diff --git a/.github/workflows/compat.yml b/.github/workflows/compat.yml
new file mode 100644
index 00000000..b626372f
--- /dev/null
+++ b/.github/workflows/compat.yml
@@ -0,0 +1,38 @@
+
+name: compat
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  run:
+    name: build
+    strategy:
+      fail-fast: true
+      matrix:
+        os:
+            - ubuntu-latest
+        ocaml-compiler:
+            - '4.03.x'
+            - '4.06.x'
+            - '4.07.x'
+            - '4.08.x'
+            - '4.13.x'
+
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use OCaml ${{ matrix.ocaml-compiler }}
+        uses: ocaml/setup-ocaml@v2
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+
+      - run: opam pin -n .
+      - run: opam depext -y containers containers-data containers-thread
+      - run: opam install containers containers-data containers-thread --deps-only
+      - run: opam exec -- dune build '@install'
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 700f1de6..c3c77594 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -1,4 +1,4 @@
-name: Build and test
+name: build and test
 
 on:
   push:
@@ -10,9 +10,9 @@ on:
 
 jobs:
   run:
-    name: Build
+    name: build
     strategy:
-      fail-fast: false
+      fail-fast: true
       matrix:
         os:
             - macos-latest
@@ -20,7 +20,6 @@ jobs:
             - windows-latest
         ocaml-compiler:
             - '4.03.x'
-            - '4.08.x'
             - '4.13.x'
 
     runs-on: ${{ matrix.os }}
@@ -39,11 +38,11 @@ jobs:
         if: matrix.os == 'ubuntu-latest'
 
       - run: |
-          opam install -t containers --deps-only
+          opam install -t containers --deps-only ;
           opam install containers-data containers-thread --deps-only  # no test deps
         if: matrix.os != 'ubuntu-latest'
 
-      - run: opam exec -- dune build
+      - run: opam exec -- dune build '@install'
 
       - run: opam exec -- dune runtest
         if: ${{ matrix.os == 'ubuntu-latest' }}
diff --git a/benchs/dune b/benchs/dune
index ff3ac80f..c8430fc7 100644
--- a/benchs/dune
+++ b/benchs/dune
@@ -4,5 +4,6 @@
              containers-thread benchmark gen iter qcheck oseq
              batteries base sek)
   (flags :standard -warn-error -3-5 -safe-string -color always -open CCShims_)
+  (optional)
   (ocamlopt_flags :standard -O3 -color always
                    -unbox-closures -unbox-closures-factor 20))
diff --git a/containers-data.opam b/containers-data.opam
index 202f4899..c30d80d4 100644
--- a/containers-data.opam
+++ b/containers-data.opam
@@ -11,7 +11,7 @@ build: [
 ]
 depends: [
   "ocaml" { >= "4.03.0" }
-  "dune" { >= "1.4" }
+  "dune" { >= "2.0" }
   "containers" { = version }
   "seq"
   "qtest" { with-test }
diff --git a/containers-thread.opam b/containers-thread.opam
index d4c32535..6af5aaf7 100644
--- a/containers-thread.opam
+++ b/containers-thread.opam
@@ -11,7 +11,7 @@ build: [
 ]
 depends: [
   "ocaml" { >= "4.03.0" }
-  "dune" { >= "1.4" }
+  "dune" { >= "2.0" }
   "base-threads"
   "dune-configurator"
   "containers" { = version }
diff --git a/containers.opam b/containers.opam
index 4330b193..59be282c 100644
--- a/containers.opam
+++ b/containers.opam
@@ -12,7 +12,7 @@ build: [
 ]
 depends: [
   "ocaml" { >= "4.03.0" }
-  "dune" { >= "1.4" }
+  "dune" { >= "2.0" }
   "dune-configurator"
   "seq" # compat
   "either" # compat
diff --git a/dune b/dune
index f8d56234..655216a2 100644
--- a/dune
+++ b/dune
@@ -1,9 +1,12 @@
 (rule
  (targets README.md.corrected)
  (deps (package containers-data) ./src/mdx_runner.exe)
+ (enabled_if (= %{system} "linux"))
  (action (run ./src/mdx_runner.exe)))
 
-(alias
- (name runtest)
+(rule
+ (alias runtest)
  (package containers-data)
+ (enabled_if (= %{system} "linux"))
+ (locks /ctest)
  (action (diff README.md README.md.corrected)))
diff --git a/dune-project b/dune-project
index f9337290..929c696e 100644
--- a/dune-project
+++ b/dune-project
@@ -1 +1 @@
-(lang dune 1.4)
+(lang dune 2.0)
diff --git a/examples/dune b/examples/dune
index 13b6e613..e8ed9dc0 100644
--- a/examples/dune
+++ b/examples/dune
@@ -4,16 +4,16 @@
   (libraries containers)
   (flags :standard -warn-error -a+8))
 
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (deps (source_tree test_data))
   (action
     (ignore-stdout
       (run ./id_sexp.exe test_data/benchpress.sexp))))
 
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (deps (source_tree test_data))
   (action
@@ -30,8 +30,8 @@
   (enabled_if (< %{ocaml_version} "4.08"))
   (action (with-stdout-to %{targets} (run echo "let() = print_endline {|ok|}"))))
 
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (deps (source_tree test_data))
   (enabled_if (>= %{ocaml_version} "4.08"))
diff --git a/fuzz/dune b/fuzz/dune
index 5f57001d..11342443 100644
--- a/fuzz/dune
+++ b/fuzz/dune
@@ -3,4 +3,5 @@
  (names     ccsexp_parse_string_does_not_crash
             ccutf8_string_uchar_to_bytes_is_same_as_simple_version
             ccsexp_csexp_reparse)
+ (optional)
  (libraries crowbar containers))
diff --git a/qtest/dune b/qtest/dune
index 5775f212..f55c6a09 100644
--- a/qtest/dune
+++ b/qtest/dune
@@ -6,8 +6,8 @@
 
 (rule
   (targets run_qtest.ml)
-  (deps make.bc (source_tree ../src))
-  (action (run ./make.bc -target %{targets} ../src/core ../src/unix/)))
+  (deps ./make.exe (source_tree ../src))
+  (action (run ./make.exe -target %{targets} ../src/core ../src/unix/)))
 
 (executable
   (name run_qtest)
@@ -17,16 +17,16 @@
   (flags :standard -warn-error -a -w -3-33-35-27-39-50)
   (libraries iter gen qcheck containers containers.unix unix uutf threads))
 
-(alias
-  (name    runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (package containers)
   (action  (run ./run_qtest.exe)))
 
 (rule
   (targets run_qtest_data.ml)
-  (deps make.bc (source_tree ../src/data))
-  (action (run ./make.bc -target %{targets} ../src/data)))
+  (deps make.exe (source_tree ../src/data))
+  (action (run ./make.exe -target %{targets} ../src/data)))
 
 (executable
   (name run_qtest_data)
@@ -36,16 +36,16 @@
   (flags :standard -warn-error -a -w -3-33-35-27-39-50)
   (libraries iter gen qcheck containers containers-data))
 
-(alias
-  (name    runtest)
+(rule
+  (alias runtest)
   (package containers-data)
   (locks /ctest)
   (action  (run ./run_qtest_data.exe)))
 
 (rule
   (targets run_qtest_thread.ml)
-  (deps make.bc (source_tree ../src/threads))
-  (action (run ./make.bc -target %{targets} ../src/threads)))
+  (deps make.exe (source_tree ../src/threads))
+  (action (run ./make.exe -target %{targets} ../src/threads)))
 
 (executable
   (name run_qtest_thread)
@@ -55,8 +55,8 @@
   (flags :standard -warn-error -a -w -3-33-35-27-39-50)
   (libraries qcheck containers containers-thread iter threads))
 
