package ppx_mixins

ppx_mixins

A ppxlib-based OCaml PPX that lets you compose module-type signatures inline using a [@@mixins ...] attribute on a type declaration.

Motivation

OCaml's include M with type t := foo idiom is the standard way to "mix in" the interface of a module type while substituting the primary type. Writing it by hand is repetitive, especially when several mixins share the same t := substitution. ppx_mixins lets you express this intent concisely:

module type S = sig
  type my_type [@@mixins Printable + Comparable]
end

expands to:

module type S = sig
  type my_type
  include Printable  with type t := my_type
  include Comparable with type t := my_type
end

Installation

Add ppx_mixins to your project's dependencies and preprocessors in dune:

(library
 (name my_lib)
 (preprocess (pps ppx_mixins)))

Syntax

type <name> [@@mixins <mixin> [+ <mixin> ...]]

where each <mixin> is:

ModuleTypeName
ModuleTypeName (param [; param ...])

and each param is one of:

Type can be any of:

Multiple mixins are separated by +. Multiple params within a mixin's parentheses are separated by ;.

Automatic t substitution

Mixin module types conventionally name their primary type t. If you do not provide any constraint that binds t (either t = ... or t := ...), ppx_mixins automatically prepends type t := <name> for you. To suppress this, supply an explicit t constraint:

type my_type [@@mixins Weird (t := my_type; u := my_type)]
(* no extra  t :=  is injected *)

Examples

Single bare mixin

module type S = sig
  type my_type [@@mixins Printable]
end
(* expands to:
   type my_type
   include Printable with type t := my_type *)

Multiple bare mixins

module type S = sig
  type my_type [@@mixins Printable + Comparable]
end
(* expands to:
   type my_type
   include Printable  with type t := my_type
   include Comparable with type t := my_type *)

Equality constraints

module type S = sig
  type my_type [@@mixins Mappable (key = string; value = int)]
end
(* expands to:
   type my_type
   include Mappable with type key = string
                     and type value = int
                     and type t := my_type *)

Mixed equality and substitution with explicit t

module type S = sig
  type my_type [@@mixins Mappable (key = string; value = int; t := my_type)]
end
(* expands to:
   type my_type
   include Mappable with type key = string
                     and type value = int
                     and type t := my_type *)

Parameterised constraint RHS

module type S = sig
  type my_type [@@mixins Container (elt = int list)]
end
(* expands to:
   type my_type
   include Container with type elt = int list and type t := my_type *)

Multi-parameter constructors use the same parenthesised tuple syntax as in type expressions:

type my_type [@@mixins Mapper (result_ = (int, string) result)]

Shorthand module type syntax

When the primary type in the expanded signature should simply be named t, you can skip the sig ... end wrapper entirely and write:

module type S = [%mixins Printable + Comparable]

which expands to:

module type S = sig
  type t
  include Printable  with type t := t
  include Comparable with type t := t
end

All the same mixin syntax (params, constraints, +-separated lists) works identically to the [@@mixins] form. The only difference is that the implicit primary type is always t -- there is no way to choose a different name with the shorthand. If you need a different primary type name, use the full [@@mixins] attribute form.

Implementation overview

The rewriter is implemented as a global Ast_traverse.map registered via Driver.V2.register_transformation. It walks every sig block in both .ml and .mli files and runs a three-phase pipeline on each annotated type declaration:

1. Parse — flatten the Pexp_apply "+" expression tree into a mixin list.
2. Inject — prepend t := <type_name> to any mixin that has no t binding.
3. Desugar — emit psig_type (without the attribute) followed by one psig_include per mixin.

See lib/ppx_mixins.ml for the full documented source.

Limitations

• Tuple types (int * string) and function types (int -> string) on the RHS of a constraint are not supported. Wrap them in a type alias if needed.
• Only sig items are rewritten; [@@mixins] on a type inside a struct is not meaningful and will be left untouched (ppxlib will warn about an unused attribute).