From 7bb2e678e312b5155cc36d9d63b54ec83dc7fb3f Mon Sep 17 00:00:00 2001
From: Josh Berdine <jjb@fb.com>
Date: Thu, 1 Jul 2021 03:30:45 -0700
Subject: [PATCH] [sledge] Add Ord.( @? ) to lexicographically compose compare
 functions

Summary:
This allows easily defining lexicographic orders as a composition of
other orders. For example, the natural ordering on `int * string`
pairs can be written:
```
Ord.Infix.((Int.compare >|= fst) @? (String.compare >|= snd))
```

The `@?` name is chosen as a hybrid between `@@` which denotes
function composition and `<?>` which is the infix operator of
`Containers.Ord` for defining lexicographic orders.

Differential Revision: D29441161

fbshipit-source-id: ae9143cdc
---
 sledge/nonstdlib/NS.mli | 12 +++++++++++-
 sledge/nonstdlib/NS0.ml | 12 +++++++++++-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/sledge/nonstdlib/NS.mli b/sledge/nonstdlib/NS.mli
index 2c2115102..9685eb97a 100644
--- a/sledge/nonstdlib/NS.mli
+++ b/sledge/nonstdlib/NS.mli
@@ -43,7 +43,17 @@ module Poly : sig
   val hash : 'a -> int
 end
 
-module Ord = Containers.Ord
+module Ord : sig
+  include module type of Containers.Ord
+
+  val ( @? ) : 'a t -> 'a t -> 'a t
+
+  module Infix : sig
+    include module type of Containers.Ord.Infix
+
+    val ( @? ) : 'a t -> 'a t -> 'a t
+  end
+end
 
 (** Function combinators *)
 
diff --git a/sledge/nonstdlib/NS0.ml b/sledge/nonstdlib/NS0.ml
index 98f454c4e..cd0331aa1 100644
--- a/sledge/nonstdlib/NS0.ml
+++ b/sledge/nonstdlib/NS0.ml
@@ -55,7 +55,17 @@ module Poly = struct
   let hash = Stdlib.Hashtbl.hash
 end
 
-module Ord = Containers.Ord
+module Ord = struct
+  include Containers.Ord
+
+  let ( @? ) c1 c2 x y = c1 x y <?> (c2, x, y)
+
+  module Infix = struct
+    include Infix
+
+    let ( @? ) = ( @? )
+  end
+end
 
 (** Function combinators *)