-(alias
-  (name    runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (package containers-thread)
   (action  (run ./run_qtest_thread.exe)))
diff --git a/qtest/make.ml b/qtest/make.ml
index bb4cb540..5c63c9d3 100644
--- a/qtest/make.ml
+++ b/qtest/make.ml
@@ -18,11 +18,11 @@ let is_code file = is_suffix ~sub:".ml" file || is_suffix ~sub:".mli" file
 let do_not_test file =
   assert (not (is_suffix ~sub:"make.ml" file));
   str_sub ~sub:"Labels.ml" file ||
+  is_suffix ~sub:".pp.ml" file ||
+  is_suffix ~sub:".pp.mli" file ||
   is_suffix ~sub:"containers.ml" file ||
   is_suffix ~sub:"_top.ml" file ||
-  is_suffix ~sub:"mkflags.ml" file ||
-  is_suffix ~sub:"mkshims.ml" file ||
-  is_suffix ~sub:"unlabel.ml" file ||
+  is_suffix ~sub:"cpp.ml" file ||
   is_suffix ~sub:"check_labelled_mods.ml" file ||
   is_suffix ~sub:"test_random.ml" file ||
   is_suffix ~sub:"test_hash.ml" file ||
diff --git a/src/codegen/tests/dune b/src/codegen/tests/dune
index 320af55e..4aeb4c37 100644
--- a/src/codegen/tests/dune
+++ b/src/codegen/tests/dune
@@ -19,6 +19,6 @@
   (flags :standard -warn-error -a+8)
   (libraries containers))
 
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (action (run ./test_bitfield.exe)))
diff --git a/src/core/CCArray.ml b/src/core/CCArray.ml
index 09a8af01..87a439d7 100644
--- a/src/core/CCArray.ml
+++ b/src/core/CCArray.ml
@@ -16,8 +16,24 @@ type 'a printer = Format.formatter -> 'a -> unit
 
 (** {2 Arrays} *)
 
-include CCShims_
-include CCShimsArray_
+open CCShims_
+
+[@@@ifge 4.8]
+
+include Array
+
+[@@@elifge 4.6]
+
+include Array
+type 'a t = 'a array
+
+[@@@else_]
+
+include Array
+module Floatarray = struct type t = float array end
+type 'a t = 'a array
+
+[@@@endif]
 
 let empty = [| |]
 
@@ -704,12 +720,15 @@ module Infix = struct
   let (--) = (--)
   let (--^) = (--^)
 
-  include CCShimsMkLet_.Make(struct
-      type 'a t = 'a array
-      let (>>=) = (>>=)
-      let (>|=) = (>|=)
-      let monoid_product a1 a2 = monoid_product (fun x y->x,y) a1 a2
-    end)
+  [@@@ifge 4.8]
+
+  type 'a t = 'a array
+  let ( let* ) = (>>=)
+  let (let+) = (>|=)
+  let[@inline] (and+) a1 a2 = monoid_product (fun x y->x,y) a1 a2
+  let ( and* ) = (and+)
+
+  [@@@endif]
 end
 
 include Infix
diff --git a/src/core/CCArray.mli b/src/core/CCArray.mli
index 99590017..34f95264 100644
--- a/src/core/CCArray.mli
+++ b/src/core/CCArray.mli
@@ -14,8 +14,31 @@ type 'a printer = Format.formatter -> 'a -> unit
 
 (** {2 Arrays} *)
 
-include module type of CCShimsArray_
-(** @inline *)
+[@@@ifge 4.8]
+
+include module type of Array
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
+
+[@@@elifge 4.6]
+
+include module type of Array
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
+
+type 'a t = 'a array
+
+[@@@else_]
+
+include module type of Array
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
+
+module Floatarray : sig type t = float array end
+
+type 'a t = 'a array
+
+[@@@endif]
 
 val empty : 'a t
 (** [empty] is the empty array, physically equal to [[||]]. *)
@@ -294,10 +317,14 @@ module Infix : sig
   (** [x --^ y] creates an array containing integers in the range [x .. y]. Right bound excluded.
       @since 0.17 *)
 
+  [@@@ifge 4.8]
+
+  include CCShims_syntax.LET with type 'a t := 'a array
   (** Let operators on OCaml >= 4.08.0, nothing otherwise
       @since 2.8
       @inline *)
-  include CCShimsMkLet_.S with type 'a t_let := 'a array
+
+  [@@@endif]
 end
 
 include module type of Infix
diff --git a/src/core/CCArrayLabels.mli b/src/core/CCArrayLabels.mli
index c385c721..8be4f679 100644
--- a/src/core/CCArrayLabels.mli
+++ b/src/core/CCArrayLabels.mli
@@ -14,8 +14,32 @@ type 'a printer = Format.formatter -> 'a -> unit
 
 (** {2 Arrays} *)
 
-include module type of CCShimsArrayLabels_
-(** @inline *)
+[@@@ifge 4.8]
+
+
+include module type of ArrayLabels with module Floatarray = Array.Floatarray
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
+
+[@@@elifge 4.6]
+
+include module type of ArrayLabels with module Floatarray = Array.Floatarray
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
+
+type 'a t = 'a array
+
+[@@@else_]
+
+include module type of ArrayLabels
+(** {{: http://caml.inria.fr/pub/docs/manual-ocaml/libref/ArrayLabels.html} Documentation for the standard ArrayLabels module}*)
+
+module Floatarray = CCArray.Floatarray
+type 'a t = 'a array
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
+
+[@@@endif]
 
 val empty : 'a t
 (** [empty] is the empty array, physically equal to [||]. *)
@@ -310,10 +334,14 @@ module Infix : sig
   (** [x --^ y] creates an array containing integers in the range [x .. y]. Right bound excluded.
       @since 0.17 *)
 
+  [@@@ifge 4.8]
+
+  include CCShims_syntax.LET with type 'a t := 'a array
   (** Let operators on OCaml >= 4.08.0, nothing otherwise
       @since 2.8
       @inline *)
-  include CCShimsMkLet_.S with type 'a t_let := 'a array
+
+  [@@@endif]
 end
 
 include module type of Infix
diff --git a/src/core/CCAtomic.ml b/src/core/CCAtomic.ml
new file mode 100644
index 00000000..dbdc8061
--- /dev/null
+++ b/src/core/CCAtomic.ml
@@ -0,0 +1,35 @@
+
+[@@@ifge 4.12]
+
+include Atomic
+
+
+[@@@else_]
+
+open CCShims_.Stdlib (* for == *)
+
+type 'a t = {mutable x: 'a}
+let[@inline] make x = {x}
+let[@inline] get {x} = x
+let[@inline] set r x = r.x <- x
+let[@inline] exchange r x =
+  let y = r.x in
+  r.x <- x;
+  y
+
+let[@inline] compare_and_set r seen v =
+  if r.x == seen then (
+    r.x <- v;
+    true
+  ) else false
+
+let[@inline] fetch_and_add r x =
+  let v = r.x in
+  r.x <- x + r.x;
+  v
+
+let[@inline] incr r = r.x <- 1 + r.x
+let[@inline] decr r = r.x <- r.x - 1
+
+
+[@@@endif]
diff --git a/src/core/CCFormat.ml b/src/core/CCFormat.ml
index 192a9597..18783909 100644
--- a/src/core/CCFormat.ml
+++ b/src/core/CCFormat.ml
@@ -362,18 +362,45 @@ let mark_close_tag st ~or_else s =
       if !color_enabled then string_of_style_list style else ""
     | exception No_such_style -> or_else s
 
