CCList: add reduce function (resolves #305)

src/core/CCList.ml

@@ -265,6 +265,27 @@ let scan_left f acc l =
   in
   aux f acc [acc] l
 
+let reduce f = function
+  | [] -> None
+  | x :: l -> Some (fold_left f x l)
+
+let reduce_exn f = function
+  | [] -> raise (Invalid_argument "CCList.reduce_exn")
+  | x :: l -> fold_left f x l
+
+(*$= & ~printer:Q.Print.(option int)
+  (Some 15) (reduce (+) [1; 2; 3; 4; 5])
+  (Some 3) (reduce CCInt.min [5; 3; 8; 9])
+*)
+
+(*$= & ~printer:Q.Print.string
+  "hello world" (reduce_exn (^) ["hello"; " "; "world"])
+*)
+
+(*$T 
+  try ignore (reduce_exn (+.) []); false with Invalid_argument _ -> true
+*)
+
 (*$= & ~printer:Q.Print.(list int)
   [0;1;3;6] (scan_left (+) 0 [1;2;3])
   [0] (scan_left (+) 0 [])

src/core/CCList.mli

@@ -81,6 +81,16 @@ val scan_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a list -> 'acc list
     @since 1.2, but only
     @since 2.2 with labels *)
 
+val reduce : ('a -> 'a -> 'a) -> 'a list -> 'a option
+(** [reduce f (hd::tl)] returns [Some (fold_left f hd tl)].  If [l] is empty,
+    then [None] is returned.
+    @since NEXT_RELEASE *)
+
+val reduce_exn : ('a -> 'a -> 'a) -> 'a list -> 'a
+(** [reduce_exn] is the unsafe version of {!reduce}.
+    @raise Invalid_argument if the given list is empty.
+    @since NEXT_RELEASE *)
+
 val fold_map2 : ('acc -> 'a -> 'b -> 'acc * 'c) -> 'acc -> 'a list -> 'b list -> 'acc * 'c list
 (** [fold_map2 f init l1 l2] is to [fold_map] what [List.map2] is to [List.map].
     @raise Invalid_argument if the lists do not have the same length.

src/core/CCListLabels.mli

@@ -85,6 +85,16 @@ val scan_left : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a list -> 'acc list
     @since 1.2, but only
     @since 2.2 with labels *)
 
+val reduce : f:('a -> 'a -> 'a) -> 'a list -> 'a option
+(** [reduce f (hd::tl)] returns [Some (fold_left f hd tl)].  If [l] is empty,
+    then [None] is returned.
+    @since NEXT_RELEASE *)
+
+val reduce_exn : f:('a -> 'a -> 'a) -> 'a list -> 'a
+(** [reduce_exn] is the unsafe version of {!reduce}.
+    @raise Invalid_argument if the given list is empty.
+    @since NEXT_RELEASE *)
+
 val fold_map2 : f:('acc -> 'a -> 'b -> 'acc * 'c) -> init:'acc -> 'a list -> 'b list -> 'acc * 'c list
 (** [fold_map2 ~f ~init l1 l2] is to [fold_map] what [List.map2] is to [List.map].
     @raise Invalid_argument if the lists do not have the same length.