package ppxlib

README.md

This repository is the merge of several ppx-related repositories, namely:

• ppx_ast;

• ppx_core;

• ppx_driver;

• ppx_metaquot;

• ppx_traverse;

• ppx_traverse_builtins;

• ppx_type_conv.

Future development will happen only in this repository, and the aforementioned ones will only contain synonym definitions to provide backward compatibility. The structure of this repository is as follows:

• ast/ contains the ppxlib.ast library, that replaces ppx_ast;

• src/ contains the ppxlib library, that replaces ppx_core, ppx_driver, and type_conv;

• metaquot/ contains the ppxlib.metaquot library, that replaces ppx_metaquot;

• metaquot_lifters/ contains the ppxlib.metaquot_lifters library, that replaces ppx_metaquot.lifters;

• print-diff/ contains the ppxlib.print_diff library, that replaces ppx_driver.print_diff;

• runner/ contains the ppxlib.runner library, that replaces ppx_driver.runner;

• runner_as_ppx/ contains the ppxlib.runner_as_ppx library, that replaces ppx_driver.runner_as_ppx;

• traverse/ contains the ppxlib.traverse library, that replaces ppx_traverse;

• traverse_builtins/ contains the ppxlib.traverse_builtins/ library, that replaces ppx_traverse_builtins.

Ast

Ppxlib_ast selects a specific version of the OCaml Abstract Syntax Tree from the ocaml-migrate-parsetree project that is not necessarily the same one as the one being used by the compiler.

It also snapshots the corresponding parser and pretty-printer from the OCaml compiler, to create a full frontend independent of the version of OCaml.

This AST is used in all Jane Street ppx rewriters, and more generally in all ppx rewriters based on Ppxlib. Using a different AST allows to "detach" the ppx code from the compiler libraries, and allow to use ppx rewriters with new compilers before upgrading the ppx code.

Ppxlib

Ppxlib is a standard library for OCaml AST transformers, that uses the AST from Ppxlib_ast. It features:

• various auto-generated AST traversal using an open recursion scheme

• helpers for building AST fragments

• helpers for matching AST fragments

• a framework for dealing with attributes and extension points

• spellchecking and other hints on misspelled/misplaced attributes and extension points

• checks for unused attributes (they are otherwise silently dropped by the compiler)

Other ASTs

If you want to write code that works with several versions of Ppxlib using different AST versions, you can use the versionned alternatives for Ast_builder and Ast_pattern. For instance:

open Ppxlib
module Ast_builder = Ast_builder_403
module Ast_pattern = Ast_pattern_403

Drivers

A driver is an executable created from a set of OCaml AST transformers linked together with a command line frontend.

The aim is to provide a tool that can be used to:

• easily view the pre-processed version of a file, no need to construct a complex command line: ppx file.ml will do;

• use a single executable to run several transformations: no need to fork many times just for pre-processing;

• improved errors for misspelled/misplaced attributes and extension points.

Using driver-based rewriters

The recommended way to use rewriters based on Ppxlib.Driver is through dune. All you need to is add this line to your (library ...) or (executables ...) stanza:

(preprocess (pps (rewriter1 rewriter2 ... ppxlib.runner)))

dune will automatically build a static driver including all these rewriters. Note the ppxlib.runner at the end of the list, it will still work if you don't put but some specific features of ppxlib won't be available.

If you are not using dune, you can build a custom driver yourself using ocamlfind.

These methods are described in the following sections.

Creating a new Ppx_driver based rewriter

If using dune, you can just use the following jbuild file:

(library
 ((name        my_ppx)
  (public_name my_ppx)
  (kind ppx_rewriter)
  (libraries (ppxlib))
  (ppx_runtime_libraries (<runtime dependencies if any>))
  (preprocess (pps (ppx_metaquot)))))

(kind ppx_driver) has two effects:

1. it links the library with -linkall. Since plugins register themselves with the Ppx_driver library by doing a toplevel side effect, you need to be sure they are linked in the static driver to be taken into accound;

2. it instructs dune to produce a special META file that is compatible with the various ways of using ppx rewriters, i.e. for people not using dune.

