prepare for 1.9

Signed-off-by: Josh Berdine <josh@berdine.net>

.github/workflows/gh-pages.yml

@@ -3,7 +3,7 @@ name: github pages
 on:
   push:
     branches:
-      - master  # Set a branch name to trigger deployment
+      - main # Set a branch name to trigger deployment
 
 jobs:
   deploy:
@@ -11,16 +11,10 @@ jobs:
     steps:
       - uses: actions/checkout@main
 
-      - name: Cache opam
-        id: cache-opam
-        uses: actions/cache@v2
+      - uses: ocaml/setup-ocaml@v3
         with:
-          path: ~/.opam
-          key: opam-ubuntu-latest-4.12.0
-
-      - uses: avsm/setup-ocaml@v1
-        with:
-          ocaml-version: '4.12.0'
+          ocaml-compiler: '4.14.x'
+          allow-prerelease-opam: true
 
       - name: Pin
         run: opam pin -n .

.github/workflows/main.yml

@@ -2,10 +2,8 @@ name: build
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
-    branches:
-      - master
 jobs:
   run:
     name: Build
@@ -17,15 +15,16 @@ jobs:
             # macOS is just too slow, ugh
           #- macos-latest
         ocaml-compiler:
-          - 4.03.x
           - 4.08.x
-          - 4.12.x
+          - 4.14.x
+            # TODO: - 5.1.x #  https://github.com/ocaml/setup-ocaml/issues/733
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
-    - uses: ocaml/setup-ocaml@v2
+    - uses: ocaml/setup-ocaml@v3
       with:
         ocaml-compiler: ${{ matrix.ocaml-compiler }}
+        allow-prerelease-opam: true
     - run: opam pin -n .
     - run: opam depext -yt iter
     - run: opam install -t . --deps-only

.ocamlformat

@@ -1,4 +1,4 @@
-version = 0.24.1
+version = 0.26.2
 profile=conventional
 margin=80
 if-then-else=k-r

CHANGELOG.md

@@ -1,4 +1,22 @@
 
+# 1.9
+
+
+- Switch exceptions used for control flow from global to local
+- Update Iter.ml to use mutable fields instead of refs (#44)
+
+# 1.8
+
+- add `Iter.map_while`
+- fix bug in `map_by_2`, add tests
+
+# 1.7
+
+- add let operators in Infix
+- require OCaml >= 4.08
+- improve docs about random iterators
+- fix: `IO.write_lines` should produce an empty file for an empty iter
+
 # 1.6
 
 - use dune 2.0

benchs/dune

@@ -2,6 +2,7 @@
  (names bench_persistent_read bench_persistent benchs)
  (libraries iter benchmark)
  (optional)
- (flags :standard -w +a-4-42-44-48-50-58-32-60@8 -safe-string -color always)
+ (flags :standard -w +a-4-42-44-48-50-58-32-60-70@8 -safe-string -color
+   always)
  (ocamlopt_flags :standard -O3 -color always -unbox-closures
    -unbox-closures-factor 20))

examples/dune

@@ -1,6 +1,7 @@
 (executable
  (name test_sexpr)
  (libraries iter)
- (flags :standard -w +a-4-42-44-48-50-58-32-60@8 -safe-string -color always)
+ (flags :standard -w +a-4-42-44-48-50-58-32-60-70@8 -safe-string -color
+   always)
  (ocamlopt_flags :standard -O3 -color always -unbox-closures
    -unbox-closures-factor 20))

iter.opam

@@ -1,6 +1,6 @@
 opam-version: "2.0"
 name: "iter"
-version: "1.6"
+version: "1.9"
 authors: ["Simon Cruanes" "Gabriel Radanne"]
 maintainer: "simon.cruanes.2007@m4x.org"
 license: "BSD-2-clause"
@@ -11,12 +11,8 @@ build: [
   ["dune" "runtest" "-p" name "-j" jobs] {with-test & arch != "arm32" & arch != "x86_32"}
 ]
 depends: [
-  "base-bytes"
-  "result"
-  "seq"
-  "ocaml" { >= "4.03.0" }
+  "ocaml" { >= "4.08.0" }
   "dune" { >= "2.0" }
-  "dune-configurator"
   "qcheck-core" {with-test}
   "ounit2" {with-test}
   "mdx" {with-test & >= "1.3" }

src/Iter.ml

@@ -1,9 +1,5 @@
 (* This file is free software, part of iter. See file "license" for more details. *)
 
