OCaml 5.4 syntax support by Octachron · Pull Request #2720 · ocaml-ppx/ocamlformat

Octachron · 2025-08-08T15:19:11Z

This PR updates the vendored parsers and related types to mirror the 5.4 version of the OCaml parser and adds a basic support for labeled tuples and bivariance.

The parser updates means that it is possible to locate comments between Longident.t component, and it is always possible to distinguish between (module M): (module S) at least for expressions.

The support for labeled expressions is bit ad-hoc and hackish in this version due to the way that labeled tuple element interact with parenthesis, for instance inside:

let t = ~x:(Fun.id 0), 1

and it could definitively be improved.

- update vendored parsers to mirror upstream at 5.4: * introduce locations for Longident.t components * distinguish (module M:S) and ((module M):(module S)) for expressions - support for new syntaxes: * bivariance * labelled tuples

Julow

Sorry for the slow reply. This is awesome :) Thanks a lot!

Julow · 2025-08-29T10:37:50Z

  | BANG MINUS  { [ mkvarinj "!" $loc($1); mkvarinj "-" $loc($2) ] }
  | INFIXOP2
-      { if ($1 = "+!") || ($1 = "-!") then [ mkvarinj $1 $sloc ]
+      { if $1 = "+!" ||  $1 = "-!" || $1 = "+-"|| $1 = "-+"


Do you think we could accept anything starting in +, - or ! here ? The standard parser will catch errors there anyway.

From the parser point of view, we could accept any INFIXOP2. And I agree that it probably better to accept this when building the CST.

Julow · 2025-08-29T15:33:09Z

      in
      let outer_wrap = has_attr && parens in
      let inner_wrap = has_attr || parens in
+      let with_label (lbl, exp) =


Instead handling special cases for ( ~foo, ~(bar : t) ), etc.. do you think we could represent the concrete syntax in the parsetree ?
I mean something like type tuple_label = Tl_pun of string loc | Tl_constraint of string loc * core_type | Tl_normal of string loc

Are case where people don't want to normalize ~x:x, ~y to ~x, ~y ? If yes, it is probably better to separate the two forms in the parsetree. Otherwise, I am not sure if this is that useful?

I think it's useful to make the formatting code simpler and less buggy. Decoding the AST was a major source of bugs in the past.
We generally do the normalization by modifying the AST: https://github.com/ocaml-ppx/ocamlformat/blob/main/lib/Extended_ast.ml#L68

I'd be happy to do the changes if you don't have the time ?

After experimenting a bit more, I agree that it feels better to not have a straight mapping of locations with punning. I will send my updated commits after some more tests.

I know I am late, but my current opinion is that every token should be represent in our CST, with its location included. This is the only way to have a bunch of features, like proper comment formatting.

~x: (* blabla *) x, ~y

is valid and should be allowed.

Octachron · 2025-09-09T15:49:06Z

 - Support for OCaml 5.4 (#2717, #2720, @Julow, @Octachron)
+  OCamlformat now supports OCaml 5.4 syntax.
+  Module packing of the form `((module M) : (module S))` are no longer
+  rewritten to `(module M : S)` because these are now two different syntaxes.


You could mention that this in preparation of modular explicits.

Julow

Thanks! Sorry for the slow reply, I missed the notification for your commits.

I added commits to your branch to fix the CI (#2721) and to add details to the changelog.
I'll wait for the CI to finish before merging.

Julow · 2025-09-09T16:06:51Z

I explain the diffs in CI (https://github.com/ocaml-ppx/ocamlformat/actions/runs/17588187443/job/49961988518?pr=2720) by a reduced number of errors (from 88 to 78 errors). Thank you for making this !