Building a custom driver using ocamlfind

To build a custom driver using ocamlfind, simply link all the AST transformers together with the ppxlib.runner package at the end:

ocamlfind ocamlopt -predicates ppx_driver -o ppx -linkpkg \
  -package ppx_sexp_conv -package ppx_bin_prot \
  -package ppxlib.runner

Normally, ppxlib.driver-based rewriters should be build with the approriate -linkall option on individual libraries. If one is missing this option, the code rewriter might not get linked in. If this is the case, a workaround is to pass -linkall when linking the custom driver.

The driver as a command line tool

It recognizes the following command-line switches:

-loc-filename <string>      File name to use in locations
-reserve-namespace <string> Mark the given namespace as reserved
-no-check                   Disable checks (unsafe)
-apply <names>              Apply these transformations in order (comma-separated list)
-dont-apply <names>         Exclude these transformations
-no-merge                   Do not merge context free transformations (better for debugging rewriters)
-as-ppx                     Run as a -ppx rewriter (must be the first argument)
--as-ppx                    Same as -as-ppx
-as-pp                      Shorthand for: -dump-ast -embed-errors
--as-pp                     Same as -as-pp
-o <filename>               Output file (use '-' for stdout)
-                           Read input from stdin
-dump-ast                   Dump the marshaled ast to the output file instead of pretty-printing it
--dump-ast                  Same as -dump-ast
-dparsetree                 Print the parsetree (same as ocamlc -dparsetree)
-embed-errors               Embed errors in the output AST (default: true when -dump-ast, false otherwise)
-null                       Produce no output, except for errors
-impl <file>                Treat the input as a .ml file
--impl <file>               Same as -impl
-intf <file>                Treat the input as a .mli file
--intf <file>               Same as -intf
-debug-attribute-drop       Debug attribute dropping
-print-transformations      Print linked-in code transformations, in the order they are applied
-print-passes               Print the actual passes over the whole AST in the order they are applied
-ite-check                  Enforce that "complex" if branches are delimited (disabled if -pp is given)
-pp <command>               Pipe sources through preprocessor <command> (incompatible with -as-ppx)
-reconcile                  (WIP) Pretty print the output using a mix of the input source and the generated code
-reconcile-with-comments    (WIP) same as -reconcile but uses comments to enclose the generated code
-no-color                   Don't use colors when printing errors
-diff-cmd                   Diff command when using code expectations
-pretty                     Instruct code generators to improve the prettiness of the generated code
-styler                     Code styler
-help                       Display this list of options
--help                      Display this list of options

When passed a file as argument, a ppx driver will pretty-print the code transformed by all its built-in AST transformers. This gives a convenient way of seeing the code generated for a given attribute/extension.

A driver can simply be used as the argument of the -pp option of the OCaml compiler, or as the argument of the -ppx option by passing -as-ppx as first argument:

$ ocamlc -c -pp "ppx -as-pp" file.ml
$ ocamlc -c -ppx "ppx -as-ppx" file.ml

Rewriters as findlib libraries

Note: if using dune, you do not need to read this as dune already does all the right things for you.

In normal operation, Ppxlib.Driver rewriters are packaged as findlib libraries. When using dune everything is simple as preprocessors and normal dependencies are separated. However historically, people have been specifying both preprocessors and normal library dependencies together. Even worse, many build system still don't use a static driver and call out to multiple ppx commands to preprocess a single file, which slow downs compilation a lot.

In order for all these different methods to work properly, you need a peculiar META file. The rules are explained below.

It is recommended to split the findlib package into two:

1. one for the main library, which almost assume it is just a normal library;

2. another sub-package one for:

   • allowing to mix preprocessors and normal dependencies;

   • the method of calling one executable per rewriter.

In the rest we'll assume we are writing a META file for a ppx_foo rewriter, that itself uses the ppxlib and re libraries, and produces code using ppx_foo.runtime-lib.

We want the META file to support all of these:

1. mix normal dependencies and preprocessors, using one executable per rewriter:

   ocamlfind ocamlc -package ppx_foo -c toto.ml
   

2. mix normal dependencies and preprocessors, using a single ppx driver:

   $ ocamlfind ocamlc -package ppx_foo -predicates custom_ppx \
      -ppx ./custom-driver.exe -c toto.ml
   

3. build a custom driver:

   $ ocamlfind ocamlc -linkpkg -package ppx_foo -predicates ppx_driver \
      -o custom-driver.exe
   

4. build systems properly specifying preprocessors as such, separated from normal dependencies, as dune does

Since preprocessors and normal dependencies are always specified separately in jbuild files, dune just always set the ppx_driver predicates.