-(** {1 Simple and Efficient Iterators} *)
-
-open Iter_shims_
-
 type 'a t = ('a -> unit) -> unit
 (** Iter abstract iterator type *)
 
@@ -118,7 +114,9 @@ let map_by_2 f seq k =
   let f y =
     match !r with
     | None -> r := Some y
-    | Some x -> k (f x y)
+    | Some x ->
+      r := None;
+      k (f x y)
   in
   seq f;
   match !r with
@@ -191,63 +189,75 @@ let keep_error seq k =
 
 (** Mutable unrolled list to serve as intermediate storage *)
 module MList = struct
-  type 'a node = Nil | Cons of 'a array * int ref * 'a node ref
+  type 'a node =
+    | Nil
+    | Cons of { a: 'a array; mutable n: int; mutable tl: 'a node }
 
   (* build and call callback on every element *)
   let of_iter_with seq k =
-    let start = ref Nil in
     let chunk_size = ref 8 in
-    (* fill the list. prev: tail-reference from previous node *)
-    let prev, cur = ref start, ref Nil in
+    let acc = ref Nil in
+    let cur = ref Nil in
+    let tail = ref Nil in
+
+    let[@inline] replace_tail () =
+      match !acc with
+      | Nil -> acc := !cur
+      | _ ->
+        (match !tail with
+        | Nil -> ()
+        | Cons r -> r.tl <- !cur)
+    in
+
     seq (fun x ->
         k x;
         (* callback *)
         match !cur with
         | Nil ->
           let n = !chunk_size in
-          if n < 4096 then chunk_size := 2 * !chunk_size;
-          cur := Cons (Array.make n x, ref 1, ref Nil)
-        | Cons (a, n, next) ->
-          assert (!n < Array.length a);
-          a.(!n) <- x;
-          incr n;
-          if !n = Array.length a then (
-            !prev := !cur;
-            prev := next;
+          if n < 4096 then chunk_size := 2 * n;
+          cur := Cons { a = Array.make n x; n = 1; tl = Nil }
+        | Cons r ->
+          assert (r.n < Array.length r.a);
+          r.a.(r.n) <- x;
+          r.n <- succ r.n;
+          if r.n = Array.length r.a then (
+            replace_tail ();
+            tail := !cur;
             cur := Nil
           ));
-    !prev := !cur;
-    !start
+    replace_tail ();
+    !acc
 
   let of_iter seq = of_iter_with seq (fun _ -> ())
 
   let rec iter f l =
     match l with
     | Nil -> ()
-    | Cons (a, n, tl) ->
-      for i = 0 to !n - 1 do
+    | Cons { a; n; tl } ->
+      for i = 0 to n - 1 do
         f a.(i)
       done;
-      iter f !tl
+      iter f tl
 
   let iteri f l =
     let rec iteri i f l =
       match l with
       | Nil -> ()
-      | Cons (a, n, tl) ->
-        for j = 0 to !n - 1 do
+      | Cons { a; n; tl } ->
+        for j = 0 to n - 1 do
           f (i + j) a.(j)
         done;
-        iteri (i + !n) f !tl
+        iteri (i + n) f tl
     in
     iteri 0 f l
 
   let rec iter_rev f l =
     match l with
     | Nil -> ()
-    | Cons (a, n, tl) ->
-      iter_rev f !tl;
-      for i = !n - 1 downto 0 do
+    | Cons { a; n; tl } ->
+      iter_rev f tl;
+      for i = n - 1 downto 0 do
         f a.(i)
       done
 
