-val set_string: key:string -> value:string -> unit
-val set_int: key:string -> value:int -> unit
-val set_float: key:string -> value:float -> unit
-val set_string_list: key:string -> value:string list -> unit
+val set_string: key:string -> value:string -> unit
+val set_int: key:string -> value:int -> unit
+val set_float: key:string -> value:float -> unit
+val set_bool: key:string -> value:bool -> unit
+val set_string_list: key:string -> value:string list -> unit
+
+(** {3 Optional values interface}
+ * Functions below took as first argument respectively a "getter" and a "setter"
+ * function. A getter is one of the get_* function above, a setter is one of the
+ * set_* function above. Returned value is a get (set) function typed as the
+ * given getter (setter) whith optional values. None is returned for missing
+ * keys and None can be assigned to a key removing it from the registry.
+ *
+ * Sample usage:
+ *
+ * match Helm_registry.get_opt Helm_registry.get_int "foo.bar" with
+ * | Some i -> ...
+ * | None -> ...
+ *)
+
+val get_opt:
+ (string -> 'a) (* getter *) ->
+ string -> 'a option
+val set_opt:
+ (key:string -> value:'a -> unit) (* setter *) ->
+ key:string -> value:'a option -> unit
+val get_opt_default: (* as get_opt with an additional default value *)
+ (string -> 'a) -> 'a -> string -> 'a