In the end the META file should look like this:

# Standard package, expect it assumes that the "ppx_driver" predicate is set
version                     = "42.0"
description                 = "interprets [%foo ...] extensions"
requires(ppx_driver)        = "ppxlib re"
archives(ppx_driver,byte)   = "ppx_foo.cma"
archives(ppx_driver,native) = "ppx_foo.cmxa"
plugin(ppx_driver,byte)     = "ppx_foo.cma"
plugin(ppx_driver,native)   = "ppx_foo.cmxs"

# This is what dune uses to find out the runtime dependencies of
# a preprocessor
ppx_runtime_deps = "ppx_foo.runtime-lib"

# This line makes things transparent for people mixing preprocessors
# and normal dependencies
requires(-ppx_driver) = "ppx_foo.deprecated-ppx-method"

package "deprecated-ppx-method" (
  description = "glue package for the deprecated method of using ppx"
  requires    = "ppx_foo.runtime-lib"
  ppx(-ppx_driver,-custom_ppx) = "./as-ppx.exe"
)

package "runtime-lib" ( ... )

You can check that this META works for all the 4 methods described above.

Derivers

The Ppxlib.Deriving module factors out functionality needed by different preprocessors that generate code from type specifications. Example libraries currently depending on Deriving:

• ppx_bin_prot;

• ppx_compare;

• ppx_fields_conv;

• ppx_sexp_conv;

• ppx_variants_conv.

Compatibility with ppx_deriving

Ppxlib.Deriving-based code generators are meant to be used with Ppxlib.Driver. However Deriving allows to export a compatible ppx_deriving plugin. By default, when not linked as part of a driver, packages using Deriving will just use ppx_deriving.

So for instance this will work as expected using ppx_deriving:

ocamlfind ocamlc -c -package ppx_sexp_conv foo.ml

For end users, the main advantage of using Deriving-based generators is that it will catch typos and attributes misplacement. For instance:

# type t = int [@@derivin sexp]
Error: Attribute `derivin' was not used
Hint: Did you mean deriving?
# type t = int [@@deriving sxp]
Error: ppxlib_deriving: 'sxp' is not a supported type deriving generator
Hint: Did you mean sexp?
# type t = int [@deriving sexp]
Error: Attribute `deriving' was not used
Hint: `deriving' is available for type declarations, type extensions
and extension constructors but is used here in the context of a core type.
Did you put it at the wrong level?"

Deriving Syntax

This section is only relevant if you are not using ppx_deriving.

Deriving interprets the [@@deriving ...] attributes on type declarations, exception declarations and extension constructor declarations:

type t = A | B [@@deriving sexp, bin_io]

sexp and bin_io are called generators. They are functions that generate code given the declaration. These functions are implemented by external libraries such as ppx_sexp_conv or ppx_bin_prot. Deriving itself provides no generator, it does only the dispatch.

Generators can take arguments. This is done using the following syntax:

type t = A | B [@@deriving foo ~arg:42]

For arguments that are just switches, it is common to use the following syntax:

type t = A | B [@@deriving foo ~bar]

Metaquot (and Metaquot_lifters)

Ppxlib_metaquot is a ppx rewriter allowing you to write values representing the OCaml AST in the OCaml syntax.

For instance:

[%expr x + 1]

is a value of type Ppxlib_ast.Ast.expression, represention the OCaml expression x + 1.

Ppxlib_metaquot is similar to ppx_tools.metaquot, expect that:

• it uses the version of the OCaml AST defined by Ppxlib_ast rather than the one from the current compiler

• it can be used simultaneously with other rewriters using Ppxlib.Driver.

Ppxlib_metaquot_lifters provides lifting functions for OCaml predefined types (int, string, list, ...).

Traverse (and Traverse_builtins)

Ppxlib_traverse is a Deriving 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. Ppxlib_traverse is in particular used to generate the open recursion classes to traverse the OCaml AST.

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, ...

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:

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