@@ -255,7 +265,7 @@ module MList = struct
     let rec len acc l =
       match l with
       | Nil -> acc
-      | Cons (_, n, tl) -> len (acc + !n) !tl
+      | Cons { n; tl; _ } -> len (acc + n) tl
     in
     len 0 l
 
@@ -263,8 +273,8 @@ module MList = struct
   let rec get l i =
     match l with
     | Nil -> raise (Invalid_argument "MList.get")
-    | Cons (a, n, _) when i < !n -> a.(i)
-    | Cons (_, n, tl) -> get !tl (i - !n)
+    | Cons { a; n; _ } when i < n -> a.(i)
+    | Cons { n; tl; _ } -> get tl (i - n)
 
   let to_iter l k = iter k l
 
@@ -275,11 +285,11 @@ module MList = struct
     let rec get_next _ =
       match !cur with
       | Nil -> None
-      | Cons (_, n, tl) when !i = !n ->
-        cur := !tl;
+      | Cons { n; tl; _ } when !i = n ->
+        cur := tl;
         i := 0;
         get_next arg
-      | Cons (a, _, _) ->
+      | Cons { a; _ } ->
         let x = a.(!i) in
         incr i;
         Some x
@@ -292,8 +302,8 @@ module MList = struct
     let rec make (l, i) () =
       match l with
       | Nil -> Seq.Nil
-      | Cons (_, n, tl) when i = !n -> make (!tl, 0) ()
-      | Cons (a, _, _) -> Seq.Cons (a.(i), make (l, i + 1))
+      | Cons { n; tl; _ } when i = n -> make (tl, 0) ()
+      | Cons { a; _ } -> Seq.Cons (a.(i), make (l, i + 1))
     in
     make (l, 0)
 end
@@ -320,9 +330,8 @@ let sort ?(cmp = Stdlib.compare) seq =
   let l = List.fast_sort cmp l in
   fun k -> List.iter k l
 
-exception Exit_sorted
-
 let sorted ?(cmp = Stdlib.compare) seq =
+  let exception Exit_sorted in
   let prev = ref None in
   try
     seq (fun x ->
@@ -546,9 +555,8 @@ let diff (type a) ?(eq = ( = )) ?(hash = Hashtbl.hash) c1 c2 =
   c2 (fun x -> Tbl.replace tbl x ());
   fun yield -> c1 (fun x -> if not (Tbl.mem tbl x) then yield x)
 
-exception Subset_exit
-
 let subset (type a) ?(eq = ( = )) ?(hash = Hashtbl.hash) c1 c2 =
+  let exception Subset_exit in
   let module Tbl = Hashtbl.Make (struct
     type t = a
 
@@ -620,9 +628,8 @@ let sumf seq : float =
       sum := t);
   !sum
 
-exception ExitHead
-
 let head seq =
+  let exception ExitHead in
   let r = ref None in
   try
     seq (fun x ->
@@ -636,9 +643,8 @@ let head_exn seq =
   | None -> invalid_arg "Iter.head_exn"
   | Some x -> x
 
-exception ExitTake
-
 let take n seq k =
+  let exception ExitTake in
   let count = ref 0 in
   try
     seq (fun x ->
@@ -647,9 +653,8 @@ let take n seq k =
         k x)
   with ExitTake -> ()
 
-exception ExitTakeWhile
-
 let take_while p seq k =
+  let exception ExitTakeWhile in
   try
     seq (fun x ->
         if p x then
@@ -658,9 +663,20 @@ let take_while p seq k =
           raise_notrace ExitTakeWhile)
   with ExitTakeWhile -> ()
 
-exception ExitFoldWhile
+let map_while f seq k =
+  let exception ExitMapWhile in
+  let consume x =
+    match f x with
+    | `Yield y -> k y
+    | `Return y ->
+      k y;
+      raise_notrace ExitMapWhile
+    | `Stop -> raise_notrace ExitMapWhile
+  in
+  try seq consume with ExitMapWhile -> ()
 
 let fold_while f s seq =
+  let exception ExitFoldWhile in
   let state = ref s in
   let consume x =
     let acc, cont = f !state x in
@@ -699,18 +715,16 @@ let rev seq =
   let l = MList.of_iter seq in
   fun k -> MList.iter_rev k l
 