@Julow

….28.1) CHANGES: ### Highlight - \* Support for OCaml 5.4 (ocaml-ppx/ocamlformat#2717, ocaml-ppx/ocamlformat#2720, ocaml-ppx/ocamlformat#2732, ocaml-ppx/ocamlformat#2733, ocaml-ppx/ocamlformat#2735, @Julow, @Octachron, @cod1r, @EmileTrotignon) OCamlformat now supports OCaml 5.4 syntax. Module packing of the form `((module M) : (module S))` are no longer rewritten to `(module M : S)` because these are now two different syntaxes. - \* Reduce indentation after `|> map (fun` (ocaml-ppx/ocamlformat#2694, @EmileTrotignon) Notably, the indentation no longer depends on the length of the infix operator, for example: ```ocaml (* before *) v |>>>>>> map (fun x -> x ) (* after *) v |>>>>>> map (fun x -> x ) ``` `@@ match` can now also be on one line. ### Added - Added option `module-indent` option (ocaml-ppx/ocamlformat#2711, @HPRIOR) to control the indentation of items within modules. This affects modules and signatures. For example, module-indent=4: ```ocaml module type M = sig type t val f : (string * int) list -> int end ``` - `exp-grouping=preserve` is now the default in `default` and `ocamlformat` profiles. This means that its now possible to use `begin ... end` without tweaking ocamlformat. (ocaml-ppx/ocamlformat#2716, @EmileTrotignon) ### Deprecated - Starting in this release, ocamlformat can use cmdliner >= 2.0.0. When that is the case, the tool no longer accepts unambiguous option names prefixes. For example, `--max-iter` is not accepted anymore, you have to pass the full option `--max-iters`. This does not apply to the keys in the `.ocamlformat` configuration files, which have always required the full name. See dbuenzli/cmdliner#200. (ocaml-ppx/ocamlformat#2680, @emillon) ### Changed - \* The formatting of infix extensions is now consistent with regular formatting by construction. This reduces indentation in `f @@ match%e` expressions to the level of indentation in `f @@ match`. Other unknown inconsistencies might also be fixed. (ocaml-ppx/ocamlformat#2676, @EmileTrotignon) - \* The spacing of infix attributes is now consistent across keywords. Every keyword but `begin` `function`, and `fun` had attributes stuck to the keyword: `match[@A]`, but `fun [@A]`. Now its also `fun[@A]`. (ocaml-ppx/ocamlformat#2676, @EmileTrotignon) - \* The formatting of`let a = b in fun ...` is now consistent with other contexts like `a ; fun ...`. A check for the syntax `let a = fun ... in ...` was made more precise. (ocaml-ppx/ocamlformat#2705, @EmileTrotignon) - \* `|> begin`, `~arg:begin`, `begin if`, `lazy begin`, `begin match`, `begin fun` and `map li begin fun` can now be printed on the same line, with one less indentation level for the body of the inner expression. (ocaml-ppx/ocamlformat#2664, ocaml-ppx/ocamlformat#2666, ocaml-ppx/ocamlformat#2671, ocaml-ppx/ocamlformat#2672, ocaml-ppx/ocamlformat#2681, ocaml-ppx/ocamlformat#2685, ocaml-ppx/ocamlformat#2693, @EmileTrotignon) For example : ```ocaml (* before *) begin fun x -> some code end (* after *) begin fun x -> some code end ``` - \* `break-struct=natural` now also applies to `sig ... end`. (ocaml-ppx/ocamlformat#2682, @EmileTrotignon) ### Fixed - Fixed `wrap-comments=true` not working with the janestreet profile (ocaml-ppx/ocamlformat#2645, @Julow) Asterisk-prefixed comments are also now formatted the same way as with the default profile. - Fixed `nested-match=align` not working with `match%ext` (ocaml-ppx/ocamlformat#2648, @EmileTrotignon) - Fixed the AST generated for bindings of the form `let pattern : type = function ...` (ocaml-ppx/ocamlformat#2651, @v-gb) - Print valid syntax for the corner case (1).a (ocaml-ppx/ocamlformat#2653, @v-gb) - `Ast_mapper.default_mapper` now iterates on the location of `in` in `let+ .. in ..` (ocaml-ppx/ocamlformat#2658, @v-gb) - Fix missing parentheses in `let+ (Cstr _) : _ = _` (ocaml-ppx/ocamlformat#2661, @Julow) This caused a crash as the generated code wasn't valid syntax. - Fix bad indentation of `let%ext { ...` (ocaml-ppx/ocamlformat#2663, @EmileTrotignon) with `dock-collection-brackets` enabled. - ocamlformat is now more robust when used as a library to print modified ASTs (ocaml-ppx/ocamlformat#2659, @v-gb) - Fix crash due to edge case with asterisk-prefixed comments (ocaml-ppx/ocamlformat#2674, @Julow) - Fix crash when formatting `mld` files that cannot be lexed as ocaml (e.g. containing LaTeX or C code) (ocaml-ppx/ocamlformat#2684, @emillon) - \* Fix double parens around module constraint in functor application : `module M = F ((A : T))` becomes `module M = F (A : T)`. (ocaml-ppx/ocamlformat#2678, @EmileTrotignon) - Fix misplaced `;;` due to interaction with floating doc comments. (ocaml-ppx/ocamlformat#2691, @EmileTrotignon) - The formatting of attributes of expression is now aware of the attributes infix or postix positions: `((fun [@A] x -> y) [@b])` is formatted without moving attributes. (ocaml-ppx/ocamlformat#2676, @EmileTrotignon) - `begin%e ... end` and `begin [@A] ... end` nodes are always preserved. (ocaml-ppx/ocamlformat#2676, @EmileTrotignon) - `begin end` syntax for `()` is now preserved. (ocaml-ppx/ocamlformat#2676, @EmileTrotignon) - Fix a crash on `type 'a t = A : 'a. {a: 'a} -> 'a t`. (ocaml-ppx/ocamlformat#2710, @EmileTrotignon) - Fix a crash where `type%e nonrec t = t` was formatted as `type nonrec%e t = t`, which is invalid syntax. (ocaml-ppx/ocamlformat#2712, @EmileTrotignon) - Fix commandline parsing being quadratic in the number of arguments (ocaml-ppx/ocamlformat#2724, @let-def) - \* Fix `;;` being added after a documentation comment (ocaml-ppx/ocamlformat#2683, @EmileTrotignon) This results in more `;;` being inserted, for example: ```ocaml (* before *) print_endline "foo" let a = 3 (* after *) print_endline "foo" ;; let a = 3 ``` - Fix dropped comment in `if then (* comment *) begin .. end` (ocaml-ppx/ocamlformat#2734, @Julow)

