Add convenience pretty printer for Either.t (added in 4.12)

stdlib/format.ml

@@ -1237,6 +1237,10 @@ let pp_print_result ~ok ~error ppf = function
 | Ok v -> ok ppf v
 | Error e -> error ppf e
 
+let pp_print_either ~left ~right ppf = function
+| Either.Left l -> left ppf l
+| Either.Right r -> right ppf r
+
  (**************************************************************)
 
 let compute_tag output tag_acc =

stdlib/format.mli

@@ -1120,6 +1120,12 @@ val pp_print_result :
 
     @since 4.08 *)
 
+val pp_print_either :
+  left:(formatter -> 'a -> unit) ->
+  right:(formatter -> 'b -> unit) -> formatter -> ('a, 'b) Either.t -> unit
+(** [pp_print_either ~left ~right ppf e] prints [e] on [ppf] using
+    [left] if [e] is [Either.Left _] and [right] if [e] is [Either.Right _]. *)
+
 (** {1:fpp Formatted pretty-printing} *)
 
 (**