# Module Ast_mapper

    module Ast_mapper: sig .. end

The interface of a -ppx rewriter

A -ppx rewriter is a program that accepts a serialized abstract syntax tree and outputs another, possibly modified, abstract syntax tree. This module encapsulates the interface between the compiler and the -ppx rewriters, handling such details as the serialization format, forwarding of command-line flags, and storing state.

`Ast_mapper.mapper` allows to implement AST rewriting using open recursion. A typical mapper would be based on `Ast_mapper.default_mapper`, a deep identity mapper, and will fall back on it for handling the syntax it does not modify. For example:

    open Asttypes
    open Parsetree
    open Ast_mapper

    let test_mapper argv =
      { default_mapper with
        expr = fun mapper expr ->
          match expr with
          | { pexp_desc = Pexp_extension ({ txt = "test" }, PStr [])} ->
            Ast_helper.Exp.constant (Const_int 42)
          | other -> default_mapper.expr mapper other; }

    let () =
      register "ppx_test" test_mapper

This -ppx rewriter, which replaces `[%test]` in expressions with the constant `42`, can be compiled using `ocamlc -o ppx_test -I +compiler-libs ocamlcommon.cma ppx_test.ml`.

## A generic Parsetree mapper

    type mapper = {

`` | `attribute : `mapper -> Parsetree.attribute -> Parsetree.attribute`;`
`` | `attributes : `mapper -> Parsetree.attribute list -> Parsetree.attribute list`;`
`` | `case : `mapper -> Parsetree.case -> Parsetree.case`;`
`` | `cases : `mapper -> Parsetree.case list -> Parsetree.case list`;`
`` | `class_declaration : `mapper -> Parsetree.class_declaration -> Parsetree.class_declaration`;`
`` | `class_description : `mapper -> Parsetree.class_description -> Parsetree.class_description`;`
`` | `class_expr : `mapper -> Parsetree.class_expr -> Parsetree.class_expr`;`
`` | `class_field : `mapper -> Parsetree.class_field -> Parsetree.class_field`;`
`` | `class_signature : `mapper -> Parsetree.class_signature -> Parsetree.class_signature`;`
`` | `class_structure : `mapper -> Parsetree.class_structure -> Parsetree.class_structure`;`
`` | `class_type : `mapper -> Parsetree.class_type -> Parsetree.class_type`;`
`` | `class_type_declaration : `mapper -> Parsetree.class_type_declaration -> Parsetree.class_type_declaration`;`
`` | `class_type_field : `mapper -> Parsetree.class_type_field -> Parsetree.class_type_field`;`
`` | `constructor_declaration : `mapper -> Parsetree.constructor_declaration -> Parsetree.constructor_declaration`;`
`` | `expr : `mapper -> Parsetree.expression -> Parsetree.expression`;`
`` | `extension : `mapper -> Parsetree.extension -> Parsetree.extension`;`
`` | `extension_constructor : `mapper -> Parsetree.extension_constructor -> Parsetree.extension_constructor`;`
`` | `include_declaration : `mapper -> Parsetree.include_declaration -> Parsetree.include_declaration`;`
`` | `include_description : `mapper -> Parsetree.include_description -> Parsetree.include_description`;`
`` | `label_declaration : `mapper -> Parsetree.label_declaration -> Parsetree.label_declaration`;`
`` | `location : `mapper -> Location.t -> Location.t`;`
`` | `module_binding : `mapper -> Parsetree.module_binding -> Parsetree.module_binding`;`
`` | `module_declaration : `mapper -> Parsetree.module_declaration -> Parsetree.module_declaration`;`
`` | `module_expr : `mapper -> Parsetree.module_expr -> Parsetree.module_expr`;`
`` | `module_type : `mapper -> Parsetree.module_type -> Parsetree.module_type`;`
`` | `module_type_declaration : `mapper -> Parsetree.module_type_declaration -> Parsetree.module_type_declaration`;`
`` | `open_description : `mapper -> Parsetree.open_description -> Parsetree.open_description`;`
`` | `pat : `mapper -> Parsetree.pattern -> Parsetree.pattern`;`
`` | `payload : `mapper -> Parsetree.payload -> Parsetree.payload`;`
`` | `signature : `mapper -> Parsetree.signature -> Parsetree.signature`;`
`` | `signature_item : `mapper -> Parsetree.signature_item -> Parsetree.signature_item`;`
`` | `structure : `mapper -> Parsetree.structure -> Parsetree.structure`;`
`` | `structure_item : `mapper -> Parsetree.structure_item -> Parsetree.structure_item`;`
`` | `typ : `mapper -> Parsetree.core_type -> Parsetree.core_type`;`
`` | `type_declaration : `mapper -> Parsetree.type_declaration -> Parsetree.type_declaration`;`
`` | `type_extension : `mapper -> Parsetree.type_extension -> Parsetree.type_extension`;`
`` | `type_kind : `mapper -> Parsetree.type_kind -> Parsetree.type_kind`;`
`` | `value_binding : `mapper -> Parsetree.value_binding -> Parsetree.value_binding`;`
`` | `value_description : `mapper -> Parsetree.value_description -> Parsetree.value_description`;`
`` | `with_constraint : `mapper -> Parsetree.with_constraint -> Parsetree.with_constraint`;`