+[@@@ifge 4.8]
+
+
+let pp_open_tag out s = pp_open_stag out (String_tag s)
+let pp_close_tag out () = pp_close_stag out ()
+
+[@@@ocaml.warning "-3"]
+let pp_get_formatter_tag_functions = pp_get_formatter_tag_functions
+let pp_set_formatter_tag_functions = pp_set_formatter_tag_functions
+
+let update_tag_funs_ funs f1 f2 =
+  { funs with
+    mark_open_tag = f1 ~or_else:funs.mark_open_tag;
+    mark_close_tag = f2 ~or_else:funs.mark_close_tag;
+  }
+
+[@@@ocaml.warning "+3"]
+
+[@@@else_]
+
+let update_tag_funs_ funs f1 f2 =
+  { funs with
+    mark_open_tag = f1 funs.mark_open_tag;
+    mark_close_tag = f2 funs.mark_close_tag;
+  }
+
+[@@@endif]
+
 (* add color handling to formatter [ppf] *)
 let set_color_tag_handling ppf =
-  let open Format in
-  let functions = CCShimsFormat_.pp_get_formatter_tag_functions ppf () in
+  let functions = pp_get_formatter_tag_functions ppf () in
   let st = Stack.create () in (* stack of styles *)
   let functions' =
-    CCShimsFormat_.cc_update_funs functions
+    update_tag_funs_ functions
       (mark_open_tag st)
       (mark_close_tag st)
   in
   pp_set_mark_tags ppf true; (* enable tags *)
-  CCShimsFormat_.pp_set_formatter_tag_functions ppf functions'
+  pp_set_formatter_tag_functions ppf functions'
 
 let set_color_default =
   let first = ref true in
@@ -398,14 +425,14 @@ let set_color_default =
 *)
 
 let with_color s pp out x =
-  CCShimsFormat_.pp_open_tag out s;
+  pp_open_tag out s;
   pp out x;
-  CCShimsFormat_.pp_close_tag out ()
+  pp_close_tag out ()
 
 let with_colorf s out fmt =
-  CCShimsFormat_.pp_open_tag out s;
+  pp_open_tag out s;
   Format.kfprintf
-    (fun out -> CCShimsFormat_.pp_close_tag out ())
+    (fun out -> pp_close_tag out ())
     out fmt
 
 (* c: whether colors are enabled *)
@@ -422,10 +449,10 @@ let with_color_ksf ~f s fmt =
   let buf = Buffer.create 64 in
   let out = Format.formatter_of_buffer buf in
   if !color_enabled then set_color_tag_handling out;
-  CCShimsFormat_.pp_open_tag out s;
+  pp_open_tag out s;
   Format.kfprintf
     (fun out ->
-       CCShimsFormat_.pp_close_tag out ();
+       pp_close_tag out ();
        Format.pp_print_flush out ();
        f (Buffer.contents buf))
     out fmt
diff --git a/src/core/CCFun.ml b/src/core/CCFun.ml
index ea7c3322..9424c667 100644
--- a/src/core/CCFun.ml
+++ b/src/core/CCFun.ml
@@ -9,7 +9,27 @@ let opaque_identity x = x
 
 include Sys
 include CCShims_.Stdlib
-include CCShimsFun_
+
+[@@@ifge 4.8]
+
+include Fun
+
+[@@@else_]
+
+external id : 'a -> 'a = "%identity"
+let[@inline] flip f x y = f y x
+let[@inline] const x _ = x
+let[@inline] negate f x = not (f x)
+let[@inline] protect ~finally f =
+  try
+    let x= f() in
+    finally();
+    x
+  with e ->
+    finally();
+    raise e
+
+[@@@endif]
 
 let compose f g x = g (f x)
 
diff --git a/src/core/CCFun.mli b/src/core/CCFun.mli
index 83b98328..4ba8fb2f 100644
--- a/src/core/CCFun.mli
+++ b/src/core/CCFun.mli
@@ -3,9 +3,24 @@
 
 (** Basic operations on Functions *)
 
-include module type of CCShimsFun_
+[@@@ifge 4.8]
+include module type of Fun
 (** @inline *)
 
+[@@@else_]
+
+(** This is an API imitating the new standard Fun module *)
+external id : 'a -> 'a = "%identity"
+val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
+val const : 'a -> _ -> 'a
+val negate : ('a -> bool) -> 'a -> bool
+
+val protect : finally:(unit -> unit) -> (unit -> 'a) -> 'a
+(* this doesn't have the exact same semantics as the stdlib's finally.
+    It will not attempt to catch exceptions raised from [finally] at all. *)
+
+[@@@endif]
+
 val compose : ('a -> 'b) -> ('b -> 'c) -> 'a -> 'c
 (** [compose f g x] is [g (f x)]. Composition. *)
 
diff --git a/src/core/CCInt.ml b/src/core/CCInt.ml
index 191a44ed..8f2e38f6 100644
--- a/src/core/CCInt.ml
+++ b/src/core/CCInt.ml
@@ -1,9 +1,17 @@
 
 (* This file is free software, part of containers. See file "license" for more details. *)
 
-open CCShims_
+[@@@ifge 4.07]
+[@@@else_]
 
-include CCShimsInt_
+module Stdlib = Pervasives
+[@@@endif]
+
+[@@@ifge 4.08]
+
+include Int
+
+[@@@endif]
 
 type t = int
 type 'a iter = ('a -> unit) -> unit
diff --git a/src/core/CCInt.mli b/src/core/CCInt.mli
index 182a51df..d430f07b 100644
--- a/src/core/CCInt.mli
+++ b/src/core/CCInt.mli
@@ -3,8 +3,14 @@
 
 (** Basic Int functions *)
 
-include module type of CCShimsInt_
-(** @inline *)
+[@@@ifge 4.08]
+
+include module type of Int
+(** @inline
+
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Int.html} Documentation for the standard Int module}*)
+
+[@@@endif]
 
 type t = int
 
diff --git a/src/core/CCList.ml b/src/core/CCList.ml
index a371b964..aa777a27 100644
--- a/src/core/CCList.ml
+++ b/src/core/CCList.ml
@@ -66,7 +66,16 @@ let rec assq_opt x = function
 
 (* end of backport *)
 
-include CCShimsList_
+[@@@ifge 4.8]
+
+include List
+
+[@@@else_]
+
+include List
+type +'a t = 'a list
+
+[@@@endif]
 
 let empty = []
 
@@ -1992,16 +2001,16 @@ module Infix = struct
   let (--) = (--)
   let (--^) = (--^)
 
-  include CCShimsMkLet_.Make(struct
-      type 'a t = 'a list
-      let (>|=) = (>|=)
-      let (>>=) = (>>=)
-      let[@inline] monoid_product l1 l2 = product (fun x y -> x,y) l1 l2
-    end)
+  [@@@ifge 4.8]
 
-  include CCShimsMkLetList_.Make(struct
-      let combine_shortest=combine_shortest
-    end)
+  let (let+) = (>|=)
+  let (let*) = (>>=)
+  let[@inline] (and+) l1 l2 = product (fun x y -> x,y) l1 l2
+  let (and*) = (and+)
+
+  let (and&) = combine_shortest
+
+  [@@@endif]
 end
 
 include Infix
diff --git a/src/core/CCList.mli b/src/core/CCList.mli
index 034b8a31..bda8c72b 100644
--- a/src/core/CCList.mli
+++ b/src/core/CCList.mli
@@ -11,10 +11,23 @@ type 'a gen = unit -> 'a option
 type 'a printer = Format.formatter -> 'a -> unit
 type 'a random_gen = Random.State.t -> 'a
 
-include module type of List
-(** {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/List.html} Documentation for the standard List module}*)
+[@@@ifge 4.8]
 
-type 'a t = 'a list
+include module type of List with type 'a t := 'a list
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/List.html} Documentation for the standard List module}*)
+
+type +'a t = 'a list
+
+[@@@else_]
+
+include module type of List
+(** @inline
+    {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/List.html} Documentation for the standard List module}*)
+
+type +'a t = 'a list
+
+[@@@endif]
 
 val empty : 'a t
 (** [empty] is [[]]. *)
@@ -925,13 +938,29 @@ module Infix : sig
   (** [i --^ j] is the infix alias for [range']. Second bound [j] excluded.
       @since 0.17 *)
 
-  (** Let operators on OCaml >= 4.08.0, nothing otherwise
-      @since 2.8
-      @inline *)
-  include CCShimsMkLet_.S with type 'a t_let := 'a list
+  [@@@ifge 4.08]
 
