From 67a40a488271c2cd991bf889529bd40576ad14f6 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Mon, 26 Dec 2016 00:51:45 +0100
Subject: [PATCH] distinction `diagonal,diagonal_l`

---
 src/sequence.ml        | 15 ++++++++++++---
 src/sequence.mli       |  9 +++++++--
 src/sequenceLabels.mli |  9 +++++++--
 3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/src/sequence.ml b/src/sequence.ml
index 1e38e3b..425bf8d 100644
--- a/src/sequence.ml
+++ b/src/sequence.ml
@@ -463,14 +463,23 @@ let product outer inner k =
 let product2 outer inner k =
   outer (fun x -> inner (fun y -> k x y))
 
-let rec diagonal l yield = match l with
+let rec diagonal_l l yield = match l with
   | [] -> ()
   | x::tail ->
     List.iter (fun y -> yield (x,y)) tail;
-    diagonal tail yield
+    diagonal_l tail yield
 
 (*$=
-  [0,1; 0,2; 1,2] (diagonal [0;1;2] |> to_list)
+  [0,1; 0,2; 1,2] (diagonal_l [0;1;2] |> to_list)
+  *)
+
+let diagonal seq =
+  let l = ref [] in
+  seq (fun x -> l := x :: !l);
+  diagonal_l (List.rev !l)
+
+(*$=
+  [0,1; 0,2; 1,2] (of_list [0;1;2] |> diagonal |> to_list)
   *)
 
 let join ~join_row s1 s2 k =
diff --git a/src/sequence.mli b/src/sequence.mli
index bfc8ac1..ce7d276 100644
--- a/src/sequence.mli
+++ b/src/sequence.mli
@@ -251,9 +251,14 @@ val product : 'a t -> 'b t -> ('a * 'b) t
     as required (several times), possibly by calling {!persistent} on it
     beforehand. *)
 
-val diagonal : 'a list -> ('a * 'a) t
+val diagonal_l : 'a list -> ('a * 'a) t
 (** All pairs of distinct positions of the list. [diagonal l] will
-    return the list of [List.nth i l, List.nth j l] if [i < j].
+    return the sequence of all [List.nth i l, List.nth j l] if [i < j].
+    @since NEXT_RELEASE *)
+
+val diagonal : 'a t -> ('a * 'a) t
+(** All pairs of distinct positions of the sequence.
+    Iterates only once on the sequence, which must be finite.
     @since NEXT_RELEASE *)
 
 val product2 : 'a t -> 'b t -> ('a, 'b) t2
diff --git a/src/sequenceLabels.mli b/src/sequenceLabels.mli
index 6ef97f3..dd80e73 100644
--- a/src/sequenceLabels.mli
+++ b/src/sequenceLabels.mli
@@ -222,9 +222,14 @@ val product : 'a t -> 'b t -> ('a * 'b) t
     as required (several times), possibly by calling {!persistent} on it
     beforehand. *)
 
-val diagonal : 'a list -> ('a * 'a) t
+val diagonal_l : 'a list -> ('a * 'a) t
 (** All pairs of distinct positions of the list. [diagonal l] will
-    return the list of [List.nth i l, List.nth j l] if [i < j].
+    return the sequence of all [List.nth i l, List.nth j l] if [i < j].
+    @since NEXT_RELEASE *)
+
+val diagonal : 'a t -> ('a * 'a) t
+(** All pairs of distinct positions of the sequence.
+    Iterates only once on the sequence, which must be finite.
     @since NEXT_RELEASE *)
 
 val product2 : 'a t -> 'b t -> ('a, 'b) t2