ppx-traverse

Ppx\_traverse is a ppx_type_conv plugin generating open recursion classes from type definition. Users can overwrite a specific method of the generated classes in order to specialize the recursion on specific nodes. Ppx\_traverse is in particular used to generate the open recursion classes to traverse the OCaml AST in ppx_core.

For instance, this is the kind of code generated (the generated code is between the [@@deriving_inline ...] and [@@@end]):

type expression =
  | Var   of string
  | Const of int
  | Add   of expression * expression
  | If    of cond * expression * expression

and cond =
  | Cond_var   of string
  | Cond_const of bool
  | Cond_and   of cond * cond

[@@deriving_inline traverse_map]
class map = object(self)
  method virtual int    : int    -> int
  method virtual string : string -> string
  method virtual int    : int    -> int

  method expression = function
    | Var x -> Var (self#string x)
    | Const x -> Const (self#int x)
    | Add (x, y) -> Add (self#expression x, self#expression y)
    | If (x, y, z) -> If (self#cond x, self#expression y, self#expression z)

  method cond = function
    | Cond_var x -> Cond_var (self#string x)
    | Cond_const x -> Cond_const (self#bool x)
    | Cond_and (x, y) -> Cond_and (self#cond x, self#cond y)
[@@end]

Now if you wanted to do a deep-copy of an expression, replacing boolean variable foo by true:

let replace_var = object
  inherit Ppx_traverse_builtins.map
  inherit map as super

  method cond = function
    | Cond_var "foo" -> Cond_const true
    | c -> super#cond c
end

let replace_var expr = replace_var replace_var#expression expr

Ppx_traverse_builtins.map contains the definition for all the builtin types, such as int, string, list, … This module is distributed in a separate ppx_traverse_bultins package to break a circular dependency between Ppx\_core and Ppx\_traverse.

Classes

Ppx\_traverse can generate the following classes: map, iter, fold, fold_map, map_with_context, lift. [@@deriving traverse] is an alias to generate all the supported classes.

lift is a special class that is mostly useful to lift an OCaml constant to the AST that represent this constant. To do so, you can use ppx_metaquot.lifters from ppx_metaquot:

type t = { x : int; y : int } [@@deriving traverse_lift]

let expression_of_t ~loc t : Ast.expression =
  let lift = object
    inherit Ppx_metaquot_lifters.expression_lifters loc
    inherit lift
  end in
  lift#t t