options: to_opt/of_opt/head/head_exn

sequence.ml

@@ -321,6 +321,17 @@ let min ?(lt=fun x y -> x < y) seq =
 
 exception ExitSequence
 
+let head seq =
+  let r = ref None in
+  try
+    seq (fun x -> r := Some x; raise ExitSequence); None
+  with ExitSequence -> !r
+
+let head_exn seq =
+  match head seq with
+  | None -> invalid_arg "Sequence.head_exn"
+  | Some x -> x
+
 let take n seq k =
   let count = ref 0 in
   try
@@ -425,6 +436,12 @@ let to_list seq = List.rev (fold (fun y x -> x::y) [] seq)
 
 let to_rev_list seq = fold (fun y x -> x :: y) [] seq
 
+let to_opt = head
+
+let of_opt o k = match o with
+  | None -> ()
+  | Some x -> k x
+
 let of_list l k = List.iter k l
 
 let to_array seq =

sequence.mli

@@ -254,6 +254,15 @@ val min : ?lt:('a -> 'a -> bool) -> 'a t -> 'a option
   (** Min element of the sequence, using the given comparison function.
       see {!max} for more details. *)
 
+val head : 'a t -> 'a option
+  (** First element, if any, otherwise [None]
+      @since NEXT_RELEASE *)
+
+val head_exn : 'a t -> 'a
+  (** First element, if any, fails
+      @raise Invalid_argument if the sequence is empty
+      @since NEXT_RELEASE *)
+
 val take : int -> 'a t -> 'a t
   (** Take at most [n] elements from the sequence. Works on infinite
       sequences. *)
@@ -310,6 +319,10 @@ val to_rev_list : 'a t -> 'a list
 
 val of_list : 'a list -> 'a t
 
+val to_opt : 'a t -> 'a option
+  (** Alias to {!head}
+      @since NEXT_RELEASE *)
+
 val to_array : 'a t -> 'a array
   (** Convert to an array. Currently not very efficient because
       an intermediate list is used. *)
@@ -325,6 +338,10 @@ val array_slice : 'a array -> int -> int -> 'a t
   (** [array_slice a i j] Sequence of elements whose indexes range
       from [i] to [j] *)
 
+val of_opt : 'a option -> 'a t
+  (** Iterate on 0 or 1 values.
+      @since NEXT_RELEASE *)
+
 val of_stream : 'a Stream.t -> 'a t
   (** Sequence of elements of a stream (usable only once) *)