From e0a95a3c3104d0e9a68059da63f3e481e1a7d234 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Mon, 7 Jul 2014 17:12:49 +0200
Subject: [PATCH] cons, snoc, take_while, drop_while, doubleton

---
 sequence.ml  | 18 +++++++++++++++++-
 sequence.mli | 18 +++++++++++++++++-
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/sequence.ml b/sequence.ml
index 496ea08..7973aa7 100644
--- a/sequence.ml
+++ b/sequence.ml
@@ -52,6 +52,11 @@ let singleton x k = k x
 let return x k = k x
 let pure f k = k f
 
+let doubleton x y k = k x; k y
+
+let cons x l k = k x; l k
+let snoc l x k = l k; k x
+
 (** Infinite sequence of the same element *)
 let repeat x = fun k -> while true do k x done
 
@@ -381,12 +386,24 @@ let take n seq =
           if !count = n then raise ExitSequence)
       with ExitSequence -> ()
 
+let take_while p seq k =
+  try
+    seq (fun x -> if p x then k x else raise ExitSequence)
+  with ExitSequence -> ()
+
 (** Drop the [n] first elements of the sequence *)
 let drop n seq =
   let count = ref 0 in
   fun k -> seq
     (fun x -> if !count >= n then k x else incr count)
 
+let drop_while p seq k =
+  let drop = ref true in
+  seq (fun x ->
+    if !drop
+    then if p x then () else (drop := false; k x)
+    else k x)
+
 (** Reverse the sequence. O(n) memory. *)
 let rev seq =
   let l = MList.of_seq seq in
@@ -476,7 +493,6 @@ let map2_2 f g seq2 =
 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
-  (** Get the list of the reversed sequence (more efficient) *)
 
 let of_list l = from_iter (fun k -> List.iter k l)
 
diff --git a/sequence.mli b/sequence.mli
index 7b3af7a..767e913 100644
--- a/sequence.mli
+++ b/sequence.mli
@@ -76,6 +76,16 @@ val empty : 'a t
 val singleton : 'a -> 'a t
   (** Singleton sequence, with exactly one element. *)
 
+val doubleton : 'a -> 'a -> 'a t
+  (** Sequence with exactly two elements *)
+
+val cons : 'a -> 'a t -> 'a t
+  (** [cons x l] yields [x], then yields from [l].
+      Same as [append (singleton x) l] *)
+
+val snoc : 'a t -> 'a -> 'a t
+  (** Same as {!cons} but yields the element after iterating on [l] *)
+
 val return : 'a -> 'a t
   (** Synonym to {!singleton} *)
 
@@ -84,7 +94,7 @@ val pure : 'a -> 'a t
 
 val repeat : 'a -> 'a t
   (** Infinite sequence of the same element. You may want to look
-      at {!take} if you iterate on it. *)
+      at {!take} and the likes if you iterate on it. *)
 
 val iterate : ('a -> 'a) -> 'a -> 'a t
   (** [iterate f x] is the infinite sequence (x, f(x), f(f(x)), ...) *)
@@ -247,9 +257,15 @@ val take : int -> 'a t -> 'a t
   (** Take at most [n] elements from the sequence. Works on infinite
       sequences. *)
 
+val take_while : ('a -> bool) -> 'a t -> 'a t
+  (** Take elements while they satisfy the predicate, then stops iterating *)
+
 val drop : int -> 'a t -> 'a t
   (** Drop the [n] first elements of the sequence. Lazy. *)
 
+val drop_while : ('a -> bool) -> 'a t -> 'a t
+  (** Predicate version of {!drop} *)
+
 val rev : 'a t -> 'a t
   (** Reverse the sequence. O(n) memory and time, needs the
       sequence to be finite. The result is persistent and does