-  include CCShimsMkLetList_.S
+  include CCShims_syntax.LET with type 'a t := 'a t
   (** @inline *)
+
+  val (and&) : 'a list -> 'b list -> ('a * 'b) list
+  (** [(and&)] is {!combine_shortest}.
+      It allows to perform a synchronized product between two lists,
+      stopping gently at the shortest. Usable both with [let+] and [let*].
+    {[
+        # let f xs ys zs =
+            let+ x = xs
+            and& y = ys
+            and& z = zs in
+            x + y + z;;
+        val f : int list -> int list -> int list -> int list = <fun>
+        # f [1;2] [5;6;7] [10;10];;
+        - : int list = [16; 18]
+    ]}
+    @since 3.1
+  *)
+
+  [@@@endif]
 end
 
 include module type of Infix
diff --git a/src/core/CCListLabels.mli b/src/core/CCListLabels.mli
index ef937b2e..6b85dfdf 100644
--- a/src/core/CCListLabels.mli
+++ b/src/core/CCListLabels.mli
@@ -852,38 +852,7 @@ val of_gen : 'a gen -> 'a t
 
     @since 0.16 *)
 
-module Infix : sig
-  val (>|=) : 'a t -> ('a -> 'b) -> 'b t
-  (** [l >|= f] is the infix version of [map] with reversed arguments. *)
-
-  val (@) : 'a t -> 'a t -> 'a t
-  (** [l1 @ l2] concatenates two lists [l1] and [l2].
-      As {!append}. *)
-
-  val (<*>) : ('a -> 'b) t -> 'a t -> 'b t
-  (** [funs <*> l] is [product (fun f x -> f x) funs l]. *)
-
-  val (<$>) : ('a -> 'b) -> 'a t -> 'b t
-  (** [f <$> l] is like {!map}. *)
-
-  val (>>=) : 'a t -> ('a -> 'b t) -> 'b t
-  (** [l >>= f] is [flat_map f l]. *)
-
-  val (--) : int -> int -> int t
-  (** [i -- j] is the infix alias for [range]. Bounds included. *)
-
-  val (--^) : int -> int -> int t
-  (** [i --^ j] is the infix alias for [range']. Second bound [j] excluded.
-      @since 0.17 *)
-
-  (** Let operators on OCaml >= 4.08.0, nothing otherwise
-      @since 2.8
-      @inline *)
-  include CCShimsMkLet_.S with type 'a t_let := 'a list
-
-  include CCShimsMkLetList_.S
-  (** @inline *)
-end
+module Infix : module type of CCList.Infix
 
 include module type of Infix
 
diff --git a/src/core/CCOption.ml b/src/core/CCOption.ml
index 327278f4..81a522ca 100644
--- a/src/core/CCOption.ml
+++ b/src/core/CCOption.ml
@@ -176,14 +176,16 @@ module Infix = struct
   let (<$>) = map
   let (<+>) = (<+>)
 
-  include CCShimsMkLet_.Make(struct
-      type 'a t = 'a option
-      let (>|=) = (>|=)
-      let (>>=) = (>>=)
-      let[@inline] monoid_product o1 o2 = match o1, o2 with
-        | Some x, Some y -> Some (x,y)
-        | _ -> None
-    end)
+  [@@@ifge 4.8]
+
+  let (let+) = (>|=)
+  let (let*) = (>>=)
+  let[@inline] (and+) o1 o2 = match o1, o2 with
+    | Some x, Some y -> Some (x,y)
+    | _ -> None
+  let (and*) = (and+)
+
+  [@@@endif]
 end
 
 include Infix
diff --git a/src/core/CCOption.mli b/src/core/CCOption.mli
index 81b8884c..0c46ddbf 100644
--- a/src/core/CCOption.mli
+++ b/src/core/CCOption.mli
@@ -171,11 +171,15 @@ module Infix : sig
   val (<+>) : 'a t -> 'a t -> 'a t
   (** [o1 <+> o2] is [o1] if [o1] is [Some _], [o2] if [o1] is [None]. *)
 
+  [@@@ifge 4.08]
+
+  include CCShims_syntax.LET with type 'a t := 'a t
   (** Let operators on OCaml >= 4.08.0, nothing otherwise
       @since 2.8
       @inline *)
-  include CCShimsMkLet_.S with type 'a t_let := 'a option
 
+
+  [@@@endif]
 end
 
 include module type of Infix
diff --git a/src/core/CCParse.ml b/src/core/CCParse.ml
index 0d33f9ea..5849a6ac 100644
--- a/src/core/CCParse.ml
+++ b/src/core/CCParse.ml
@@ -355,12 +355,14 @@ module Infix = struct
   let (|||) = both
   let[@inline] (<?>) p msg = set_error_message msg p
 
-  include CCShimsMkLet_.Make(struct
-      type nonrec 'a t = 'a t
-      let (>>=) = (>>=)
-      let (>|=) = (>|=)
-      let monoid_product = both
-    end)
+  [@@@ifge 4.8]
+
+  let (let+) = (>|=)
+  let (let*) = (>>=)
+  let (and+) = both
+  let (and*) = (and+)
+
+  [@@@endif]
 end
 
 include Infix
diff --git a/src/core/CCParse.mli b/src/core/CCParse.mli
index 86d59a9d..f34bc00a 100644
--- a/src/core/CCParse.mli
+++ b/src/core/CCParse.mli
@@ -665,10 +665,14 @@ module Infix : sig
       [a ||| b] parses [a], then [b], then returns the pair of their results.
       @since 3.6 *)
 
+  [@@@ifge 4.08]
+
+  include CCShims_syntax.LET with type 'a t := 'a t
   (** Let operators on OCaml >= 4.08.0, nothing otherwise
       @since 2.8
       @inline *)
-  include CCShimsMkLet_.S with type 'a t_let := 'a t
+
+  [@@@endif]
 end
 
 include module type of Infix
diff --git a/src/core/CCRandom.ml b/src/core/CCRandom.ml
index 0f24d77b..c1567b65 100644
--- a/src/core/CCRandom.ml
+++ b/src/core/CCRandom.ml
@@ -196,12 +196,14 @@ let pure x _st = x
 
 let (<*>) f g st = f st (g st)
 
-include CCShimsMkLet_.Make(struct
-    type nonrec 'a t = 'a t
-    let (>>=) = (>>=)
-    let (>|=) = (>|=)
-    let monoid_product a1 a2 st = a1 st, a2 st
-  end)
+[@@@ifge 4.8]
+
+let (let+) = (>|=)
+let (let*) = (>>=)
+let[@inline] (and+) a1 a2 st = a1 st, a2 st
+let (and*) = (and+)
+
+[@@@endif]
 
 let __default_state = Random.State.make_self_init ()
 
diff --git a/src/core/CCRandom.mli b/src/core/CCRandom.mli
index b8a86626..f351f8e7 100644
--- a/src/core/CCRandom.mli
+++ b/src/core/CCRandom.mli
@@ -150,10 +150,14 @@ val pure : 'a -> 'a t
 
 val (<*>) : ('a -> 'b) t -> 'a t -> 'b t
 
+[@@@ifge 4.08]
+
+include CCShims_syntax.LET with type 'a t := 'a t
 (** Let operators on OCaml >= 4.08.0, nothing otherwise
     @since 2.8
     @inline *)
-include CCShimsMkLet_.S with type 'a t_let := 'a t
+
+[@@@endif]
 
 (** {4 Run a generator} *)
 
diff --git a/src/core/CCResult.ml b/src/core/CCResult.ml
index 90e6159d..469c143a 100644
--- a/src/core/CCResult.ml
+++ b/src/core/CCResult.ml
@@ -295,15 +295,17 @@ module Infix = struct
   let (>>=) e f = flat_map f e
   let (<*>) = (<*>)
 
-  include CCShimsMkLet_.Make2(struct
-      type ('a,'e) t = ('a,'e) result
-      let (>>=) = (>>=)
-      let (>|=) = (>|=)
-      let monoid_product x1 x2 = match x1, x2 with
-        | Ok x, Ok y -> Ok (x,y)
-        | Error e, _ -> Error e
-        | _, Error e -> Error e
-    end)
+  [@@@ifge 4.8]
+
+  let (let+) = (>|=)
+  let (let*) = (>>=)
+  let[@inline] (and+) x1 x2 = match x1, x2 with
+    | Ok x, Ok y -> Ok (x,y)
+    | Error e, _ -> Error e
+    | _, Error e -> Error e
+  let (and*) = (and+)
+
+  [@@@endif]
 end
 
 include Infix
diff --git a/src/core/CCResult.mli b/src/core/CCResult.mli
index 78f21ef8..ac6b19d4 100644
--- a/src/core/CCResult.mli
+++ b/src/core/CCResult.mli
@@ -194,10 +194,21 @@ module Infix : sig
       [Ok (a b)]. Otherwise, it fails, and the error of [a] is chosen
       over the error of [b] if both fail. *)
 
