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NAME
Ast_mapper - The interface of a -ppx rewriter
Module
Module Ast_mapper
Documentation
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 enables 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 .
Warning: this module is unstable and part of Compiler_libs .
A generic Parsetree mapper
type mapper = {
attribute : mapper -> Parsetree.attribute -> Parsetree.attribute ;
attributes : mapper -> Parsetree.attribute list -> Parsetree.attribute list ;
binding_op : mapper -> Parsetree.binding_op -> Parsetree.binding_op ;
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 ;
constant : mapper -> Parsetree.constant -> Parsetree.constant ;
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_substitution : mapper -> Parsetree.module_substitution ->
Parsetree.module_substitution ;
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_declaration : mapper -> Parsetree.open_declaration -> Parsetree.open_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_exception : mapper -> Parsetree.type_exception -> Parsetree.type_exception ;
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 , 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