From 16f90a7ef50bcfddee8f2f35f0ccaa04bc2b8334 Mon Sep 17 00:00:00 2001
From: Joseph Ferano <joseph@ferano.io>
Date: Sun, 20 Sep 2026 18:14:58 +0700
Subject: [PATCH] The dot advice says assign where the form is an assignment

(set p.x 1) was being told that a field is read with an accessor, which is a
sentence that does not apply to the form it is printed under. The place
spelling is (set (.x p) 1), checked to be a real form.
---
 lib/check.ml      | 31 +++++++++++++++++++------------
 test/test_flan.ml |  5 +++++
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/lib/check.ml b/lib/check.ml
index 6713d33..3d0b36c 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -4413,8 +4413,8 @@ and value_candidates ctx =
    dot-infix habit.
 
    The second is the near miss, over values only — see [value_candidates]. *)
-and unknown_name : 'a. ctx -> Loc.t -> string -> 'a =
-  fun ctx loc name ->
+and unknown_name : 'a. ?setting:bool -> ctx -> Loc.t -> string -> 'a =
+  fun ?(setting = false) ctx loc name ->
   let dot = String.index_opt name '.' in
   let head, field =
     match dot with
@@ -4440,25 +4440,32 @@ and unknown_name : 'a. ctx -> Loc.t -> string -> 'a =
     | Some sn, _ ->
       let s = Option.get (fields_named ctx.env sn) in
       let notes = declared_note ctx.env sn in
+      (* In a [set] the accessor is the *place*, so the spelling to give is
+         [(set (.x p) 1)] and not [(.x p)] on its own. Saying "read" at an
+         assignment would be a sentence that does not apply to the form it is
+         printed under. *)
+      let how =
+        if setting then Printf.sprintf "a field is assigned through an \
+                                        accessor, so write (set (.%s %s) ...)"
+            field head
+        else Printf.sprintf "a field is read with an accessor, so write (.%s %s)"
+            field head
+      in
       if Tast.field_index s field <> None then
-        Loc.failk "check/dot-access" loc ~notes
-          "unknown name %s — a field is read with an accessor, so write (.%s %s)"
-          name field head
+        Loc.failk "check/dot-access" loc ~notes "unknown name %s — %s" name how
       else
         Loc.failk "check/dot-access" loc ~notes
-          "unknown name %s — a field is read with an accessor, (.%s %s), and \
-           %s has no field %s"
-          name field head sn field
+          "unknown name %s — %s, and %s has no field %s" name how sn field
     | None, Some t ->
       Loc.failk "check/dot-access" loc
         "unknown name %s — a dot is part of the name here, not field access. \
-         Fields are read with an accessor, (.%s %s), and %s is %s, which has \
-         no fields"
+         A field is reached through an accessor, (.%s %s), and %s is %s, \
+         which has no fields"
         name field head head (Types.to_string t)
     | None, None ->
       Loc.failk "check/unknown-name" loc
         "unknown name %s — nothing named %s is in scope either. A field is \
-         read with an accessor, (.%s %s), not with a dot"
+         reached through an accessor, (.%s %s), not with a dot"
         name head field head
   end
   else
@@ -4578,7 +4585,7 @@ and check_place ctx loc (p : Ast.place) : Tast.place * Types.t =
             into the image and there is nothing to assign to. Declare it with \
             defvar if it has to change" name
        | Some (ty, false) -> Tast.Pglobal name, ty
-       | None -> captured ctx loc name; unknown_name ctx loc name)
+       | None -> captured ctx loc name; unknown_name ~setting:true ctx loc name)
   | Ast.Pfield (target, name) ->
     let target, sname = struct_target ctx target in
     let s = Option.get (fields_named ctx.env sname) in
diff --git a/test/test_flan.ml b/test/test_flan.ml
index f09a568..a95d824 100644
--- a/test/test_flan.ml
+++ b/test/test_flan.ml
@@ -1739,6 +1739,11 @@ let () =
   rejects_check "and says so when the field is not there either"
     "(defstruct P [x i32]) (defn f [] i32 (let [p (P {.x 1})] p.z))"
     ~needle:"(.z p), and P has no field z";
+  rejects_check "and in a set it is the place that is spelled"
+    "(defstruct P [x i32]) (defn f [] i32 (let [p (P {.x 1})] (set p.x 2) 0))"
+    ~needle:"a field is assigned through an accessor, so write (set (.x p) ...)";
+  accepts "which is a real form"
+    "(defstruct P [x i32]) (defn f [] i32 (let [p (P {.x 1})] (set (.x p) 2) (.x p)))";
   rejects_check "a dotted head that is not a struct says what it is"
     "(defn f [] i32 (let [n 1] n.x))" ~needle:"n is i32, which has no fields";
   (* A capitalised head keeps the case spelling it always had: [Shape.Circle]