dialect

(dialect ...)

Declare a new dialect.

(name <name>)

The name of the dialect being defined. It must be unique in a given project.

This field is required.

(implementation ...)

Details related to the implementation files (corresponding to *.ml).

Changed in version 3.9: This field is made optional.

(extension <string>)

Specify the file extension used for this dialect.

The extension string must not start with a period and be unique in a given project (so that a given extension can be mapped back to a corresponding dialect). In Dune 3.9 and later, the extension string may contain periods (e.g., cppo.ml).

This field is required.

(preprocess <action>)

Run <action> to produce a valid OCaml abstract syntax tree.

This action is expected to read the file given in the variable named %{input-file} and output a binary abstract syntax tree on its standard output.

If the field is not present, it is assumed that the corresponding source code is already valid OCaml code and can be passed to the OCaml compiler as-is.

See also

Preprocessing With Actions

(format <action>)

Run <action> to format source code for this dialect.

The action is expected to read the file given in the variable named %{input-file} and output the formatted source code on its standard output.

If the field is not present, the behavior depends on the presence of (preprocess): if it is also not present (that is, the dialect consists of valid OCaml code), then the dialect will be formatted as any other OCaml code. Otherwise no special formatting will be done.

See also

How to Set up Automatic Formatting

(interface ...)

Details related to the interface files (corresponding to *.mli).

This field supports the same sub-fields as implementation.

Changed in version 3.9: This field is made optional.

(merlin_reader <program> <args>...)

Configure Merlin to use <program> <args>… as READER. Merlin’s READER is a mechanism to extend Merlin to support OCaml dialects by providing a program that transforms a dialect AST into an OCaml AST.

See also

merlin/src/extend/extend_protocol.ml for the protocol specification.

This field is optional.

Added in version 3.16.

Default dialects

Dune ships with two dialects pre-configured and enabled:

• ocaml for the default OCaml syntax which consumes .ml and .mli files and uses ocamlformat for formatting.

• reason for the Reason syntax and enabled in .re/.rei files. refmt is used for formatting.

A third dialect, rescript, is added when Melange support (see JavaScript Compilation With Melange) is enabled in the project.