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5.5 support: Structure item embedded in expressions#2780

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Julow merged 14 commits intoocaml-ppx:mainfrom
Julow:55-expr-let-str
Mar 13, 2026
Merged

5.5 support: Structure item embedded in expressions#2780
Julow merged 14 commits intoocaml-ppx:mainfrom
Julow:55-expr-let-str

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@Julow Julow commented Mar 13, 2026

Closes #2700

This backports the Pexp_struct_item AST node, which replaces Pexp_letexception, Pexp_letmodule and
Pexp_letopen. The code for the old constructs is removed and replaced by a call to fmt_structure_item. This requires a lot of refactoring.

This brings many regression but some of them can be considered to be improvements instead. Some of them are:

  • Items with the disable attribute flow with the surrounding expression better:
-let f () = let module [@ocamlformat "disable"] X = Y
-  in
+let f () =
+  let module [@ocamlformat "disable"] X = Y in
   ()
  • Attributes on let module, let open, etc.. are rewritten in the short form:
 let g =
   M.f
-    ((let open M in
-      x
-     ) [@attr]
+    (let[@attr] open M in
+     x
     )

Julow added 14 commits March 11, 2026 19:07
@Julow
parser-std: Backport let-str-item
4131c7a
@Julow
parser-ext: Backport let-str-item
72def28
@Julow
Format Pexp_struct_item
a86f41e 
`Pexp_struct_item` replaces `Pexp_letexception`, `Pexp_letmodule` and
`Pexp_letopen`. The code for the old constructs is removed and replaced
by a call to `fmt_structure_item`.

The code for formatting structure items is changed to propagate the
`~pro` and `~epi` arguments.

Tweak break before 'in'
@Julow
Promote tests
01a9a15
@Julow
Restore fitting of 'in' in some 'let module' expressions
c6dea86
@Julow
Tweak fitting of 'in' after functors
2846f18
@Julow
Tweak comments around Pexp_struct_item
01edc4d
@Julow
Fix closing parens exceeding the margin
5d93bce
@Julow
Restore 'in' behavior for the janestreet profile
55a7bdb
@Julow
Fix let-module=sparse breaking regular module formatting
40dd946
@Julow
Extend the context for embedded struct items
f6f2be2 
The `Str_exp` context is needed to apply the `let_module` option. The
`Mb` context constructor is extended to carry the parent context, which
would otherwise be erased before `fmt_module` gets called.
@Julow
Restore closing parens behavior
4303bf4
@Julow
Tweak for the janestreet profile
9a10fa1
@Julow
Update CHANGES
d217024 
Listing the changes.
@Julow Julow merged commit 5582edd into ocaml-ppx:main Mar 13, 2026
4 of 9 checks passed
Julow added a commit to Julow/opam-repository that referenced this pull request Mar 17, 2026
@Julow
[new release] ocamlformat (4 packages) (0.29.0)
f62a590 
CHANGES:

### Highlight

- \* Support OCaml 5.5 syntax
  (ocaml-ppx/ocamlformat#2772, ocaml-ppx/ocamlformat#2774, ocaml-ppx/ocamlformat#2775, ocaml-ppx/ocamlformat#2777, ocaml-ppx/ocamlformat#2780, ocaml-ppx/ocamlformat#2781, ocaml-ppx/ocamlformat#2782, ocaml-ppx/ocamlformat#2783, @Julow)
  The update brings several tiny changes, they are listed below.

- \* Update Odoc's parser to 3.0 (ocaml-ppx/ocamlformat#2757, @Julow)
  The indentation of code-blocks containing OCaml code is reduced by 2 to avoid
  changing the generated documentation. The indentation within code-blocks is
  now significative in Odoc and shows up in generated documentation.

### Added

- Added option `letop-punning` (ocaml-ppx/ocamlformat#2746, @WardBrian) to control whether
  punning is used in extended binding operators.
  For example, the code `let+ x = x in ...` can be formatted as
  `let+ x in ...` when `letop-punning=always`. With `letop-punning=never`, it
  becomes `let+ x = x in ...`. The default is `preserve`, which will
  only use punning when it exists in the source.
  This also applies to `let%ext` bindings (ocaml-ppx/ocamlformat#2747, @WardBrian).

