From 72df07652237e8d66765782a20c572f1b8915ccc Mon Sep 17 00:00:00 2001
From: Valentin Gatien-Baron <valentin.gatienbaron@gmail.com>
Date: Mon, 10 Jun 2024 19:26:37 +0200
Subject: [PATCH] make Ast_builder.{e,p}list able to build lists such as `a ::
 b :: c`

Signed-off-by: Valentin Gatien-Baron <valentin.gatienbaron@gmail.com>
---
 CHANGES.md              |  3 +++
 src/ast_builder.ml      | 24 ++++++++++++++++--------
 src/ast_builder_intf.ml |  4 ++--
 3 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index e3458f2d9..756af5062 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,9 @@
 unreleased
 ----------
 
+- Ast_builder.{e,p}list now take a `?tail:expression` parameter, so they
+  can build ASTs like `a :: b :: c` instead of only `[ a; b ]`.
+
 - Fix `Longident.parse` so it also handles indexing operators such as
   `.!()`, `.%(;..)<-`, or `Vec.(.%())` (#494, @octachron)
 
diff --git a/src/ast_builder.ml b/src/ast_builder.ml
index 99a2e4992..980672ddf 100644
--- a/src/ast_builder.ml
+++ b/src/ast_builder.ml
@@ -127,21 +127,29 @@ module Default = struct
   let econstruct cd arg =
     pexp_construct ~loc:cd.pcd_loc (Located.map_lident cd.pcd_name) arg
 
-  let rec elist ~loc l =
+  let rec elist ~loc ?tail l =
     match l with
-    | [] -> pexp_construct ~loc (Located.mk ~loc (Longident.Lident "[]")) None
+    | [] -> (
+        match tail with
+        | Some e -> e
+        | None ->
+            pexp_construct ~loc (Located.mk ~loc (Longident.Lident "[]")) None)
     | x :: l ->
         pexp_construct ~loc
           (Located.mk ~loc (Longident.Lident "::"))
-          (Some (pexp_tuple ~loc [ x; elist ~loc l ]))
+          (Some (pexp_tuple ~loc [ x; elist ~loc ?tail l ]))
 
-  let rec plist ~loc l =
+  let rec plist ~loc ?tail l =
     match l with
-    | [] -> ppat_construct ~loc (Located.mk ~loc (Longident.Lident "[]")) None
+    | [] -> (
+        match tail with
+        | Some p -> p
+        | None ->
+            ppat_construct ~loc (Located.mk ~loc (Longident.Lident "[]")) None)
     | x :: l ->
         ppat_construct ~loc
           (Located.mk ~loc (Longident.Lident "::"))
-          (Some (ppat_tuple ~loc [ x; plist ~loc l ]))
+          (Some (ppat_tuple ~loc [ x; plist ~loc ?tail l ]))
 
   let unapplied_type_constr_conv_without_apply ~loc (ident : Longident.t) ~f =
     match ident with
@@ -386,8 +394,8 @@ end) : S = struct
   let eapply e el = Default.eapply ~loc e el
   let eabstract ps e = Default.eabstract ~loc ps e
   let esequence el = Default.esequence ~loc el
-  let elist l = Default.elist ~loc l
-  let plist l = Default.plist ~loc l
+  let elist ?tail l = Default.elist ~loc ?tail l
+  let plist ?tail l = Default.plist ~loc ?tail l
 
   let type_constr_conv ident ~f args =
     Default.type_constr_conv ~loc ident ~f args
diff --git a/src/ast_builder_intf.ml b/src/ast_builder_intf.ml
index 652828d93..9bd42fefc 100644
--- a/src/ast_builder_intf.ml
+++ b/src/ast_builder_intf.ml
@@ -42,8 +42,8 @@ module type Additional_helpers = sig
   val pexp_tuple_opt : (expression list -> expression option) with_loc
   val pconstruct : constructor_declaration -> pattern option -> pattern
   val econstruct : constructor_declaration -> expression option -> expression
-  val elist : (expression list -> expression) with_loc
-  val plist : (pattern list -> pattern) with_loc
+  val elist : (?tail:expression -> expression list -> expression) with_loc
+  val plist : (?tail:pattern -> pattern list -> pattern) with_loc
 
   val pstr_value_list :
     loc:Location.t ->