Octachron mentioned this pull request Jul 29, 2025

OCaml 5.4.0 release readiness ocaml/opam-repository#27916

Closed

18 tasks

Octachron added 4 commits August 8, 2025 17:30

Basic OCaml 5.4 support

a07793a

- update vendored parsers to mirror upstream at 5.4: * introduce locations for Longident.t components * distinguish (module M:S) and ((module M):(module S)) for expressions - support for new syntaxes: * bivariance * labelled tuples

Add test for labeled tuples

9d99992

Test bivariance

6deaae3

update CHANGES.md

201949e

Octachron force-pushed the OCaml_5.4_support branch from eabfcee to 201949e Compare August 8, 2025 15:49

Julow reviewed Aug 29, 2025

View reviewed changes

Octachron added 5 commits September 1, 2025 13:21

review: lax rule for variance annotation

8689441

review: test comments inside tuple types

8d418b0

review: track labels locations in tuple types

f438e4b

WIP: locations for labels

c8325a9

Concrete node for tuple element punning

7cc3534

Octachron force-pushed the OCaml_5.4_support branch from fced3c8 to 7cc3534 Compare September 3, 2025 12:24

Julow added 2 commits September 9, 2025 17:44

Merge branch 'MAIN' into pr/2720

32379ce

Mention new syntax in CHANGES

07fff22

Octachron commented Sep 9, 2025

View reviewed changes

Julow approved these changes Sep 9, 2025

View reviewed changes

Julow merged commit 3d98dcd into ocaml-ppx:main Sep 9, 2025
10 of 12 checks passed

Octachron mentioned this pull request Oct 6, 2025

[5.4 compat] effects keyword can be an attribute id #2714

Merged

Julow mentioned this pull request Oct 14, 2025

Distinguish module packing annotations #2656

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Julow mentioned this pull request Oct 21, 2025

[new release] ocamlformat-lib, ocamlformat and ocamlformat-rpc-lib (0.28.0) ocaml/opam-repository#28752

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EmileTrotignon mentioned this pull request Oct 22, 2025

Last fn arguments shifted to the right with js profile (0.28 change) #2729

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Julow mentioned this pull request Oct 24, 2025

[new release] ocamlformat-lib, ocamlformat and ocamlformat-rpc-lib (0.28.1) ocaml/opam-repository#28774

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OCaml 5.4 syntax support#2720

OCaml 5.4 syntax support#2720
Julow merged 11 commits intoocaml-ppx:mainfrom
Octachron:OCaml_5.4_support

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