- Support the unnamed functor parameters syntax in module types
  (ocaml-ppx/ocamlformat#2755, ocaml-ppx/ocamlformat#2759, @Julow)
  ```ocaml
  module type F = ARG -> S
  ```
  The following lines are now formatted as they are in the source file:
  ```ocaml
  module M : (_ : S) -> (_ : S) -> S = N
  module M : S -> S -> S = N
  (* The preceding two lines are no longer turned into this: *)
  module M : (_ : S) (_ : S) -> S = N
  ```

### Fixed

- Fix dropped comment in `(function _ -> x (* cmt *))` (ocaml-ppx/ocamlformat#2739, @Julow)

- \* `cases-matching-exp-indent=compact` does not impact `begin end` nodes that
  don't have a match inside. (ocaml-ppx/ocamlformat#2742, @EmileTrotignon)
  ```ocaml
  (* before *)
  begin match () with
  | () -> begin
    f x
  end
  end
  (* after *)
  begin match () with
  | () -> begin
      f x
    end
  end
  ```

- `Ast_mapper` now iterates on *all* locations inside of Longident.t,
  instead of only some.
  (ocaml-ppx/ocamlformat#2737, @v-gb)

- Remove line break in `M with module N = N (* cmt *)` (ocaml-ppx/ocamlformat#2779, @Julow)

### Internal

- Added information on writing tests to `CONTRIBUTING.md` (ocaml-ppx/ocamlformat#2838, @WardBrian)