-  (** Let operators on OCaml >= 4.08.0, nothing otherwise
-      @since 2.8
-      @inline *)
-  include CCShimsMkLet_.S2 with type ('a,'e) t_let2 := ('a,'e) result
+  [@@@ifge 4.08]
+
+  val (let+) : ('a,'e) t -> ('a -> 'b) -> ('b,'e) t
+  (** @since 2.8 *)
+
+  val (and+) : ('a,'e) t -> ('b,'e) t -> ('a * 'b, 'e) t
+  (** @since 2.8 *)
+
+  val (let*) : ('a,'e) t -> ('a -> ('b,'e) t) -> ('b,'e) t
+  (** @since 2.8 *)
+
+  val (and*) : ('a,'e) t -> ('b,'e) t -> ('a * 'b,'e) t
+  (** @since 2.8 *)
+
+  [@@@endif]
 end
 
 include module type of Infix
diff --git a/src/core/CCShims_.ml b/src/core/CCShims_.ml
new file mode 100644
index 00000000..7fd31b22
--- /dev/null
+++ b/src/core/CCShims_.ml
@@ -0,0 +1,10 @@
+
+[@@@ifge 4.07]
+
+module Stdlib = Stdlib
+
+[@@@else_]
+
+module Stdlib = Pervasives
+
+[@@@endif]
diff --git a/src/core/CCShims_syntax.mli b/src/core/CCShims_syntax.mli
new file mode 100644
index 00000000..5b789d85
--- /dev/null
+++ b/src/core/CCShims_syntax.mli
@@ -0,0 +1,19 @@
+
+[@@@ifge 4.8]
+
+(** Let operators on OCaml >= 4.08.0, nothing otherwise
+  @since 2.8
+  @inline *)
+module type LET = sig
+  type 'a t
+
+  val (let+) : 'a t -> ('a -> 'b) -> 'b t
+
+  val (and+) : 'a t -> 'b t -> ('a * 'b) t
+
+  val (let*) : 'a t -> ('a -> 'b t) -> 'b t
+
+  val (and*) : 'a t -> 'b t -> ('a * 'b) t
+end
+
+[@@@endif]
diff --git a/src/core/CCString.ml b/src/core/CCString.ml
index a651349b..84991e40 100644
--- a/src/core/CCString.ml
+++ b/src/core/CCString.ml
@@ -469,9 +469,14 @@ module Split = struct
   *)
 end
 
+[@@@ifge 4.04]
+[@@@else_]
+
 let split_on_char c s: _ list =
   Split.list_cpy ~drop:Split.no_drop ~by:(String.make 1 c) s
 
+[@@@endif]
+
 (*$= & ~printer:Q.Print.(list string)
   ["a"; "few"; "words"; "from"; "our"; "sponsors"] \
     (split_on_char ' ' "a few words from our sponsors")
diff --git a/src/core/CCUnit.ml b/src/core/CCUnit.ml
new file mode 100644
index 00000000..9aee1240
--- /dev/null
+++ b/src/core/CCUnit.ml
@@ -0,0 +1,13 @@
+
+[@@@ifge 4.08]
+
+include Unit
+
+[@@@else_]
+
+type t = unit
+let[@inline] equal (_:t) (_:t) = true
+let[@inline] compare (_:t) (_:t) = 0
+let to_string () = "()"
+
+[@@@endif]
diff --git a/src/core/CCVector.ml b/src/core/CCVector.ml
index 29f9a01c..77fb42fa 100644
--- a/src/core/CCVector.ml
+++ b/src/core/CCVector.ml
@@ -1159,9 +1159,13 @@ let pp ?(pp_start=fun _ () -> ()) ?(pp_stop=fun _ () -> ())
     ) v;
   pp_stop fmt ()
 
-include CCShimsMkLet_.Make2(struct
-    type nonrec ('a,'e) t = ('a,'e) t
-    let (>|=) = (>|=)
-    let (>>=) = (>>=)
-    let monoid_product a1 a2 = monoid_product (fun x y->x,y) a1 a2
-  end)
+[@@@ifge 4.8]
+
+let (let+) = (>|=)
+let (let*) = (>>=)
+let[@inline] (and+) a1 a2 = monoid_product (fun x y->x,y) a1 a2
+let (and*) = (and+)
+
+[@@@endif]
+
+
diff --git a/src/core/CCVector.mli b/src/core/CCVector.mli
index 08221d73..e7d63c19 100644
--- a/src/core/CCVector.mli
+++ b/src/core/CCVector.mli
@@ -389,7 +389,20 @@ val pp : ?pp_start:unit printer -> ?pp_stop:unit printer -> ?pp_sep:unit printer
     By defaults [pp_start] and [pp_stop] does nothing and [pp_sep] defaults to
     (fun out -> Format.fprintf out ",@ "). *)
 
-(** Let operators on OCaml >= 4.08.0, nothing otherwise
-    @since 2.8
-    @inline *)
-include CCShimsMkLet_.S2 with type ('a,'e) t_let2 := ('a,'e) t
+
+
+[@@@ifge 4.08]
+
+val (let+) : ('a,'e) t -> ('a -> 'b) -> ('b,'e) t
+(** @since 2.8 *)
+
+val (and+) : ('a,'e) t -> ('b,'e) t -> ('a * 'b, 'e) t
+(** @since 2.8 *)
+
+val (let*) : ('a,'e) t -> ('a -> ('b,'e) t) -> ('b,'e) t
+(** @since 2.8 *)
+
+val (and*) : ('a,'e) t -> ('b,'e) t -> ('a * 'b,'e) t
+(** @since 2.8 *)
+
+[@@@endif]
diff --git a/src/core/cpp/cpp.ml b/src/core/cpp/cpp.ml
new file mode 100644
index 00000000..03c85c20
--- /dev/null
+++ b/src/core/cpp/cpp.ml
@@ -0,0 +1,111 @@
+
+module C = Configurator.V1
+
+type op = Le | Ge
+
+type line =
+  | If of op * int * int
+  | Elseif of op * int * int
+  | Else
+  | Endif
+  | Raw of string
+  | Eof
+
+let prefix ~pre s =
+  let len = String.length pre in
+  if len > String.length s then false
+  else (
+    let rec check i =
+      if i=len then true
+      else if (String.unsafe_get s i) <> (String.unsafe_get pre i) then false
+      else check (i+1)
+    in
+    check 0
+  )
+
+let eval ~major ~minor op i j =
+  match op with
+    | Le -> (major,minor) <= (i,j)
+    | Ge -> (major,minor) >= (i,j)
+
+let preproc_lines ~file ~major ~minor (ic:in_channel) : unit =
+  let pos = ref 0 in
+  let fail msg = failwith (Printf.sprintf "at line %d: %s" !pos msg) in
+  let pp_pos () = Printf.printf "#%d %S\n" !pos file in
+
+  let parse_line () : line =
+    match input_line ic with
+      | exception End_of_file -> Eof
+      | line ->
+        let line' = String.trim line in
+        incr pos;
+        if line' <> "" && line'.[0] = '[' then (
+          if prefix line' ~pre:"[@@@ifle" then
+            Scanf.sscanf line' "[@@@ifle %d.%d]" (fun x y -> If(Le,x,y))
+          else if prefix line' ~pre:"[@@@ifge" then
+            Scanf.sscanf line' "[@@@ifge %d.%d]" (fun x y -> If(Ge,x,y))
+          else if prefix line' ~pre:"[@@@elifle" then
+            Scanf.sscanf line' "[@@@elifle %d.%d]" (fun x y -> Elseif(Le,x,y))
+          else if prefix line' ~pre:"[@@@elifge" then
+            Scanf.sscanf line' "[@@@elifge %d.%d]" (fun x y -> Elseif(Ge,x,y))
+          else if line'="[@@@else_]" then Else
+          else if line'="[@@@endif]" then Endif
+          else Raw line
+        ) else Raw line
+  in
+
+  (* entry point *)
+  let rec top () =
+    match parse_line () with
+      | Eof -> ()
+      | If (op,i,j) ->
+        if eval ~major ~minor op i j then (
+          pp_pos();
+          cat_block ()
+        ) else skip_block ~elseok:true ()
+      | Raw s -> print_endline s; top()
+      | Elseif _ | Else | Endif ->
+        fail "unexpected elseif|else|endif"
+
+  (* current block is the valid one *)
+  and cat_block () =
+    match parse_line () with
+      | Eof -> fail "unexpected EOF"
+      | If _ -> fail "nested if not supported"
+      | Raw s -> print_endline s; cat_block()
+      | Endif -> pp_pos(); top()
+      | Elseif _ | Else -> skip_block ~elseok:false ()
+
+  (* skip current block.
+     @param elseok if true, we should evaluate "elseif" *)
+  and skip_block ~elseok () =
+    match parse_line () with
+      | Eof -> fail "unexpected EOF"
+      | If _ -> fail "nested if not supported"
+      | Raw _ -> skip_block ~elseok ()
+      | Endif -> pp_pos(); top()
+      | Elseif (op,i,j) ->
+        if elseok && eval ~major ~minor op i j then (
+          pp_pos();
+          cat_block ()
+        ) else skip_block ~elseok ()
+      | Else ->
+        if elseok then (
+          pp_pos();
+          cat_block()
+        ) else skip_block ~elseok ()
+  in
+  top()
+
+let () =
+  let t0 = Unix.gettimeofday()in
+  let file = Sys.argv.(1) in
+  let c = C.create "main" in
+  let version = C.ocaml_config_var_exn c "version" in
+  let major, minor = Scanf.sscanf version "%u.%u" (fun maj min -> maj, min) in
+
+  let ic = open_in file in
+  preproc_lines ~file ~major ~minor ic;
+
+  Printf.printf "(* file preprocessed in %.3fs *)\n" (Unix.gettimeofday() -. t0);
+  ()
diff --git a/src/core/cpp/dune b/src/core/cpp/dune
new file mode 100644
index 00000000..dff668db
--- /dev/null
+++ b/src/core/cpp/dune
@@ -0,0 +1,6 @@
+; our little preprocessor
+(executable
+  (name cpp)
+  (flags :standard -warn-error -a+8)
+  (modes native)
+  (libraries dune.configurator))
diff --git a/src/core/dune b/src/core/dune
index ca13aa78..8d7c1a4e 100644
--- a/src/core/dune
+++ b/src/core/dune
@@ -1,24 +1,12 @@
 
