PR#5644: a dumbed-down fix to fix the observable effect (Stream.of_string) with no change to the underlying code (to avoid regressions)

As I learned the hard way, it is essentially impossible to get
a satisfying behavior in presence of a mix of updates, concatenations
and counts. The best thing to do is thus to not change anything
(a good way to preserve compatibility), and only fix the
Stream.of_string function to assume nothing of the count passed to the
[from] function.

I also modified the mli documentation of [Stream.from] to warn other
users of this potential pitfall.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13915 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

2 files changed, 18 insertions, 2 deletions

diff --git a/stdlib/stream.ml b/stdlib/stream.ml
index 0a4af3d6e..753bce005 100644
--- a/stdlib/stream.ml
+++ b/stdlib/stream.ml
@@ -145,7 +145,18 @@ let of_list l =
 ;;
 
 let of_string s =
-  from (fun c -> if c < String.length s then Some s.[c] else None)
+  let count = ref 0 in
+  from (fun _ ->
+    (* We cannot use the index passed by the [from] function directly
+       because it returns the current stream count, with absolutely no
+       guarantee that it will start from 0. For example, in the case
+       of [Stream.icons 'c' (Stream.from_string "ab")], the first
+       access to the string will be made with count [1] already.
+    *)
+    let c = !count in
+    if c < String.length s
+    then (incr count; Some s.[c])
+    else None)
 ;;
 
 let of_channel ic =
diff --git a/stdlib/stream.mli b/stdlib/stream.mli
index 1098a2765..aeb0da1e8 100644
--- a/stdlib/stream.mli
+++ b/stdlib/stream.mli
@@ -32,7 +32,12 @@ val from : (int -> 'a option) -> 'a t
    To create a new stream element, the function [f] is called with
    the current stream count. The user function [f] must return either
    [Some <value>] for a value or [None] to specify the end of the
-   stream. *)
+   stream.
+
+   Do note that the indices passed to [f] may not start at [0] in the
+   general case. For example, [[< '0; '1; Stream.from f >]] would call
+   [f] the first time with count [2].
+*)
 
 val of_list : 'a list -> 'a t
 (** Return the stream holding the elements of the list in the same