### Changed

- indentation of the `end` keyword in a match-case is now always at least 2. (ocaml-ppx/ocamlformat#2742, @EmileTrotignon)
  ```ocaml
  (* before *)
  begin match () with
  | () -> begin
    match () with
    | () -> ()
  end
  end
  (* after *)
  begin match () with
  | () -> begin
    match () with
    | () -> ()

- \* use shortcut `begin end` in `match` cases and `if then else` body. (ocaml-ppx/ocamlformat#2744, @EmileTrotignon)
  ```ocaml
  (* before *)
  match () with
  | () -> begin
      match () with
      | () ->
    end
  end
  (* after *)
  match () with
  | () ->
    begin match () with
      | () ->
    end
  end
  ```

- \* Set the `ocaml-version` to `5.4` by default (ocaml-ppx/ocamlformat#2750, @EmileTrotignon)
  The main difference is that the `effect` keyword is recognized without having
  to add `ocaml-version=5.3` to the configuration.
  In exchange, code that use `effect` as an identifier must use
  `ocaml-version=5.2`.

- The work to support OCaml 5.5 come with several improvements:
  + Improve the indentation of `let structure-item` with the
    `[@ocamlformat "disable"]` attribute.
    `let structure-item` means `let module`, `let open`, `let include` and
    `let exception`.
  + `(let open M in e)[@A]` is turned into `let[@A] open M in e`.
  + Long `let open ... in` no longer exceed the margin.
  + Improve indentation of `let structure-item` within parentheses:
    ```ocaml
    (* before *)
    (let module M = M in
    M.foo)
    (* after *)
    (let module M = M in
     M.foo)
    ```
Julow added a commit to Julow/opam-repository that referenced this pull request Mar 17, 2026
@Julow
[new release] ocamlformat (4 packages) (0.29.0)
e67854d 
CHANGES:

### Highlight

- \* Support OCaml 5.5 syntax
  (ocaml-ppx/ocamlformat#2772, ocaml-ppx/ocamlformat#2774, ocaml-ppx/ocamlformat#2775, ocaml-ppx/ocamlformat#2777, ocaml-ppx/ocamlformat#2780, ocaml-ppx/ocamlformat#2781, ocaml-ppx/ocamlformat#2782, ocaml-ppx/ocamlformat#2783, @Julow)
  The update brings several tiny changes, they are listed below.

- \* Update Odoc's parser to 3.0 (ocaml-ppx/ocamlformat#2757, @Julow)
  The indentation of code-blocks containing OCaml code is reduced by 2 to avoid
  changing the generated documentation. The indentation within code-blocks is
  now significative in Odoc and shows up in generated documentation.

### Added

- Added option `letop-punning` (ocaml-ppx/ocamlformat#2746, @WardBrian) to control whether
  punning is used in extended binding operators.
  For example, the code `let+ x = x in ...` can be formatted as
  `let+ x in ...` when `letop-punning=always`. With `letop-punning=never`, it
  becomes `let+ x = x in ...`. The default is `preserve`, which will
  only use punning when it exists in the source.
  This also applies to `let%ext` bindings (ocaml-ppx/ocamlformat#2747, @WardBrian).

- Support the unnamed functor parameters syntax in module types
  (ocaml-ppx/ocamlformat#2755, ocaml-ppx/ocamlformat#2759, @Julow)
  ```ocaml
  module type F = ARG -> S
  ```
  The following lines are now formatted as they are in the source file:
  ```ocaml
  module M : (_ : S) -> (_ : S) -> S = N
  module M : S -> S -> S = N
  (* The preceding two lines are no longer turned into this: *)
  module M : (_ : S) (_ : S) -> S = N
  ```

### Fixed

- Fix dropped comment in `(function _ -> x (* cmt *))` (ocaml-ppx/ocamlformat#2739, @Julow)

- \* `cases-matching-exp-indent=compact` does not impact `begin end` nodes that
  don't have a match inside. (ocaml-ppx/ocamlformat#2742, @EmileTrotignon)
  ```ocaml
  (* before *)
  begin match () with
  | () -> begin
    f x
  end
  end
  (* after *)
  begin match () with
  | () -> begin
      f x
    end
  end
  ```

- `Ast_mapper` now iterates on *all* locations inside of Longident.t,
  instead of only some.
  (ocaml-ppx/ocamlformat#2737, @v-gb)

- Remove line break in `M with module N = N (* cmt *)` (ocaml-ppx/ocamlformat#2779, @Julow)

### Internal

- Added information on writing tests to `CONTRIBUTING.md` (ocaml-ppx/ocamlformat#2838, @WardBrian)

### Changed

- indentation of the `end` keyword in a match-case is now always at least 2. (ocaml-ppx/ocamlformat#2742, @EmileTrotignon)
  ```ocaml
  (* before *)
  begin match () with
  | () -> begin
    match () with
    | () -> ()
  end
  end
  (* after *)
  begin match () with
  | () -> begin
    match () with
    | () -> ()

- \* use shortcut `begin end` in `match` cases and `if then else` body. (ocaml-ppx/ocamlformat#2744, @EmileTrotignon)
  ```ocaml
  (* before *)
  match () with
  | () -> begin
      match () with
      | () ->
    end
  end
  (* after *)
  match () with
  | () ->
    begin match () with
      | () ->
    end
  end
  ```

- \* Set the `ocaml-version` to `5.4` by default (ocaml-ppx/ocamlformat#2750, @EmileTrotignon)
  The main difference is that the `effect` keyword is recognized without having
  to add `ocaml-version=5.3` to the configuration.
  In exchange, code that use `effect` as an identifier must use
  `ocaml-version=5.2`.

- The work to support OCaml 5.5 come with several improvements:
  + Improve the indentation of `let structure-item` with the
    `[@ocamlformat "disable"]` attribute.
    `let structure-item` means `let module`, `let open`, `let include` and
    `let exception`.
  + `(let open M in e)[@A]` is turned into `let[@A] open M in e`.
  + Long `let open ... in` no longer exceed the margin.
  + Improve indentation of `let structure-item` within parentheses:
    ```ocaml
    (* before *)
    (let module M = M in
    M.foo)
    (* after *)
    (let module M = M in
     M.foo)
    ```
@Julow Julow mentioned this pull request Mar 17, 2026
Merged
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5.5 support: Structure item embedded in expressions

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@Julow