-exception ExitForall
-
 let for_all p seq =
+  let exception ExitForall in
   try
     seq (fun x -> if not (p x) then raise_notrace ExitForall);
     true
   with ExitForall -> false
 
-exception ExitExists
-
 (** Exists there some element satisfying the predicate? *)
 let exists p seq =
+  let exception ExitExists in
   try
     seq (fun x -> if p x then raise_notrace ExitExists);
     false
@@ -718,9 +732,8 @@ let exists p seq =
 
 let mem ?(eq = ( = )) x seq = exists (eq x) seq
 
-exception ExitFind
-
 let find_map f seq =
+  let exception ExitFind in
   let r = ref None in
   (try
      seq (fun x ->
@@ -735,6 +748,7 @@ let find_map f seq =
 let find = find_map
 
 let find_mapi f seq =
+  let exception ExitFind in
   let i = ref 0 in
   let r = ref None in
   (try
@@ -768,9 +782,8 @@ let[@inline] length seq =
   seq (fun _ -> incr r);
   !r
 
-exception ExitIsEmpty
-
 let is_empty seq =
+  let exception ExitIsEmpty in
   try
     seq (fun _ -> raise_notrace ExitIsEmpty);
     true
@@ -1018,9 +1031,14 @@ module Map = struct
     let of_iter_ seq = fold (fun m (k, v) -> M.add k v m) M.empty seq
     let keys m = from_iter (fun k -> M.iter (fun x _ -> k x) m)
     let values m = from_iter (fun k -> M.iter (fun _ y -> k y) m)
+
+    [@@@ocaml.warning "-32"]
+
     let of_list l = of_iter_ (of_list l)
     let to_list x = to_list (to_iter_ x)
 
+    [@@@ocaml.warning "+32"]
+
     include M
 
     let to_iter = to_iter_
@@ -1119,6 +1137,10 @@ module Infix = struct
   let[@inline] ( >|= ) x f = map f x
   let[@inline] ( <*> ) funs args k = funs (fun f -> args (fun x -> k (f x)))
   let ( <+> ) = append
+  let[@inline] ( let+ ) x f = map f x
+  let[@inline] ( let* ) x f = flat_map f x
+  let ( and+ ) = product
+  let ( and* ) = product
 end
 
 include Infix
@@ -1192,22 +1214,28 @@ module IO = struct
       close_in_noerr ic;
       raise e
 
-  let write_bytes_to ?(mode = 0o644) ?(flags = [ Open_creat; Open_wronly ])
-      filename seq =
+  let with_out_ ?(mode = 0o644) ?(flags = [ Open_creat; Open_wronly ]) filename
+      f =
     let oc = open_out_gen flags mode filename in
     try
-      seq (fun s -> output oc s 0 (Bytes.length s));
+      f oc;
       close_out oc
     with e ->
       close_out oc;
       raise e
 
+  let write_bytes_to ?mode ?flags filename it =
+    with_out_ ?mode ?flags filename (fun oc ->
+        it (fun s -> output oc s 0 (Bytes.length s)))
+
   let write_to ?mode ?flags filename seq =
     write_bytes_to ?mode ?flags filename (map Bytes.unsafe_of_string seq)
 
-  let write_bytes_lines ?mode ?flags filename seq =
-    let ret = Bytes.unsafe_of_string "\n" in
-    write_bytes_to ?mode ?flags filename (snoc (intersperse ret seq) ret)
+  let write_bytes_lines ?mode ?flags filename it =
+    with_out_ ?mode ?flags filename (fun oc ->
+        it (fun s ->
+            output oc s 0 (Bytes.length s);
+            output_char oc '\n'))
 
   let write_lines ?mode ?flags filename seq =
     write_bytes_lines ?mode ?flags filename (map Bytes.unsafe_of_string seq)

src/Iter.mli

@@ -259,11 +259,11 @@ val keep_some : 'a option t -> 'a t
     Same as [filter_map (fun x->x)]
     @since 1.0 *)
 