-(executable
- (name mkshims)
- (modules mkshims)
- (libraries dune.configurator))
-
-(rule
- (targets CCShims_.ml CCShimsList_.ml CCShimsFun_.ml CCShimsFun_.mli
-   CCShimsArray_.ml CCShimsFormat_.ml CCShimsMkLet_.ml CCShimsMkLetList_.ml
-   CCShimsArrayLabels_.ml CCShimsInt_.ml CCAtomic.ml CCUnit.ml)
- (deps ./mkshims.exe)
- (action
-  (run ./mkshims.exe)))
-
 (library
  (name containers)
  (public_name containers)
  (wrapped false)
- (modules :standard \ mkshims)
- (flags :standard -warn-error -a+8 -w -32 -safe-string -strict-sequence -nolabels -open
-   CCMonomorphic)
+ (modules_without_implementation CCShims_syntax)
+ (preprocess (action (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
+ (flags :standard -warn-error -a+8 -w -32-70 -safe-string
+        -strict-sequence -nolabels -open CCMonomorphic)
  (libraries seq either containers.monomorphic))
 
 (ocamllex (modules CCSexp_lex))
diff --git a/src/core/mkshims.ml b/src/core/mkshims.ml
deleted file mode 100644
index 8cb652b3..00000000
--- a/src/core/mkshims.ml
+++ /dev/null
@@ -1,270 +0,0 @@
-module C = Configurator.V1
-
-let write_file f s =
-  let out = open_out f in
-  output_string out s; flush out; close_out out
-
-let shims_pre_407 = "module Stdlib = Pervasives"
-
-let shims_post_407 = "module Stdlib = Stdlib"
-
-let shims_fmt_pre_408 = "
-include Format
-let cc_update_funs funs f1 f2 =
-  let open Format in
-  {
-    funs with
-    mark_open_tag = f1 funs.mark_open_tag;
-    mark_close_tag = f2 funs.mark_close_tag;
-  }
-
-"
-let shims_fmt_post_408 = "
-open Format
-
-[@@@ocaml.warning \"-3\"]
-
-let pp_open_tag = pp_open_tag
-let pp_close_tag = pp_close_tag
-let pp_get_formatter_tag_functions = pp_get_formatter_tag_functions
-let pp_set_formatter_tag_functions = pp_set_formatter_tag_functions
-
-let cc_update_funs funs f1 f2 =
-  let open Format in
-  {
-    funs with
-    mark_open_tag = f1 ~or_else:funs.mark_open_tag;
-    mark_close_tag = f2 ~or_else:funs.mark_close_tag;
-  }
-"
-
-let shims_fun_pre_408 = "
-  external id : 'a -> 'a = \"%identity\"
-  let[@inline] flip f x y = f y x
-  let[@inline] const x _ = x
-  let[@inline] negate f x = not (f x)
-  let[@inline] protect ~finally f =
-    try
-      let x= f() in
-      finally();
-      x
-    with e ->
-      finally();
-      raise e
-
-"
-let shims_fun_mli_pre_408 = "
-  (** This is an API imitating the new standard Fun module *)
-  external id : 'a -> 'a = \"%identity\"
-  val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
-  val const : 'a -> _ -> 'a
-  val negate : ('a -> bool) -> 'a -> bool
-
-  val protect : finally:(unit -> unit) -> (unit -> 'a) -> 'a
-  (* this doesn't have the exact same semantics as the stdlib's finally.
-      It will not attempt to catch exceptions raised from [finally] at all. *)
-"
-
-let shims_fun_post_408 = "include Fun"
-let shims_fun_mli_post_408 = "include module type of Fun"
-
-let shims_list_pre_408 = "
-  include List
-  type +'a t = 'a list
-"
-let shims_list_post_408 = "include List"
-
-let shims_array_pre_406 = "
-  include Array
-  (** {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
-
-  module Floatarray = struct type t = float array end
-  type 'a t = 'a array
-  "
-
-let shims_array_label_pre_406 = "
-  include ArrayLabels
-  (** {{: http://caml.inria.fr/pub/docs/manual-ocaml/libref/ArrayLabels.html} Documentation for the standard ArrayLabels module}*)
-
-  module Floatarray = CCShimsArray_.Floatarray
-  type 'a t = 'a array
-  "
-
-let shims_array_label_406_408 = "
-  include (ArrayLabels : module type of ArrayLabels with module Floatarray = Array.Floatarray)
-  (** {{: http://caml.inria.fr/pub/docs/manual-ocaml/libref/ArrayLabels.html} Documentation for the standard ArrayLabels module}*)
-
-  type 'a t = 'a array
-  "
-
-let shims_array_406_408 = "
-  include Array
-  (** {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
-
-  type 'a t = 'a array
-"
-let shims_array_post_408 = "
-  include Array
-  (** {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Array.html} Documentation for the standard Array module}*)
-"
-let shims_array_label_post_408 = "
-  include (ArrayLabels : module type of ArrayLabels with module Floatarray = Array.Floatarray)
-  (** {{: http://caml.inria.fr/pub/docs/manual-ocaml/libref/ArrayLabels.html} Documentation for the standard ArrayLabels module}*)
-"
-
-let shims_let_op_pre_408 =
-  "
-   (** glue code for let-operators on OCaml < 4.08 (auto generated) *)
-   module type S = sig type 'a t_let end
-   module Make(X:sig type 'a t end) = struct type 'a t_let = 'a X.t end
-
-   module type S2 = sig type ('a,'b) t_let2 end
-   module Make2(X:sig type ('a,'b) t end) = struct type ('a,'b) t_let2 = ('a,'b) X.t end
-"
-let shims_let_op_post_408 =
-  " (** glue code for let-operators on OCaml >= 4.08 (auto generated) *)
-    module type S = sig
-      type 'a t_let
-      val (let+) : 'a t_let -> ('a -> 'b) -> 'b t_let
-      val (and+) : 'a t_let -> 'b t_let -> ('a * 'b) t_let
-      val (let*) : 'a t_let -> ('a -> 'b t_let) -> 'b t_let
-      val (and*) : 'a t_let -> 'b t_let -> ('a * 'b) t_let
-    end
-   module Make(X:sig
-    type 'a t
-    val (>|=) : 'a t -> ('a -> 'b) -> 'b t
-    val monoid_product : 'a t -> 'b t -> ('a * 'b) t
-    val (>>=) : 'a t -> ('a -> 'b t) -> 'b t
-    end) : S with type 'a t_let = 'a X.t = struct
-      type 'a t_let = 'a X.t
-      let (let+) = X.(>|=)
-      let (and+) = X.monoid_product
-      let (let*) = X.(>>=)
