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bh=i8HAiV3RypL6m644aK7y2wwi+ypVoeQjgs3WavWkilo=; h=Date:Subject:To:References:From:In-Reply-To:From; b=CpZIv7f2QtwotPIkAu0RmEMk8GdfUikBkwf5ABKGUmHLuMhXTgCo5CU5pr8xAjqYu 4dOBVDuN0LZVwrVILBaI9B4f3ZEYL9efzunIbm2anrz3nkGfX0ueQebIjThLNiDS0d WILiP9YkBsHAYADB53ORSIqt2QKz7m167SnJUs3Y= Content-Type: multipart/alternative; boundary="------------rea6kTVuL0Q0J2bT0j10jr9n" Message-ID: <62c6640c-d06c-4103-b479-5dace4b5cfaf@polychoron.fr> Date: Tue, 3 Jun 2025 11:23:08 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird To: caml-list@inria.fr References: <26E93D7A-BAF3-46FB-8CAF-8DA7F7DEAF6B@mpi-sws.org> Content-Language: en-US From: Florian Angeletti In-Reply-To: <26E93D7A-BAF3-46FB-8CAF-8DA7F7DEAF6B@mpi-sws.org> Subject: Re: [Caml-list] Warning 69 (unused record fields) and polymorphic reads Reply-To: Florian Angeletti X-Loop: caml-list@inria.fr X-Sequence: 19334 Errors-To: caml-list-owner@inria.fr Precedence: list Precedence: bulk Sender: caml-list-request@inria.fr X-no-archive: yes List-Id: List-Help: List-Subscribe: List-Unsubscribe: List-Post: List-Owner: List-Archive: Archived-At: This is a multi-part message in MIME format. --------------rea6kTVuL0Q0J2bT0j10jr9n Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Those warnings can be suppressed on the type definition by adding a `@@warning` attribute module M: sig type t end = struct   type t = { x:int; y:int } [@@warning "-unused-field"] end With this attribute, the compiler will not register those fields for the usage checker and thus no unused warning fields will be emitted for the fields `x` and `y` For this specific warning, the compiler even supports disabling the usage checker field-by-field. For instance, this code will only disable the warning for the `M.x` field, and thus warns module M: sig type t end = struct   type t = { x:int[@warning "-unused-field"] ; y:int } end  that the field `y` is being unused while omitting the check for the  field `x`. — Florian. Le 03/06/2025 à 11:11, Andreas Rossberg a écrit : > Today, I ran into a slight annoyance with warning 69 (unused record fields). Obviously, the warning does not consider uses of polymorphic operators like `=` or `compare`, which technically are reads of the fields. Unfortunately, it turns out that there are reasonable use cases where these are the _only_ reads, resulting in bogus warnings. > > There probably isn't much that can be done about it(?), since such access could hide in any polymorphic function invocation. Hence I didn’t file a bug. But for the record, I thought I'll show the counter example anyway. > > Consider code that implements some processing akin to SQL `group by`, as in: > ``` > SELECT artist, album, COUNT(*), SUM(time), ... FROM Tracks GROUP BY artist, album; > ``` > Intuitively, this extracts all known albums from a list of track (song) meta data, and computes their total running time, among other values. > > Here is a sketch of how to achieve something similar in OCaml: > ``` > module GroupKey = > struct > type t = {artist : string; title : string} > let compare = compare > end > module GroupMap = Map.Make(GroupKey) > > type track = ... > type acc = ... (* result type *) > val empty_acc : acc > val accumulate : entry -> acc -> acc (* combine result *) > > let albums = > tracks > |> List.fold_left (fun map (entry : track) -> > let group = {artist = entry.artist; title = entry.title} in > let acc = Option.value (GroupMap.find_opt group map) ~default: empty_acc in > GroupMap.add group (accumulate acc entry) map > ) GroupMap.empty > |> GroupMap.bindings |> List.map snd > ``` > The only purpose of the `GroupKey.t` type in this code is to identify entries belonging to the same group. Its fields are read implicitly by `GroupMap.find/add`, which invokes `compare` on them. Yet, this code produces warnings that `artist` and `title` are never read, which technically isn’t quite correct. > > In my actual code, the key record has more fields, which is why I didn’t want to replace it with a tuple. > > Perhaps there is some annotation magic I’m missing that could be applied to the type definition to suppress the warning? > > /Andreas > --------------rea6kTVuL0Q0J2bT0j10jr9n Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit Those warnings can be suppressed on the type definition by adding a `@@warning` attribute
module M: sig type t end = struct
  type t = { x:int; y:int } [@@warning "-unused-field"]
end
With this attribute, the compiler will not register those fields for the usage checker and thus no unused
warning fields will be emitted for the fields `x` and `y`

For this specific warning, the compiler even supports disabling the usage checker field-by-field.
For instance, this code will only disable the warning for the `M.x` field, and thus warns
module M: sig type t end = struct
  type t = { x:int[@warning "-unused-field"] ; y:int }
end
 that the field `y` is being unused while omitting the check for the  field `x`.

— Florian.

Le 03/06/2025 à 11:11, Andreas Rossberg a écrit :
Today, I ran into a slight annoyance with warning 69 (unused record fields). Obviously, the warning does not consider uses of polymorphic operators like `=` or `compare`, which technically are reads of the fields. Unfortunately, it turns out that there are reasonable use cases where these are the _only_ reads, resulting in bogus warnings.

There probably isn't much that can be done about it(?), since such access could hide in any polymorphic function invocation. Hence I didn’t file a bug. But for the record, I thought I'll show the counter example anyway.

Consider code that implements some processing akin to SQL `group by`, as in:
```
SELECT artist, album, COUNT(*), SUM(time), ... FROM Tracks GROUP BY artist, album;
```
Intuitively, this extracts all known albums from a list of track (song) meta data, and computes their total running time, among other values.

Here is a sketch of how to achieve something similar in OCaml:
```
module GroupKey =
   struct
      type t = {artist : string; title : string}
      let compare = compare
   end
module GroupMap = Map.Make(GroupKey)

type track = ...
type acc = ...  (* result type *)
val empty_acc : acc
val accumulate : entry -> acc -> acc  (* combine result *)

let albums =
   tracks
   |> List.fold_left (fun map (entry : track) ->
         let group = {artist = entry.artist; title = entry.title} in
         let acc = Option.value (GroupMap.find_opt group map) ~default: empty_acc in
         GroupMap.add group (accumulate acc entry) map
      ) GroupMap.empty
   |> GroupMap.bindings |> List.map snd
```
The only purpose of the `GroupKey.t` type in this code is to identify entries belonging to the same group. Its fields are read implicitly by `GroupMap.find/add`, which invokes `compare` on them. Yet, this code produces warnings that `artist` and `title` are never read, which technically isn’t quite correct.

In my actual code, the key record has more fields, which is why I didn’t want to replace it with a tuple.

Perhaps there is some annotation magic I’m missing that could be applied to the type definition to suppress the warning?

/Andreas

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