-val keep_ok : ('a, _) Result.result t -> 'a t
+val keep_ok : ('a, _) result t -> 'a t
 (** [keep_ok l] retains only elements of the form [Ok x].
     @since 1.0 *)
 
-val keep_error : (_, 'e) Result.result t -> 'e t
+val keep_error : (_, 'e) result t -> 'e t
 (** [keep_error l] retains only elements of the form [Error x].
     @since 1.0 *)
 
@@ -487,6 +487,18 @@ val take_while : ('a -> bool) -> 'a t -> 'a t
     Will work on an infinite iterator [s] if the predicate is false for at
     least one element of [s]. *)
 
+val map_while : ('a -> [ `Yield of 'b | `Return of 'b | `Stop ]) -> 'a t -> 'b t
+(** Maps over elements of the iterator, stopping early if the mapped function
+    returns [`Stop] or [`Return x]. At each iteration:
+    {ul
+      {- If [f] returns [`Yield y], [y] is added to the sequence and the
+         iteration continues.}
+      {- If [f] returns [`Stop], nothing is added to the sequence and the
+         iteration stops.}
+      {- If [f] returns [`Return y], [y] is added to the sequence and the
+         iteration stops.}}
+    @since 1.8 *)
+
 val fold_while : ('a -> 'b -> 'a * [ `Stop | `Continue ]) -> 'a -> 'b t -> 'a
 (** Folds over elements of the iterator, stopping early if the accumulator
     returns [('a, `Stop)]
@@ -707,6 +719,8 @@ end
 
 (** {1 Random iterators} *)
 
+(** {2 Generating} *)
+
 val random_int : int -> int t
 (** Infinite iterator of random integers between 0 and
     the given higher bound (see Random.int) *)
@@ -745,6 +759,26 @@ val sample : int -> 'a t -> 'a array
       It returns an array of size [min (length seq) n].
       @since 0.7 *)
 
+(** {2 Seeding}
+
+    Random iterators use [Random.int], [Random.float], [Random.bool],
+    etc., under the hood, so they will respect seeding of the random
+    generator in the usual way.  I.e., if you do not initialize the
+    random generator with one of [Random.init], [Random.full_init], or
+    [Random.self_init] before calling these functions, they will yield
+    the same values across seperate invocations of your program.
+
+    Example:
+
+    {[
+      (* Ensure a fresh random seed each time the program is executed. *)
+      let () = Random.self_init ()
+
+      (* Generate random values. *)
+      let l = Iter.random_int 1000 |> Iter.take 3 |> Iter.to_list
+    ]}
+    *)
+
 (** {1 Infix functions} *)
 
 module Infix : sig
@@ -772,6 +806,22 @@ module Infix : sig
   val ( <+> ) : 'a t -> 'a t -> 'a t
   (** Concatenation of iterators
       @since 0.5 *)
+
+  val ( let+ ) : 'a t -> ('a -> 'b) -> 'b t
+  (** Alias for {!map}
+      @since 1.7 *)
+
+  val ( and+ ) : 'a t -> 'b t -> ('a * 'b) t
+  (** Alias for {!product}
+      @since 1.7 *)
+
+  val ( let* ) : 'a t -> ('a -> 'b t) -> 'b t
+  (** Alias for {!flat_map}
+      @since 1.7 *)
+
+  val ( and* ) : 'a t -> 'b t -> ('a * 'b) t
+  (** Alias for {!product}
+      @since 1.7 *)
 end
 
 include module type of Infix

src/IterLabels.mli

@@ -227,11 +227,11 @@ val keep_some : 'a option t -> 'a t
     Same as [filter_map (fun x->x)]
     @since 1.0 *)
 
-val keep_ok : ('a, _) Result.result t -> 'a t
+val keep_ok : ('a, _) result t -> 'a t
 (** [keep_ok l] retains only elements of the form [Ok x].
     @since 1.0 *)
 
