formatter = Format.formatter -> 'a -> unit
+
+(** {2 Basics} *)
+
+type +'a t =
+  [ `Ok of 'a
+  | `Error of string
+  ]
+
+let return x = `Ok x
+
+let fail s = `Error s
+
+let of_exn e = `Error (Printexc.to_string e)
+
+let map f e = match e with
+  | `Ok x -> `Ok (f x)
+  | `Error s -> `Error s
+
+let flat_map f e = match e with
+  | `Ok x -> f x
+  | `Error s -> `Error s
+
+let guard f =
+  try
+    return (f ())
+  with e -> of_exn e
+
+let (>|=) e f = map f e
+
+let (>>=) e f = flat_map f e
+
+let equal eq a b = match a, b with
+  | `Ok x, `Ok y -> eq x y
+  | `Error s, `Error s' -> s = s'
+  | _ -> false
+
+let compare cmp a b = match a, b with
+  | `Ok x, `Ok y -> cmp x y
+  | `Ok _, _  -> 1
+  | _, `Ok _ -> -1
+  | `Error s, `Error s' -> String.compare s s'
+
+(** {2 Collections} *)
+
+let map_l f l =
+  let rec map acc l = match l with
+  | [] -> `Ok (List.rev acc)
+  | x::l' ->
+      match f x with
+      | `Error s -> `Error s
+      | `Ok y -> map (y::acc) l'
+  in map [] l
+
+exception LocalExit of string
+
+let fold_seq f acc seq =
+  try
+    let acc = ref acc in
+    seq
+      (fun x -> match f !acc x with
+        | `Error s -> raise (LocalExit s)
+        | `Ok y -> acc := y
+      );
+    `Ok !acc
+  with LocalExit s -> `Error s
+
+let fold_l f acc l = fold_seq f acc (fun k -> List.iter k l)
+
+(** {2 Conversions} *)
+
+let to_opt = function
+  | `Ok x -> Some x
+  | `Error _ -> None
+
+let of_opt = function
+  | None -> `Error "of_opt"
+  | Some x -> `Ok x
+
+let to_seq e k = match e with
+  | `Ok x -> k x
+  | `Error _ -> ()
+
+(** {2 IO} *)
+
+let pp pp_x buf e = match e with
+  | `Ok x -> Printf.bprintf buf "ok(%a)" pp_x x
+  | `Error s -> Printf.bprintf buf "error(%s)" s
+
+let print pp_x fmt e = match e with
+  | `Ok x -> Format.fprintf fmt "ok(%a)" pp_x x
+  | `Error s -> Format.fprintf fmt "error(%s)" s
diff --git a/core/CCError.mli b/core/CCError.mli
new file mode 100644
index 00000000..76a49065
--- /dev/null
+++ b/core/CCError.mli
@@ -0,0 +1,82 @@
+
+(*
+copyright (c) 2013-2014, 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 Error Monad} *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a equal = 'a -> 'a -> bool
+type 'a ord = 'a -> 'a -> int
+type 'a printer = Buffer.t -> 'a -> unit
+type 'a formatter = Format.formatter -> 'a -> unit
+
+(** {2 Basics} *)
+
+type +'a t =
+  [ `Ok of 'a
+  | `Error of string
+  ]
+
+val return : 'a -> 'a t
+
+val fail : string -> 'a t
+
+val of_exn : exn -> 'a t
+
+val map : ('a -> 'b) -> 'a t -> 'b t
+
+val flat_map : ('a -> 'b t) -> 'a t -> 'b t
+
+val guard : (unit -> 'a) -> 'a t
+
+val (>|=) : 'a t -> ('a -> 'b) -> 'b t
+
+val (>>=) : 'a t -> ('a -> 'b t) -> 'b t
+
+val equal : 'a equal -> 'a t equal
+
+val compare : 'a ord -> 'a t ord
+
+(** {2 Collections} *)
+
+val map_l : ('a -> 'b t) -> 'a list -> 'b list t
+
+val fold_l : ('acc -> 'a -> 'acc t) -> 'acc -> 'a list -> 'acc t
+
+val fold_seq : ('acc -> 'a -> 'acc t) -> 'acc -> 'a sequence -> 'acc t
+
+(** {2 Conversions} *)
+
+val to_opt : 'a t -> 'a option
+
+val of_opt : 'a option -> 'a t
+
+val to_seq : 'a t -> 'a sequence
+
+(** {2 IO} *)
+
+val pp : 'a printer -> 'a t printer
+
+val print : 'a formatter -> 'a t formatter