From b649ac9dc5bfff01e7f2412ac31376cc683f9d4d Mon Sep 17 00:00:00 2001
From: Fardale <fardale+git@crans.org>
Date: Wed, 25 Dec 2024 14:27:47 +0100
Subject: [PATCH] CCFun(cleanup): align CCFun.compose with the stdlib

Conditionally define CCFun.compose and align its definition with the
stdlib. The arguments are now swapped.
---
 CHANGELOG.md       |  5 +++--
 src/core/CCFun.ml  | 10 ++++++++--
 src/core/CCFun.mli | 12 +++++++++---
 3 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3909b6fb..3eb08dac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,7 +1,8 @@
 
 ## main
-- change the semantic of CCFloat.{min,max} with respect to NaN to follow the Stdlib
-- change the semantic of CCInt.rem with respect to negative number to follow the Stdlib
+- breaking: invert the argument of CCFun.compose to align it with the Stdlib
+- breaking: change the semantic of CCFloat.{min,max} with respect to NaN to follow the Stdlib
+- breaking: change the semantic of CCInt.rem with respect to negative number to follow the Stdlib
 
 ## 3.17
 
diff --git a/src/core/CCFun.ml b/src/core/CCFun.ml
index 96484352..1bf3a4d0 100644
--- a/src/core/CCFun.ml
+++ b/src/core/CCFun.ml
@@ -10,7 +10,13 @@ include Fun
 
 let[@inline] and_pred f g x = f x && g x
 let[@inline] or_pred f g x = f x || g x
-let[@inline] compose f g x = g (f x)
+
+[@@@iflt 5.2]
+
+let[@inline] compose f g x = f (g x)
+
+[@@@endif]
+
 let[@inline] compose_binop f g x y = g (f x) (f y)
 let[@inline] curry f x y = f (x, y)
 let[@inline] uncurry f (x, y) = f x y
@@ -70,7 +76,7 @@ let[@inline] with_return (type ret) f : ret =
 
 module Infix = struct
   (* default implem for some operators *)
-  let ( %> ) = compose
+  let ( %> ) f g = compose g f
   let[@inline] ( % ) f g x = f (g x)
   let ( let@ ) = ( @@ )
   let ( ||> ) (a, b) f = f a b
diff --git a/src/core/CCFun.mli b/src/core/CCFun.mli
index 2d6a5b0e..b024f6f5 100644
--- a/src/core/CCFun.mli
+++ b/src/core/CCFun.mli
@@ -17,8 +17,13 @@ val or_pred : ('a -> bool) -> ('a -> bool) -> 'a -> bool
     @since 3.13.1
 *)
 
-val compose : ('a -> 'b) -> ('b -> 'c) -> 'a -> 'c
-(** [compose f g x] is [g (f x)]. Composition. *)
+[@@@iflt 5.2]
+
+val compose : ('b -> 'c) -> ('a -> 'b) -> 'a -> 'c
+(** [compose f g x] is [f (g x)]. Composition.
+    @since NEXT_RELEASE arguments are inversted *)
+
+[@@@endif]
 
 val compose_binop : ('a -> 'b) -> ('b -> 'b -> 'c) -> 'a -> 'a -> 'c
 (** [compose_binop f g] is [fun x y -> g (f x) (f y)].
@@ -100,7 +105,8 @@ let find_array arr x =
 
 module Infix : sig
   val ( %> ) : ('a -> 'b) -> ('b -> 'c) -> 'a -> 'c
-  (** [(f %> g) x] or [(%>) f g x] is [g (f x)]. Alias to [compose]. *)
+  (** [(f %> g) x] or [(%>) f g x] is [g (f x)]. Infix version of [compose].
+      The order of the arguments of [%>] and {!compose} are inverted. *)
 
   val ( % ) : ('b -> 'c) -> ('a -> 'b) -> 'a -> 'c
   (** [(f % g) x] or [(%) f g x] is [f (g x)]. Mathematical composition. *)