-val keep_error : (_, 'e) Result.result t -> 'e t
+val keep_error : (_, 'e) result t -> 'e t
 (** [keep_error l] retains only elements of the form [Error x].
     @since 1.0 *)
 
@@ -676,6 +676,8 @@ end
 
 (** {2 Random iterators} *)
 
+(** {3 Generating} *)
+
 val random_int : int -> int t
 (** Infinite iterator of random integers between 0 and
     the given higher bound (see Random.int) *)
@@ -714,6 +716,26 @@ val sample : n:int -> 'a t -> 'a array
       It returns an array of size [min (length seq) n].
       @since 0.7 *)
 
+(** {3 Seeding}
+
+    Random iterators use [Random.int], [Random.float], [Random.bool],
+    etc., under the hood, so they will respect seeding of the random
+    generator in the usual way.  I.e., if you do not initialize the
+    random generator with one of [Random.init], [Random.full_init], or
+    [Random.self_init] before calling these functions, they will yield
+    the same values across seperate invocations of your program.
+
+    Example:
+
+    {[
+      (* Ensure a fresh random seed each time the program is executed. *)
+      let () = Random.self_init ()
+
+      (* Generate random values. *)
+      let l = Iter.random_int 1000 |> Iter.take 3 |> Iter.to_list
+    ]}
+    *)
+
 (** {2 Infix functions} *)
 
 module Infix : sig

src/bigarray/IterBigarray.ml

@@ -2,7 +2,8 @@
 
 (** {1 Interface and Helpers for bigarrays} *)
 
-open! IterBigarrayShims_
+let bigarray_map_file fd ty lay b len =
+  Unix.map_file fd ty lay b [| len |] |> Bigarray.array1_of_genarray
 
 let of_bigarray b yield =
   let len = Bigarray.Array1.dim b in

src/bigarray/dune

@@ -2,19 +2,6 @@
  (name iter_bigarray)
  (public_name iter.bigarray)
  (libraries iter bigarray unix)
- (modules IterBigarray IterBigarrayShims_)
+ (modules IterBigarray)
  (wrapped false)
  (optional))
-
-(executable
- (name mkshims)
- (modules mkshims)
- (libraries dune.configurator))
-
-(rule
- (targets IterBigarrayShims_.ml)
- (deps mkshims.exe)
- (action
-  (with-stdout-to
-   %{targets}
-   (run ./mkshims.exe))))

src/bigarray/mkshims.ml

@@ -1,21 +0,0 @@
-module C = Configurator.V1
-
-let shims_pre_408 =
-  "\nopen! Bigarray\nlet bigarray_map_file = Bigarray.Array1.map_file\n"
-
-let shims_post_408 =
-  "\n\
-   let bigarray_map_file fd ty lay b len =\n\
-  \  Unix.map_file fd ty lay b [| len |] |> Bigarray.array1_of_genarray\n"
-
-let () =
-  C.main ~name:"mkshims" (fun c ->
-      let version = C.ocaml_config_var_exn c "version" in
-      let major, minor =
-        Scanf.sscanf version "%u.%u" (fun maj min -> maj, min)
-      in
-      print_endline
-        (if (major, minor) >= (4, 8) then
-          shims_post_408
-        else
-          shims_pre_408))

src/dune

@@ -1,23 +1,9 @@
-(executable
- (name mkshims)
- (modules mkshims)
- (libraries dune.configurator))
-
-(rule
- (targets Iter_shims_.ml)
- (deps mkshims.exe)
- (action
-  (with-stdout-to
-   %{targets}
-   (run ./mkshims.exe))))
-
 (library
  (name iter)
  (public_name iter)
  (wrapped false)
- (modules Iter IterLabels Iter_shims_)
- (flags :standard -w +a -warn-error -a+8 -nolabels)
- (libraries bytes result seq))
+ (modules Iter IterLabels)
+ (flags :standard -w +a-4 -warn-error -a+8 -nolabels))
 