}

A mapper record implements one "method" per syntactic category, using an open recursion style: each method takes as its first argument the mapper to be applied to children in the syntax tree.

    val default_mapper : mapper

A default mapper, which implements a "deep identity" mapping.

## Apply mappers to compilation units

    val tool_name : unit -> string

Can be used within a ppx preprocessor to know which tool is calling it `"ocamlc"`, `"ocamlopt"`, `"ocamldoc"`, `"ocamldep"`, `"ocaml"`, ... Some global variables that reflect command-line options are automatically synchronized between the calling tool and the ppx preprocessor: `Clflags.include_dirs`, `Config.load_path`, `Clflags.open_modules`, `Clflags.for_package`, `Clflags.debug`.

    val apply : source:string -> target:string -> mapper -> unit

Apply a mapper (parametrized by the unit name) to a dumped parsetree found in the `source` file and put the result in the `target` file. The `structure` or `signature` field of the mapper is applied to the implementation or interface.

    val run_main : (string list -> mapper) -> unit

Entry point to call to implement a standalone -ppx rewriter from a mapper, parametrized by the command line arguments. The current unit name can be obtained from `Location.input_name`. This function implements proper error reporting for uncaught exceptions.

## Registration API

    val register_function : (string -> (string list -> mapper) -> unit) ref

    val register : string -> (string list -> mapper) -> unit

Apply the `register_function`. The default behavior is to run the mapper immediately, taking arguments from the process command line. This is to support a scenario where a mapper is linked as a stand-alone executable.

It is possible to overwrite the `register_function` to define "-ppx drivers", which combine several mappers in a single process. Typically, a driver starts by defining `register_function` to a custom implementation, then lets ppx rewriters (linked statically or dynamically) register themselves, and then run all or some of them. It is also possible to have -ppx drivers apply rewriters to only specific parts of an AST.

The first argument to `register` is a symbolic name to be used by the ppx driver.

## Convenience functions to write mappers

    val map_opt : ('a -> 'b) -> 'a option -> 'b option

    val extension_of_error : Location.error -> Parsetree.extension

Encode an error into an 'ocaml.error' extension node which can be inserted in a generated Parsetree. The compiler will be responsible for reporting the error.

    val attribute_of_warning : Location.t -> string -> Parsetree.attribute

Encode a warning message into an 'ocaml.ppwarning' attribute which can be inserted in a generated Parsetree. The compiler will be responsible for reporting the warning.

## Helper functions to call external mappers

    val add_ppx_context_str : tool_name:string -> Parsetree.structure -> Parsetree.structure

Extract information from the current environment and encode it into an attribute which is prepended to the list of structure items in order to pass the information to an external processor.

    val add_ppx_context_sig : tool_name:string -> Parsetree.signature -> Parsetree.signature

Same as `add_ppx_context_str`, but for signatures.

    val drop_ppx_context_str : restore:bool -> Parsetree.structure -> Parsetree.structure

Drop the ocaml.ppx.context attribute from a structure. If `restore` is true, also restore the associated data in the current process.

    val drop_ppx_context_sig : restore:bool -> Parsetree.signature -> Parsetree.signature

Same as `drop_ppx_context_str`, but for signatures.

## Cookies

Cookies are used to pass information from a ppx processor to a further invocation of itself, when called from the OCaml toplevel (or other tools that support cookies).

    val set_cookie : string -> Parsetree.expression -> unit

    val get_cookie : string -> Parsetree.expression option