-      let (and*) = X.monoid_product
-  end[@@inline]
-
-    module type S2 = sig
-      type ('a,'e) t_let2
-      val (let+) : ('a,'e) t_let2 -> ('a -> 'b) -> ('b,'e) t_let2
-      val (and+) : ('a,'e) t_let2 -> ('b,'e) t_let2 -> ('a * 'b, 'e) t_let2
-      val (let*) : ('a,'e) t_let2 -> ('a -> ('b,'e) t_let2) -> ('b,'e) t_let2
-      val (and*) : ('a,'e) t_let2 -> ('b,'e) t_let2 -> ('a * 'b,'e) t_let2
-    end
-
-   module Make2(X:sig
-    type ('a,'b) t
-    val (>|=) : ('a,'e) t -> ('a -> 'b) -> ('b,'e) t
-    val monoid_product : ('a,'e) t -> ('b,'e) t -> ('a * 'b, 'e) t
-    val (>>=) : ('a,'e) t -> ('a -> ('b,'e) t) -> ('b,'e) t
-    end) : S2 with type ('a,'e) t_let2 = ('a,'e) X.t = struct
-      type ('a,'e) t_let2 = ('a,'e) X.t
-      let (let+) = X.(>|=)
-      let (and+) = X.monoid_product
-      let (let*) = X.(>>=)
-      let (and*) = X.monoid_product
-  end[@@inline]
-"
-
-let shims_let_op_list_pre_408 =
-  "
-   (** glue code for let-operators on OCaml < 4.08 (auto generated) *)
-   module type S = sig end
-   module Make(X:sig end) = struct end
-"
-let shims_let_op_list_post_408 =
-  "module type S = sig
-   val (and&) : 'a list -> 'b list -> ('a * 'b) list
-   (** [(and&)] is {!combine_shortest}.
-      It allows to perform a synchronized product between two lists,
-        stopping gently at the shortest. Usable both with [let+] and [let*].
-    {[
-        # let f xs ys zs =
-            let+ x = xs
-            and& y = ys
-            and& z = zs in
-            x + y + z;;
-        val f : int list -> int list -> int list -> int list = <fun>
-        # f [1;2] [5;6;7] [10;10];;
-        - : int list = [16; 18]
-    ]}
-    @since 3.1
-  *)
-  end
-
-  module Make(X:sig
-    val combine_shortest : 'a list -> 'b list -> ('a*'b) list
-  end) = struct
-    let (and&) = X.combine_shortest
-  end
-"
-
-let shims_int_pre_408 = ""
-let shims_int_post_408 = "
-  include Int
-  (** {{: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Int.html} Documentation for the standard Int module}*)
-"
-
-let shims_unit_before_408 = {|
-type t = unit
-let[@inline] equal (_:t) (_:t) = true
-let[@inline] compare (_:t) (_:t) = 0
-let to_string () = "()"
-|}
-
-let shims_unit_after_408 = "include Unit"
-
-let shims_atomic_before_412 = {|
-  open CCShims_.Stdlib (* for == *)
-
-  type 'a t = {mutable x: 'a}
-  let[@inline] make x = {x}
-  let[@inline] get {x} = x
-  let[@inline] set r x = r.x <- x
-  let[@inline] exchange r x =
-    let y = r.x in
-    r.x <- x;
-    y
-
-  let[@inline] compare_and_set r seen v =
-    if r.x == seen then (
-      r.x <- v;
-      true
-    ) else false
-
-  let[@inline] fetch_and_add r x =
-    let v = r.x in
-    r.x <- x + r.x;
-    v
-
-  let[@inline] incr r = r.x <- 1 + r.x
-  let[@inline] decr r = r.x <- r.x - 1
-  |}
-
-let shims_atomic_after_412 = {|include Atomic|}
-
-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
-    write_file "CCShims_.ml" (if (major, minor) >= (4,7) then shims_post_407 else shims_pre_407);
-    write_file "CCShimsList_.ml" (if (major, minor) >= (4,8) then shims_list_post_408 else shims_list_pre_408);
-    write_file "CCShimsArray_.ml"
-      (if (major, minor) >= (4,8) then shims_array_post_408
-       else if (major, minor) >= (4,6) then shims_array_406_408
-       else shims_array_pre_406);
-    write_file "CCShimsArrayLabels_.ml"
-      (if (major, minor) >= (4,8) then shims_array_label_post_408
-       else if (major, minor) >= (4,6) then shims_array_label_406_408
-       else shims_array_label_pre_406);
-    write_file "CCShimsFormat_.ml" (if (major, minor) >= (4,8) then shims_fmt_post_408 else shims_fmt_pre_408);
-    write_file "CCShimsFun_.ml" (if (major, minor) >= (4,8) then shims_fun_post_408 else shims_fun_pre_408);
-    write_file "CCShimsFun_.mli" (if (major, minor) >= (4,8) then shims_fun_mli_post_408 else shims_fun_mli_pre_408);
-    write_file "CCShimsMkLet_.ml" (if (major, minor) >= (4,8) then shims_let_op_post_408 else shims_let_op_pre_408);
-    write_file "CCShimsMkLetList_.ml" (if (major, minor) >= (4,8) then shims_let_op_list_post_408 else shims_let_op_list_pre_408);
-    write_file "CCShimsInt_.ml"
-      (if (major, minor) >= (4,8) then shims_int_post_408 else shims_int_pre_408);
-    write_file "CCAtomic.ml"
-      (if (major, minor) >= (4,12) then shims_atomic_after_412 else shims_atomic_before_412);
-    write_file "CCUnit.ml"
-      (if (major, minor) >= (4,8) then shims_unit_after_408 else shims_unit_before_408);
-  )
diff --git a/src/core/tests/check_labelled_mods.ml b/src/core/tests/check_labelled_mods.ml
index 7745095b..05d4dc91 100644
--- a/src/core/tests/check_labelled_mods.ml
+++ b/src/core/tests/check_labelled_mods.ml
@@ -2,7 +2,7 @@
 
 module A = struct
   (* test consistency of interfaces *)
-  module FA = CCShimsArray_.Floatarray
+  module FA = CCArray.Floatarray
   module type L = module type of CCArray with module Floatarray := FA
   module type LL = module type of CCArrayLabels with module Floatarray := FA
 
diff --git a/src/core/tests/dune b/src/core/tests/dune
index 03ca9a0d..7a26dadc 100644
--- a/src/core/tests/dune
+++ b/src/core/tests/dune
@@ -24,21 +24,21 @@
   (modules test_csexp)
   (libraries containers csexp qcheck-core qcheck))
 
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (package containers)
   (action (run ./test_random.exe)))
 
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (package containers)
   (action (run ./test_csexp.exe)))
 