 (env
  (_

src/mkshims.ml

@@ -1,16 +0,0 @@
-module C = Configurator.V1
-
-let shims_pre_408 = "module Stdlib = Pervasives"
-let shims_post_408 = "module Stdlib = Stdlib"
-
-let () =
-  C.main ~name:"mkshims" (fun c ->
-      let version = C.ocaml_config_var_exn c "version" in
-      let major, minor =
-        Scanf.sscanf version "%u.%u" (fun maj min -> maj, min)
-      in
-      print_endline
-        (if (major, minor) >= (4, 8) then
-          shims_post_408
-        else
-          shims_pre_408))

tests/unit/dune

@@ -1,4 +1,3 @@
-
 (tests
  (names t_iter)
  (libraries iter qcheck-core qcheck-core.runner ounit2))

tests/unit/t_iter.ml

@@ -237,6 +237,31 @@ let () =
   OUnit.assert_equal 2 n;
   ()
 
+let list_equal eq l1 l2 =
+  List.length l1 = List.length l2 && List.for_all2 eq l1 l2
+
+let () =
+  OUnit.assert_equal ~cmp:(list_equal Int.equal)
+    (1 -- 10
+    |> map_while (fun x ->
+           if x = 7 then
+             `Return (x + 1)
+           else
+             `Yield (x - 1))
+    |> to_list)
+    [ 0; 1; 2; 3; 4; 5; 8 ]
+
+let () =
+  OUnit.assert_equal ~cmp:(list_equal Int.equal)
+    (1 -- 10
+    |> map_while (fun x ->
+           if x = 7 then
+             `Stop
+           else
+             `Yield (x - 1))
+    |> to_list)
+    [ 0; 1; 2; 3; 4; 5 ]
+
 let () = 1 -- 5 |> drop 2 |> to_list |> OUnit.assert_equal [ 3; 4; 5 ]
 let () = 1 -- 5 |> rev |> to_list |> OUnit.assert_equal [ 5; 4; 3; 2; 1 ]
 
@@ -328,6 +353,43 @@ let () =
   OUnit.assert_bool "not empty" (not (is_empty s));
   ()
 
+let with_tmp_file f =
+  let path = Filename.temp_file "test_iter" "data" in
+  try
+    let x = f path in
+    (try Sys.remove path with _ -> ());
+    x
+  with e ->
+    (try Sys.remove path with _ -> ());
+    raise e
+
+let () =
+  with_tmp_file @@ fun path ->
+  Iter.IO.write_lines path Iter.empty;
+  let l = Iter.IO.lines_of path |> Iter.to_list in
+  OUnit.assert_equal ~printer:Q.Print.(list @@ Printf.sprintf "%S") [] l
+
 let () =
   let errcode = QCheck_base_runner.run_tests ~colors:false !qchecks in
   if errcode <> 0 then exit errcode
+
+(* map_by_2 tests *)
+let test = OUnit.assert_equal ~printer:Q.Print.(list int)
+let () = test [] (map_by_2 (fun a _ -> a) (of_list []) |> to_list)
+
+(* Test empty iterator *)
+let () = test [ 1 ] (map_by_2 (fun _ b -> b) (of_list [ 1 ]) |> to_list)
+let () = test [ 3 ] (map_by_2 (fun _ b -> b) (1 -- 3) |> drop 1 |> to_list)
+let () = test [ 9 ] (map_by_2 (fun _ b -> b) (1 -- 9) |> drop 4 |> to_list)
+
+(* Odd number of elements should leave the last element in the iterator.
+   For an increasing integer range [1,2k] (fun _ b -> b) returns only
+   even numbers so this is sufficient to test that this element is left
+   in the iterator. *)
+let () = test [ 1 ] (map_by_2 (fun a _ -> a) (1 -- 2) |> to_list)
+let () = test [ 2 ] (map_by_2 (fun _ b -> b) (1 -- 2) |> to_list)
+
+(* Test two elements *)
+let () = test [ 1; 3; 5; 7; 9 ] (map_by_2 (fun a _ -> a) (1 -- 10) |> to_list)
+let () = test [ 2; 4; 6; 8; 10 ] (map_by_2 (fun _ b -> b) (1 -- 10) |> to_list)
+(* Test more than two elements *)