Add CCResult.iter_err

Closes #238

AUTHORS.adoc

@@ -33,3 +33,4 @@
 - Metin Akat (@loxs)
 - Francois Berenger (@UnixJunkie)
 - Hongchang Wu (@hongchangwu)
+- Nathan Rebours (@NathanReb)

src/core/CCResult.ml

@@ -83,6 +83,19 @@ let iter f e = match e with
   | Ok x -> f x
   | Error _ -> ()
 
+let iter_err f e = match e with
+  | Ok _ -> ()
+  | Error err -> f err
+
+(*$R iter_err
+  let called_with = ref None in
+  let f e = called_with := Some e in
+  iter_err f (Ok 1);
+  assert_bool "should not apply when Ok" (!called_with = None);
+  iter_err f (Error 1);
+  assert_bool "should apply f to Error" (!called_with = Some 1)
+*)
+
 exception Get_error
 
 let get_exn = function

src/core/CCResult.mli

@@ -73,6 +73,10 @@ val map2 : ('a -> 'b) -> ('err1 -> 'err2) -> ('a, 'err1) t -> ('b, 'err2) t
 val iter : ('a -> unit) -> ('a, _) t -> unit
 (** Apply the function only in case of [Ok]. *)
 
+val iter_err : ('err -> unit) -> (_, 'err) t -> unit
+(** Apply the function in case of [Error].
+    @since NEXT_RELEASE *)
+
 exception Get_error
 
 val get_exn : ('a, _) t -> 'a