diff --git a/core/CCError.ml b/core/CCError.ml
index 8418f21f..da7f7888 100644
--- a/core/CCError.ml
+++ b/core/CCError.ml
@@ -49,6 +49,10 @@ let map f e = match e with
   | `Ok x -> `Ok (f x)
   | `Error s -> `Error s
 
+let map2 f g e = match e with
+  | `Ok x -> `Ok (f x)
+  | `Error s -> `Error (g s)
+
 let flat_map f e = match e with
   | `Ok x -> f x
   | `Error s -> `Error s
diff --git a/core/CCError.mli b/core/CCError.mli
index 49462101..61e8e98a 100644
--- a/core/CCError.mli
+++ b/core/CCError.mli
@@ -47,6 +47,10 @@ val of_exn : exn -> 'a t
 
 val map : ('a -> 'b) -> 'a t -> 'b t
 
+val map2 : ('a -> 'b) -> (string -> string) -> 'a t -> 'b t
+(** Same as {!map}, but also with a function that can transform
+    the error message in case of failure *)
+
 val flat_map : ('a -> 'b t) -> 'a t -> 'b t
 
 val guard : (unit -> 'a) -> 'a t