 ; what matters is that it compiles
-(alias
-  (name runtest)
+(rule
+  (alias runtest)
   (locks /ctest)
   (package containers)
   (action  (run ./check_labelled_mods.exe)))
diff --git a/src/data/CCMutHeap_intf.ml b/src/data/CCMutHeap_intf.ml
index fc64ce92..ec5aa81e 100644
--- a/src/data/CCMutHeap_intf.ml
+++ b/src/data/CCMutHeap_intf.ml
@@ -5,9 +5,14 @@
 
 module type RANKED = sig
   type t
-  val idx: t -> int (** Index in heap. return -1 if never set *)
-  val set_idx : t -> int -> unit (** Update index in heap *)
-  val lt : t -> t -> bool (** [cmp a b] is true iff [a < b] *)
+  val idx: t -> int
+  (** Index in heap. return -1 if never set *)
+
+  val set_idx : t -> int -> unit
+  (** Update index in heap *)
+
+  val lt : t -> t -> bool
+  (** [cmp a b] is true iff [a < b] *)
 end
 
 module type S = sig
diff --git a/src/data/CCPersistentArray.mli b/src/data/CCPersistentArray.mli
index 9fc760a9..729aa74a 100644
--- a/src/data/CCPersistentArray.mli
+++ b/src/data/CCPersistentArray.mli
@@ -78,6 +78,7 @@ val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
 val iter : ('a -> unit) -> 'a t -> unit
 (** [iter f t] applies function [f] to all elements of the persistent array,
     in order from element [0] to element [length t - 1]. *)
+
 val iteri : (int -> 'a -> unit) -> 'a t -> unit
 
 val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a
diff --git a/src/data/dune b/src/data/dune
index 7c5c1868..fe2d6cc0 100644
--- a/src/data/dune
+++ b/src/data/dune
@@ -3,5 +3,6 @@
   (name containers_data)
   (public_name containers-data)
   (wrapped false)
-  (flags :standard -w +a-4-42-44-48-50-58-32-60@8 -safe-string -open CCShims_)
+  (flags :standard -warn-error -3 -w -70 -color always
+         -safe-string -strict-sequence -open CCShims_)
   (libraries containers))
diff --git a/src/dune b/src/dune
index 21bb22a5..8cff22c0 100644
--- a/src/dune
+++ b/src/dune
@@ -1,7 +1,7 @@
 
 (env
   (_
-    (flags :standard -warn-error -3 -color always)
+    (flags :standard -warn-error -a+8 -w -32-70 -color always -safe-string -strict-sequence)
     (ocamlopt_flags :standard -O3 -unbox-closures -unbox-closures-factor 20
                     -inline 100)
      ))
diff --git a/src/mdx_runner.ml b/src/mdx_runner.ml
index e485d0ca..719a366d 100644
--- a/src/mdx_runner.ml
+++ b/src/mdx_runner.ml
@@ -20,7 +20,7 @@ let () =
       printf "warning: ocaml-mdx exited with code %d\n" e;
       just_copy();
     ) else (
-      print_endline "ocaml-mdx returned 0";
+      print_endline "ocaml-mdx returned 0 ✔";
     )
   with Sys_error e ->
     printf "error when running mdx: %s\n" e;
diff --git a/src/monomorphic/CCMonomorphic.ml b/src/monomorphic/CCMonomorphic.ml
index 462cf2b1..14bf732e 100644
--- a/src/monomorphic/CCMonomorphic.ml
+++ b/src/monomorphic/CCMonomorphic.ml
@@ -1,7 +1,10 @@
 
 (* This file is free software, part of containers. See file "license" for more details. *)
 
-open CCMonomorphicShims_
+[@@@ifge 4.07]
+[@@@else_]
+module Stdlib = Pervasives
+[@@@endif]
 
 let (=) : int -> int -> bool = Stdlib.(=)
 let (<>) : int -> int -> bool  = Stdlib.(<>)
diff --git a/src/monomorphic/dune b/src/monomorphic/dune
index c3a3bdbd..be29afd5 100644
--- a/src/monomorphic/dune
+++ b/src/monomorphic/dune
@@ -1,17 +1,6 @@
-
-(executable
-  (name mkshims)
-  (modules mkshims)
-  (libraries dune.configurator))
-
-(rule
-  (targets CCMonomorphicShims_.ml)
-  (deps ./mkshims.exe)
-  (action (with-stdout-to %{targets} (run ./mkshims.exe))))
-
 (library
   (name containers_monomorphic)
   (public_name containers.monomorphic)
-  (modules CCMonomorphic CCMonomorphicShims_)
-  (wrapped false)
-  (flags :standard -w +a-4-42-44-48-50-58-32-60@8 -safe-string))
+  (preprocess (action (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
+  (modules CCMonomorphic)
+  (wrapped false))
diff --git a/src/monomorphic/mkshims.ml b/src/monomorphic/mkshims.ml
deleted file mode 100644
index f391a32c..00000000
--- a/src/monomorphic/mkshims.ml
+++ /dev/null
@@ -1,11 +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))
diff --git a/src/threads/CCPool.ml b/src/threads/CCPool.ml
index eee1dd73..60e8687d 100644
--- a/src/threads/CCPool.ml
+++ b/src/threads/CCPool.ml
@@ -710,13 +710,14 @@ module Make(P : PARAM) = struct
       let (>|=) a f = map f a
       let (<*>) = app
 
+      [@@@ifge 4.8]
 
-    include CCShimsMkLet_.Make(struct
-        type nonrec 'a t = 'a t
-        let (>>=) = (>>=)
-        let (>|=) = (>|=)
-        let monoid_product a1 a2 = monoid_product (fun x y->x,y) a1 a2
-      end)
+      let (let+) = (>|=)
+      let (let*) = (>>=)
+      let[@inline] (and+) a1 a2 = monoid_product (fun x y->x,y) a1 a2
+      let (and*) = (and+)
+
+      [@@@endif]
     end
 
     include Infix
diff --git a/src/threads/CCPool.mli b/src/threads/CCPool.mli
index 603cc1e3..b9892fc2 100644
--- a/src/threads/CCPool.mli
+++ b/src/threads/CCPool.mli
@@ -154,23 +154,15 @@ module Make(P : PARAM) : sig
       val (>|=) : 'a t -> ('a -> 'b) -> 'b t
       val (<*>) : ('a -> 'b) t -> 'a t -> 'b t
 
+      [@@@ifge 4.08]
+
+      include CCShims_syntax.LET with type 'a t := 'a t
       (** Let operators on OCaml >= 4.08.0, nothing otherwise
           @since 2.8 *)
-      include CCShimsMkLet_.S with type 'a t_let := 'a t
+
+      [@@@endif]
     end
 
-    val (>>=) : 'a t -> ('a -> 'b t) -> 'b t
-
-    val (>>) : 'a t -> (unit -> 'b t) -> 'b t
-
-    val (>|=) : 'a t -> ('a -> 'b) -> 'b t
-    (** Alias to {!map}. *)
-
-    val (<*>): ('a -> 'b) t -> 'a t -> 'b t
-    (** Alias to {!app}. *)
-
-    (** Let operators on OCaml >= 4.08.0, nothing otherwise
-        @since 2.8 *)
-    include CCShimsMkLet_.S with type 'a t_let := 'a t
+    include module type of Infix
   end
 end
diff --git a/src/threads/dune b/src/threads/dune
index f27b5e65..1959f433 100644
--- a/src/threads/dune
+++ b/src/threads/dune
@@ -5,5 +5,6 @@
   (wrapped false)
   (optional)
   (flags :standard -warn-error -a+8 -w -32 -safe-string -open CCShims_)
+  (preprocess (action (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
   (libraries containers threads))
 
diff --git a/src/top/dune b/src/top/dune
index 29a6a6b4..5c8f3724 100644
--- a/src/top/dune
+++ b/src/top/dune
@@ -4,5 +4,4 @@
   (name containers_top)
   (public_name containers.top)
   (wrapped false)
-  (flags :standard -w +a-4-42-44-48-50-58-32-60@8 -safe-string)
   (libraries compiler-libs.common containers containers.unix))